diff --git a/generate_release.py b/generate_release.py
index 5ee0a51..76c979d 100644
--- a/generate_release.py
+++ b/generate_release.py
@@ -33,7 +33,7 @@ def check_subp(x):
         raise Exception('Got nonzero exit code from command')
 
 def log(line):
-    print('[generate_release.py] ' + line)
+    print(f'[generate_release.py] {line}')
 
 # https://stackoverflow.com/questions/7833715/python-deleting-certain-file-extensions
 def remove_files_with_extensions(path, extensions):
@@ -43,23 +43,23 @@ def remove_files_with_extensions(path, extensions):
                 os.remove(os.path.join(root, file))
 
 def download_if_not_exists(file_name, url, sha256=None):
-    if not os.path.exists('./' + file_name):
+    if not os.path.exists(f'./{file_name}'):
         # Reject non-https URLs so a mistaken constant cannot cause a
         # plaintext download (bandit B310 hardening).
         if not url.startswith('https://'):
-            raise Exception('Refusing to download over non-https URL: ' + url)
-        log('Downloading ' + file_name + '..')
+            raise Exception(f'Refusing to download over non-https URL: {url}')
+        log(f'Downloading {file_name}..')
         data = urllib.request.urlopen(url).read()
-        log('Finished downloading ' + file_name)
-        with open('./' + file_name, 'wb') as f:
+        log(f'Finished downloading {file_name}')
+        with open(f'./{file_name}', 'wb') as f:
             f.write(data)
         if sha256:
             digest = hashlib.sha256(data).hexdigest()
             if digest != sha256:
-                log('Error: ' + file_name + ' has wrong hash: ' + digest)
+                log(f'Error: {file_name} has wrong hash: {digest}')
                 sys.exit(1)
     else:
-        log('Using existing ' + file_name)
+        log(f'Using existing {file_name}')
 
 def wine_run_shell(command):
     # Keep argv-style invocation (no shell) to avoid command injection.
@@ -120,7 +120,7 @@ if len(os.listdir('./yt-local')) == 0:
 # ----------- Generate embedded python distribution -----------
 os.environ['PYTHONDONTWRITEBYTECODE'] = '1'     # *.pyc files double the size of the distribution
 get_pip_url = 'https://bootstrap.pypa.io/get-pip.py'
-latest_dist_url = 'https://www.python.org/ftp/python/' + latest_version + '/python-' + latest_version
+latest_dist_url = f'https://www.python.org/ftp/python/{latest_version}/python-{latest_version}'
 if bitness == '32':
     latest_dist_url += '-embed-win32.zip'
 else:
@@ -142,7 +142,7 @@ else:
 
 download_if_not_exists('get-pip.py', get_pip_url)
 
-python_dist_name = 'python-dist-' + latest_version + '-' + bitness + '.zip'
+python_dist_name = f'python-dist-{latest_version}-{bitness}.zip'
 
 download_if_not_exists(python_dist_name, latest_dist_url)
 download_if_not_exists(visual_c_name,
@@ -203,7 +203,7 @@ and replaced with a .pth. Isolated mode will have to be specified manually.
 
 log('Removing ._pth')
 major_release = latest_version.split('.')[1]
-os.remove(r'./python/python3' + major_release + '._pth')
+os.remove(rf'./python/python3{major_release}._pth')
 
 log('Adding path_fixes.pth')
 with open(r'./python/path_fixes.pth', 'w', encoding='utf-8') as f:
@@ -214,7 +214,7 @@ with open(r'./python/path_fixes.pth', 'w', encoding='utf-8') as f:
 #  Need to add the directory where packages are installed,
 # and the parent directory (which is where the yt-local files are)
 major_release = latest_version.split('.')[1]
-with open('./python/python3' + major_release + '._pth', 'a', encoding='utf-8') as f:
+with open(rf'./python/python3{major_release}._pth', 'a', encoding='utf-8') as f:
     f.write('.\\Lib\\site-packages\n')
     f.write('..\n')'''
 
@@ -255,10 +255,10 @@ log('Copying python distribution into release folder')
 shutil.copytree(r'./python', r'./yt-local/python')
 
 # ----------- Create release zip -----------
-output_filename = 'yt-local-' + release_tag + '-' + suffix + '.zip'
-if os.path.exists('./' + output_filename):
+output_filename = f'yt-local-{release_tag}-{suffix}.zip'
+if os.path.exists(f'./{output_filename}'):
     log('Removing previous zipped release')
-    os.remove('./' + output_filename)
+    os.remove(f'./{output_filename}')
 log('Zipping release')
 check_subp(subprocess.run(['7z', '-mx=9', 'a', output_filename, './yt-local']))
 
diff --git a/server.py b/server.py
index 36ddae5..7f0100c 100644
--- a/server.py
+++ b/server.py
@@ -32,9 +32,9 @@ def youtu_be(env, start_response):
     id = env['PATH_INFO'][1:]
     env['PATH_INFO'] = '/watch'
     if not env['QUERY_STRING']:
-        env['QUERY_STRING'] = 'v=' + id
+        env['QUERY_STRING'] = f'v={id}'
     else:
-        env['QUERY_STRING'] += '&v=' + id
+        env['QUERY_STRING'] += f'&v={id}'
     yield from yt_app(env, start_response)
 
 
@@ -64,12 +64,12 @@ def proxy_site(env, start_response, video=False):
     if 'HTTP_RANGE' in env:
         send_headers['Range'] = env['HTTP_RANGE']
 
-    url = "https://" + env['SERVER_NAME'] + env['PATH_INFO']
+    url = f"https://{env['SERVER_NAME']}{env['PATH_INFO']}"
     # remove /name portion
     if video and '/videoplayback/name/' in url:
         url = url[0:url.rfind('/name/')]
     if env['QUERY_STRING']:
-        url += '?' + env['QUERY_STRING']
+        url += f'?{env["QUERY_STRING"]}'
 
     try_num = 1
     first_attempt = True
@@ -96,7 +96,7 @@ def proxy_site(env, start_response, video=False):
                                 +[('Access-Control-Allow-Origin', '*')])
 
         if first_attempt:
-            start_response(str(response.status) + ' ' + response.reason,
+            start_response(f"{response.status} {response.reason}",
                            response_headers)
 
         content_length = int(dict(response_headers).get('Content-Length', 0))
@@ -136,9 +136,8 @@ def proxy_site(env, start_response, video=False):
                     fail_byte = start + total_received
                     send_headers['Range'] = 'bytes=%d-%d' % (fail_byte, end)
                     print(
-                        'Warning: YouTube closed the connection before byte',
-                        str(fail_byte) + '.', 'Expected', start+content_length,
-                        'bytes.'
+                        f'Warning: YouTube closed the connection before byte {fail_byte}. '
+                        f'Expected {start+content_length} bytes.'
                     )
 
                     retry = True
@@ -185,7 +184,7 @@ def split_url(url):
     # python STILL doesn't have a proper regular expression engine like grep uses built in...
     match = re.match(r'(?:https?://)?([\w-]+(?:\.[\w-]+)+?)(/.*|$)', url)
     if match is None:
-        raise ValueError('Invalid or unsupported url: ' + url)
+        raise ValueError(f'Invalid or unsupported url: {url}')
 
     return match.group(1), match.group(2)
 
@@ -238,7 +237,7 @@ def site_dispatch(env, start_response):
             if base_name == '':
                 base_name = domain
             else:
-                base_name = domain + '.' + base_name
+                base_name = f"{domain}.{base_name}"
 
             try:
                 handler = site_handlers[base_name]
diff --git a/settings.py b/settings.py
index 3fd96ac..677a33a 100644
--- a/settings.py
+++ b/settings.py
@@ -397,14 +397,14 @@ acceptable_targets = SETTINGS_INFO.keys() | {
 def comment_string(comment):
     result = ''
     for line in comment.splitlines():
-        result += '# ' + line + '\n'
+        result += f'# {line}\n'
     return result
 
 
 def save_settings(settings_dict):
     with open(settings_file_path, 'w', encoding='utf-8') as file:
         for setting_name, setting_info in SETTINGS_INFO.items():
-            file.write(comment_string(setting_info['comment']) + setting_name + ' = ' + repr(settings_dict[setting_name]) + '\n\n')
+            file.write(f"{comment_string(setting_info['comment'])}{setting_name} = {repr(settings_dict[setting_name])}\n\n")
 
 
 def add_missing_settings(settings_dict):
@@ -481,7 +481,7 @@ upgrade_functions = {
 
 
 def log_ignored_line(line_number, message):
-    print('WARNING: Ignoring settings.txt line ' + str(line_number) + ' (' + message + ')')
+    print(f'WARNING: Ignoring settings.txt line {line_number} ({message})')
 
 
 if os.path.isfile("settings.txt"):
@@ -535,7 +535,7 @@ else:
                 continue
 
             if target.id not in acceptable_targets:
-                log_ignored_line(node.lineno,  target.id + " is not a valid setting")
+                log_ignored_line(node.lineno,  f"{target.id} is not a valid setting")
                 continue
 
             if type(node.value) not in attributes:
@@ -645,6 +645,6 @@ def settings_page():
         for func, old_value, value in to_call:
             func(old_value, value)
 
-        return flask.redirect(util.URL_ORIGIN + '/settings', 303)
+        return flask.redirect(f'{util.URL_ORIGIN}/settings', 303)
     else:
         flask.abort(400)
diff --git a/tests/test_shorts.py b/tests/test_shorts.py
index c5b7301..c413a8b 100644
--- a/tests/test_shorts.py
+++ b/tests/test_shorts.py
@@ -27,7 +27,7 @@ class TestChannelCtokenV5:
 
     def _decode_outer(self, ctoken):
         """Decode the outer protobuf layer of a ctoken."""
-        raw = base64.urlsafe_b64decode(ctoken + '==')
+        raw = base64.urlsafe_b64decode(f'{ctoken}==')
         return {fn: val for _, fn, val in proto.read_protobuf(raw)}
 
     def test_shorts_token_generates_without_error(self):
@@ -68,8 +68,8 @@ class TestChannelCtokenV5:
         assert t_with_shorts != t_without_shorts
 
         # Decode and verify the filter is present
-        raw_with_shorts = base64.urlsafe_b64decode(t_with_shorts + '==')
-        raw_without_shorts = base64.urlsafe_b64decode(t_without_shorts + '==')
+        raw_with_shorts = base64.urlsafe_b64decode(f'{t_with_shorts}==')
+        raw_without_shorts = base64.urlsafe_b64decode(f'{t_without_shorts}==')
 
         # Parse the outer protobuf structure
         import youtube.proto as proto
@@ -95,8 +95,8 @@ class TestChannelCtokenV5:
         decoded_without = urllib.parse.unquote(encoded_inner_without.decode('ascii'))
 
         # Decode the base64 data
-        decoded_with_bytes = base64.urlsafe_b64decode(decoded_with + '==')
-        decoded_without_bytes = base64.urlsafe_b64decode(decoded_without + '==')
+        decoded_with_bytes = base64.urlsafe_b64decode(f'{decoded_with}==')
+        decoded_without_bytes = base64.urlsafe_b64decode(f'{decoded_without}==')
 
         # Parse the decoded protobuf data
         fields_with = list(proto.read_protobuf(decoded_with_bytes))
diff --git a/youtube/__init__.py b/youtube/__init__.py
index b0e7cd3..885cadc 100644
--- a/youtube/__init__.py
+++ b/youtube/__init__.py
@@ -76,7 +76,7 @@ theme_names = {
 @yt_app.context_processor
 def inject_theme_preference():
     return {
-        'theme_path': '/youtube.com/static/' + theme_names[settings.theme] + '.css',
+        'theme_path': f'/youtube.com/static/{theme_names[settings.theme]}.css',
         'settings': settings,
         # Detect version
         'current_version': app_version()['version'],
@@ -145,9 +145,9 @@ def error_page(e):
                 ' exit node is overutilized. Try getting a new exit node by'
                 ' using the New Identity button in the Tor Browser.')
             if fetch_err.error_message:
-                error_message += '\n\n' + fetch_err.error_message
+                error_message += f'\n\n{fetch_err.error_message}'
             if fetch_err.ip:
-                error_message += '\n\nExit node IP address: ' + fetch_err.ip
+                error_message += f'\n\nExit node IP address: {fetch_err.ip}'
             return flask.render_template('error.html', error_message=error_message, slim=slim), 502
 
         elif error_code == '429':
@@ -157,7 +157,7 @@ def error_page(e):
                 '• Enable Tor routing in Settings for automatic IP rotation\n'
                 '• Use a VPN to change your IP address')
             if fetch_err.ip:
-                error_message += '\n\nYour IP: ' + fetch_err.ip
+                error_message += f'\n\nYour IP: {fetch_err.ip}'
             return flask.render_template('error.html', error_message=error_message, slim=slim), 429
 
         elif error_code == '502' and ('Failed to resolve' in str(fetch_err) or 'Failed to establish' in str(fetch_err)):
@@ -179,7 +179,7 @@ def error_page(e):
             # Catch-all for any other FetchError (400, etc.)
             error_message = f'Error communicating with YouTube ({error_code}).'
             if fetch_err.error_message:
-                error_message += '\n\n' + fetch_err.error_message
+                error_message += f'\n\n{fetch_err.error_message}'
             return flask.render_template('error.html', error_message=error_message, slim=slim), 502
 
     return flask.render_template('error.html', traceback=traceback.format_exc(),
diff --git a/youtube/channel.py b/youtube/channel.py
index 14a565e..8baf588 100644
--- a/youtube/channel.py
+++ b/youtube/channel.py
@@ -253,7 +253,7 @@ def get_channel_tab(channel_id, page="1", sort=3, tab='videos', view=1,
     # For now it seems to be constant for the API endpoint, not dependent
     # on the browsing session or channel
     key = 'AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8'
-    url = 'https://www.youtube.com/youtubei/v1/browse?key=' + key
+    url = f'https://www.youtube.com/youtubei/v1/browse?key={key}'
 
     data = {
         'context': {
@@ -285,8 +285,8 @@ def get_number_of_videos_channel(channel_id):
         return 1000
 
     # Uploads playlist
-    playlist_id = 'UU' + channel_id[2:]
-    url = 'https://m.youtube.com/playlist?list=' + playlist_id + '&pbj=1'
+    playlist_id = f'UU{channel_id[2:]}'
+    url = f'https://m.youtube.com/playlist?list={playlist_id}&pbj=1'
 
     try:
         response = util.fetch_url(url, headers_mobile,
@@ -328,7 +328,7 @@ def get_channel_id(base_url):
     # method that gives the smallest possible response at ~4 kb
     # needs to be as fast as possible
     base_url = base_url.replace('https://www', 'https://m') # avoid redirect
-    response = util.fetch_url(base_url + '/about?pbj=1', headers_mobile,
+    response = util.fetch_url(f'{base_url}/about?pbj=1', headers_mobile,
         debug_name='get_channel_id', report_text='Got channel id').decode('utf-8')
     match = channel_id_re.search(response)
     if match:
@@ -372,7 +372,7 @@ def get_channel_search_json(channel_id, query, page):
     ctoken = base64.urlsafe_b64encode(proto.nested(80226972, ctoken)).decode('ascii')
 
     key = 'AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8'
-    url = 'https://www.youtube.com/youtubei/v1/browse?key=' + key
+    url = f'https://www.youtube.com/youtubei/v1/browse?key={key}'
 
     data = {
         'context': {
@@ -414,18 +414,18 @@ def post_process_channel_info(info):
 
 def get_channel_first_page(base_url=None, tab='videos', channel_id=None, sort=None):
     if channel_id:
-        base_url = 'https://www.youtube.com/channel/' + channel_id
+        base_url = f'https://www.youtube.com/channel/{channel_id}'
 
     # Build URL with sort parameter
     # YouTube URL sort params: p=popular, dd=newest, lad=newest no shorts
     # Note: 'da' (oldest) was removed by YouTube in January 2026
-    url = base_url + '/' + tab + '?pbj=1&view=0'
+    url = f'{base_url}/{tab}?pbj=1&view=0'
     if sort:
         # Map sort values to YouTube's URL parameter values
         sort_map = {'3': 'dd', '4': 'lad'}
-        url += '&sort=' + sort_map.get(sort, 'dd')
+        url += f'&sort={sort_map.get(sort, "dd")}'
 
-    return util.fetch_url(url, headers_desktop, debug_name='gen_channel_' + tab)
+    return util.fetch_url(url, headers_desktop, debug_name=f'gen_channel_{tab}')
 
 
 playlist_sort_codes = {'2': "da", '3': "dd", '4': "lad"}
@@ -462,7 +462,7 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
         if page_number == 1:
             tasks = (
                 gevent.spawn(playlist.playlist_first_page,
-                             'UU' + channel_id[2:],
+                             f'UU{channel_id[2:]}',
                              report_text='Retrieved channel videos'),
                 gevent.spawn(get_metadata, channel_id),
             )
@@ -477,11 +477,11 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
                 set_cached_number_of_videos(channel_id, number_of_videos)
         else:
             tasks = (
-                gevent.spawn(playlist.get_videos, 'UU' + channel_id[2:],
+                gevent.spawn(playlist.get_videos, f'UU{channel_id[2:]}',
                              page_number, include_shorts=True),
                 gevent.spawn(get_metadata, channel_id),
                 gevent.spawn(get_number_of_videos_channel, channel_id),
-                gevent.spawn(playlist.playlist_first_page, 'UU' + channel_id[2:],
+                gevent.spawn(playlist.playlist_first_page, f'UU{channel_id[2:]}',
                              report_text='Retrieved channel video count'),
             )
             gevent.joinall(tasks)
@@ -567,10 +567,10 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
     elif tab == 'search' and channel_id:
         polymer_json = get_channel_search_json(channel_id, query, page_number)
     elif tab == 'search':
-        url = base_url + '/search?pbj=1&query=' + urllib.parse.quote(query, safe='')
+        url = f'{base_url}/search?pbj=1&query={urllib.parse.quote(query, safe="")}'
         polymer_json = util.fetch_url(url, headers_desktop, debug_name='gen_channel_search')
     elif tab != 'videos':
-        flask.abort(404, 'Unknown channel tab: ' + tab)
+        flask.abort(404, f'Unknown channel tab: {tab}')
 
     if polymer_json is not None and info is None:
         info = yt_data_extract.extract_channel_info(
@@ -583,7 +583,7 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
         return flask.render_template('error.html', error_message=info['error'])
 
     if channel_id:
-        info['channel_url'] = 'https://www.youtube.com/channel/' + channel_id
+        info['channel_url'] = f'https://www.youtube.com/channel/{channel_id}'
         info['channel_id'] = channel_id
     else:
         channel_id = info['channel_id']
@@ -663,22 +663,22 @@ def get_channel_page_general_url(base_url, tab, request, channel_id=None):
 @yt_app.route('/channel/<channel_id>/')
 @yt_app.route('/channel/<channel_id>/<tab>')
 def get_channel_page(channel_id, tab='videos'):
-    return get_channel_page_general_url('https://www.youtube.com/channel/' + channel_id, tab, request, channel_id)
+    return get_channel_page_general_url(f'https://www.youtube.com/channel/{channel_id}', tab, request, channel_id)
 
 
 @yt_app.route('/user/<username>/')
 @yt_app.route('/user/<username>/<tab>')
 def get_user_page(username, tab='videos'):
-    return get_channel_page_general_url('https://www.youtube.com/user/' + username, tab, request)
+    return get_channel_page_general_url(f'https://www.youtube.com/user/{username}', tab, request)
 
 
 @yt_app.route('/c/<custom>/')
 @yt_app.route('/c/<custom>/<tab>')
 def get_custom_c_page(custom, tab='videos'):
-    return get_channel_page_general_url('https://www.youtube.com/c/' + custom, tab, request)
+    return get_channel_page_general_url(f'https://www.youtube.com/c/{custom}', tab, request)
 
 
 @yt_app.route('/<custom>')
 @yt_app.route('/<custom>/<tab>')
 def get_toplevel_custom_page(custom, tab='videos'):
-    return get_channel_page_general_url('https://www.youtube.com/' + custom, tab, request)
+    return get_channel_page_general_url(f'https://www.youtube.com/{custom}', tab, request)
diff --git a/youtube/comments.py b/youtube/comments.py
index 8d03f22..c4e30dd 100644
--- a/youtube/comments.py
+++ b/youtube/comments.py
@@ -104,20 +104,19 @@ def post_process_comments_info(comments_info):
                 comment['replies_url'] = None
             comment['replies_url'] = concat_or_none(
                 util.URL_ORIGIN,
-                '/comments?replies=1&ctoken=' + ctoken)
+                f'/comments?replies=1&ctoken={ctoken}')
 
         if reply_count == 0:
             comment['view_replies_text'] = 'Reply'
         elif reply_count == 1:
             comment['view_replies_text'] = '1 reply'
         else:
-            comment['view_replies_text'] = str(reply_count) + ' replies'
+            comment['view_replies_text'] = f'{reply_count} replies'
 
         if comment['approx_like_count'] == '1':
             comment['likes_text'] = '1 like'
         else:
-            comment['likes_text'] = (str(comment['approx_like_count'])
-                                     + ' likes')
+            comment['likes_text'] = f"{comment['approx_like_count']} likes"
 
     comments_info['include_avatars'] = settings.enable_comment_avatars
     if comments_info['ctoken']:
@@ -163,14 +162,13 @@ def video_comments(video_id, sort=0, offset=0, lc='', secret_key=''):
     comments_info = {'error': None}
     try:
         other_sort_url = (
-            util.URL_ORIGIN + '/comments?ctoken='
-            + make_comment_ctoken(video_id, sort=1 - sort, lc=lc)
+            f"{util.URL_ORIGIN}/comments?ctoken="
+            f"{make_comment_ctoken(video_id, sort=1 - sort, lc=lc)}"
         )
-        other_sort_text = 'Sort by ' + ('newest' if sort == 0 else 'top')
+        other_sort_text = f'Sort by {"newest" if sort == 0 else "top"}'
 
-        this_sort_url = (util.URL_ORIGIN
-                         + '/comments?ctoken='
-                         + make_comment_ctoken(video_id, sort=sort, lc=lc))
+        this_sort_url = (f"{util.URL_ORIGIN}/comments?ctoken="
+                         f"{make_comment_ctoken(video_id, sort=sort, lc=lc)}")
 
         comments_info['comment_links'] = [
             (other_sort_text, other_sort_url),
@@ -188,17 +186,16 @@ def video_comments(video_id, sort=0, offset=0, lc='', secret_key=''):
         if e.code == '429' and settings.route_tor:
             comments_info['error'] = 'Error: YouTube blocked the request because the Tor exit node is overutilized.'
             if e.error_message:
-                comments_info['error'] += '\n\n' + e.error_message
-            comments_info['error'] += '\n\nExit node IP address: %s' % e.ip
+                comments_info['error'] += f'\n\n{e.error_message}'
+            comments_info['error'] += f'\n\nExit node IP address: {e.ip}'
         else:
-            comments_info['error'] = 'YouTube blocked the request. Error: %s' % str(e)
+            comments_info['error'] = f'YouTube blocked the request. Error: {e}'
 
     except Exception as e:
-        comments_info['error'] = 'YouTube blocked the request. Error: %s' % str(e)
+        comments_info['error'] = f'YouTube blocked the request. Error: {e}'
 
     if comments_info.get('error'):
-        print('Error retrieving comments for ' + str(video_id) + ':\n' +
-              comments_info['error'])
+        print(f'Error retrieving comments for {video_id}:\n{comments_info["error"]}')
 
     return comments_info
 
@@ -218,12 +215,10 @@ def get_comments_page():
             other_sort_url = None
         else:
             other_sort_url = (
-                util.URL_ORIGIN
-                + '/comments?ctoken='
-                + make_comment_ctoken(comments_info['video_id'],
-                                      sort=1-comments_info['sort'])
+                f'{util.URL_ORIGIN}/comments?ctoken='
+                f'{make_comment_ctoken(comments_info["video_id"], sort=1-comments_info["sort"])}'
             )
-        other_sort_text = 'Sort by ' + ('newest' if comments_info['sort'] == 0 else 'top')
+        other_sort_text = f'Sort by {"newest" if comments_info["sort"] == 0 else "top"}'
         comments_info['comment_links'] = [(other_sort_text, other_sort_url)]
 
     return flask.render_template(
diff --git a/youtube/local_playlist.py b/youtube/local_playlist.py
index e9b0b20..44207d2 100644
--- a/youtube/local_playlist.py
+++ b/youtube/local_playlist.py
@@ -92,9 +92,7 @@ def add_extra_info_to_videos(videos, playlist_name):
         util.add_extra_html_info(video)
         if video['id'] + '.jpg' in thumbnails:
             video['thumbnail'] = (
-                '/https://youtube.com/data/playlist_thumbnails/'
-                + playlist_name
-                + '/' + video['id'] + '.jpg')
+                f'/https://youtube.com/data/playlist_thumbnails/{playlist_name}/{video["id"]}.jpg')
         else:
             video['thumbnail'] = util.get_thumbnail_url(video['id'])
             missing_thumbnails.append(video['id'])
diff --git a/youtube/playlist.py b/youtube/playlist.py
index 3b784ba..e1e1342 100644
--- a/youtube/playlist.py
+++ b/youtube/playlist.py
@@ -20,7 +20,7 @@ def playlist_ctoken(playlist_id, offset, include_shorts=True):
 
     continuation_info = proto.string(3, proto.percent_b64encode(offset))
 
-    playlist_id = proto.string(2, 'VL' + playlist_id)
+    playlist_id = proto.string(2, f'VL{playlist_id}')
     pointless_nest = proto.string(80226972, playlist_id + continuation_info)
 
     return base64.urlsafe_b64encode(pointless_nest).decode('ascii')
@@ -30,7 +30,7 @@ def playlist_first_page(playlist_id, report_text="Retrieved playlist",
                         use_mobile=False):
     # Use innertube API (pbj=1 no longer works for many playlists)
     key = 'AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8'
-    url = 'https://www.youtube.com/youtubei/v1/browse?key=' + key
+    url = f'https://www.youtube.com/youtubei/v1/browse?key={key}'
 
     data = {
         'context': {
@@ -41,7 +41,7 @@ def playlist_first_page(playlist_id, report_text="Retrieved playlist",
                 'clientVersion': '2.20240327.00.00',
             },
         },
-        'browseId': 'VL' + playlist_id,
+        'browseId': f'VL{playlist_id}',
     }
 
     content_type_header = (('Content-Type', 'application/json'),)
@@ -58,7 +58,7 @@ def get_videos(playlist_id, page, include_shorts=True, use_mobile=False,
     page_size = 100
 
     key = 'AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8'
-    url = 'https://www.youtube.com/youtubei/v1/browse?key=' + key
+    url = f'https://www.youtube.com/youtubei/v1/browse?key={key}'
 
     ctoken = playlist_ctoken(playlist_id, (int(page)-1)*page_size,
                              include_shorts=include_shorts)
@@ -97,7 +97,7 @@ def get_playlist_page():
     if playlist_id.startswith('RD'):
         first_video_id = playlist_id[2:]  # video ID after 'RD' prefix
         return flask.redirect(
-            util.URL_ORIGIN + '/watch?v=' + first_video_id + '&list=' + playlist_id,
+            f'{util.URL_ORIGIN}/watch?v={first_video_id}&list={playlist_id}',
             302
         )
 
@@ -132,9 +132,9 @@ def get_playlist_page():
         if 'id' in item and not item.get('thumbnail'):
             item['thumbnail'] = f"{settings.img_prefix}https://i.ytimg.com/vi/{item['id']}/hqdefault.jpg"
 
-        item['url'] += '&list=' + playlist_id
+        item['url'] += f'&list={playlist_id}'
         if item['index']:
-            item['url'] += '&index=' + str(item['index'])
+            item['url'] += f'&index={item["index"]}'
 
     video_count = yt_data_extract.deep_get(info, 'metadata', 'video_count')
     if video_count is None:
diff --git a/youtube/proto.py b/youtube/proto.py
index db83a06..72a8a94 100644
--- a/youtube/proto.py
+++ b/youtube/proto.py
@@ -76,7 +76,7 @@ def read_varint(data):
         except IndexError:
             if i == 0:
                 raise EOFError()
-            raise Exception('Unterminated varint starting at ' + str(data.tell() - i))
+            raise Exception(f'Unterminated varint starting at {data.tell() - i}')
         result |= (byte & 127) << 7*i
         if not byte & 128:
             break
@@ -118,7 +118,7 @@ def read_protobuf(data):
         elif wire_type == 5:
             value = data.read(4)
         else:
-            raise Exception("Unknown wire type: " + str(wire_type) + " at position " + str(data.tell()))
+            raise Exception(f"Unknown wire type: {wire_type} at position {data.tell()}")
         yield (wire_type, field_number, value)
 
 
@@ -170,8 +170,7 @@ def _make_protobuf(data):
             elif field[0] == 2:
                 result += string(field[1], _make_protobuf(field[2]))
             else:
-                raise NotImplementedError('Wire type ' + str(field[0])
-                    + ' not implemented')
+                raise NotImplementedError(f'Wire type {field[0]} not implemented')
         return result
     return data
 
@@ -218,4 +217,4 @@ def b64_to_bytes(data):
     if isinstance(data, bytes):
         data = data.decode('ascii')
     data = data.replace("%3D", "=")
-    return base64.urlsafe_b64decode(data + "="*((4 - len(data) % 4) % 4))
+    return base64.urlsafe_b64decode(f'{data}={"=" * ((4 - len(data) % 4) % 4)}')
diff --git a/youtube/proto_debug.py b/youtube/proto_debug.py
index 927b385..bb8da7c 100644
--- a/youtube/proto_debug.py
+++ b/youtube/proto_debug.py
@@ -179,7 +179,7 @@ def read_varint(data):
         except IndexError:
             if i == 0:
                 raise EOFError()
-            raise Exception('Unterminated varint starting at ' + str(data.tell() - i))
+            raise Exception(f'Unterminated varint starting at {data.tell() - i}')
         result |= (byte & 127) << 7*i
         if not byte & 128:
             break
@@ -235,8 +235,7 @@ def _make_protobuf(data):
             elif field[0] == 2:
                 result += string(field[1], _make_protobuf(field[2]))
             else:
-                raise NotImplementedError('Wire type ' + str(field[0])
-                    + ' not implemented')
+                raise NotImplementedError(f'Wire type {field[0]} not implemented')
         return result
     return data
 
@@ -286,7 +285,7 @@ def b64_to_bytes(data):
     if isinstance(data, bytes):
         data = data.decode('ascii')
     data = data.replace("%3D", "=")
-    return base64.urlsafe_b64decode(data + "="*((4 - len(data) % 4) % 4))
+    return base64.urlsafe_b64decode(f'{data}={"=" * ((4 - len(data) % 4) % 4)}')
 # --------------------------------------------------------------------
 
 
@@ -344,7 +343,7 @@ fromhex = bytes.fromhex
 
 
 def aligned_ascii(data):
-    return ' '.join(' ' + chr(n) if n in range(32, 128) else ' _' for n in data)
+    return ' '.join(f' {chr(n)}' if n in range(32, 128) else ' _' for n in data)
 
 
 def parse_protobuf(data, mutable=False, spec=()):
@@ -372,7 +371,7 @@ def parse_protobuf(data, mutable=False, spec=()):
         elif wire_type == 5:
             value = data.read(4)
         else:
-            raise Exception("Unknown wire type: " + str(wire_type) + ", Tag: " + bytes_to_hex(varint_encode(tag)) + ", at position " + str(data.tell()))
+            raise Exception(f"Unknown wire type: {wire_type}, Tag: {bytes_to_hex(varint_encode(tag))}, at position {data.tell()}")
         if mutable:
             yield [wire_type, field_number, value]
         else:
@@ -453,7 +452,7 @@ def b32decode(s, casefold=False, map01=None):
     if map01 is not None:
         map01 = _bytes_from_decode_data(map01)
         assert len(map01) == 1, repr(map01)
-        s = s.translate(bytes.maketrans(b'01', b'O' + map01))
+        s = s.translate(bytes.maketrans(b'01', f'O{map01.decode("ascii")}'))
     if casefold:
         s = s.upper()
     # Strip off pad characters from the right.  We need to count the pad
@@ -494,7 +493,7 @@ def b32decode(s, casefold=False, map01=None):
 def dec32(data):
     if isinstance(data, bytes):
         data = data.decode('ascii')
-    return b32decode(data + "="*((8 - len(data)%8)%8))
+    return b32decode(f'{data}={"=" * ((8 - len(data)%8)%8)}')
 
 
 _patterns = [
@@ -563,9 +562,7 @@ def _pp(obj, indent):   # not my best work
         if len(obj) == 3:   # (wire_type, field_number, data)
             return obj.__repr__()
         else:   # (base64, [...])
-            return ('(' + obj[0].__repr__() + ',\n'
-                    + indent_lines(_pp(obj[1], indent), indent) + '\n'
-                    + ')')
+            return f"({obj[0].__repr__()},\n{indent_lines(_pp(obj[1], indent), indent)}\n)"
     elif isinstance(obj, list):
         # [wire_type, field_number, data]
         if (len(obj) == 3
@@ -577,13 +574,11 @@ def _pp(obj, indent):   # not my best work
         elif (len(obj) == 3
             and not any(isinstance(x, (list, tuple)) for x in obj[0:2])
         ):
-            return ('[' + obj[0].__repr__() + ', ' + obj[1].__repr__() + ',\n'
-                    + indent_lines(_pp(obj[2], indent), indent) + '\n'
-                    + ']')
+            return f"[{obj[0].__repr__()}, {obj[1].__repr__()},\n{indent_lines(_pp(obj[2], indent), indent)}\n]"
         else:
             s = '[\n'
             for x in obj:
-                s += indent_lines(_pp(x, indent), indent) + ',\n'
+                s += f"{indent_lines(_pp(x, indent), indent)},\n"
             s += ']'
             return s
     else:
diff --git a/youtube/search.py b/youtube/search.py
index 6e62e28..7573595 100644
--- a/youtube/search.py
+++ b/youtube/search.py
@@ -51,7 +51,7 @@ def get_search_json(query, page, autocorrect, sort, filters):
         'X-YouTube-Client-Name': '1',
         'X-YouTube-Client-Version': '2.20180418',
     }
-    url += "&pbj=1&sp=" + page_number_to_sp_parameter(page, autocorrect, sort, filters).replace("=", "%3D")
+    url += f"&pbj=1&sp={page_number_to_sp_parameter(page, autocorrect, sort, filters).replace('=', '%3D')}"
     content = util.fetch_url(url, headers=headers, report_text="Got search results", debug_name='search_results')
     info = json.loads(content)
     return info
diff --git a/youtube/subscriptions.py b/youtube/subscriptions.py
index dafea58..7d3efab 100644
--- a/youtube/subscriptions.py
+++ b/youtube/subscriptions.py
@@ -126,7 +126,7 @@ def delete_thumbnails(to_delete):
                 os.remove(os.path.join(thumbnails_directory, thumbnail))
                 existing_thumbnails.remove(video_id)
         except Exception:
-            print('Failed to delete thumbnail: ' + thumbnail)
+            print(f'Failed to delete thumbnail: {thumbnail}')
             traceback.print_exc()
 
 
@@ -184,7 +184,7 @@ def _get_videos(cursor, number_per_page, offset, tag=None):
             'time_published': exact_timestamp(db_video[3]) if db_video[4] else posix_to_dumbed_down(db_video[3]),
             'author':   db_video[5],
             'author_id': db_video[6],
-            'author_url': '/https://www.youtube.com/channel/' + db_video[6],
+            'author_url': f'/https://www.youtube.com/channel/{db_video[6]}',
         })
 
     return videos, pseudo_number_of_videos
@@ -304,9 +304,9 @@ def posix_to_dumbed_down(posix_time):
         if delta >= unit_time:
             quantifier = round(delta/unit_time)
             if quantifier == 1:
-                return '1 ' + unit_name + ' ago'
+                return f'1 {unit_name} ago'
             else:
-                return str(quantifier) + ' ' + unit_name + 's ago'
+                return f'{quantifier} {unit_name}s ago'
     else:
         raise Exception()
 
@@ -363,7 +363,7 @@ def autocheck_dispatcher():
             time_until_earliest_job = earliest_job['next_check_time'] - time.time()
 
             if time_until_earliest_job <= -5:   # should not happen unless we're running extremely slow
-                print('ERROR: autocheck_dispatcher got job scheduled in the past, skipping and rescheduling: ' + earliest_job['channel_id'] + ', ' + earliest_job['channel_name'] + ', ' + str(earliest_job['next_check_time']))
+                print(f'ERROR: autocheck_dispatcher got job scheduled in the past, skipping and rescheduling: {earliest_job["channel_id"]}, {earliest_job["channel_name"]}, {earliest_job["next_check_time"]}')
                 next_check_time = time.time() + 3600*secrets.randbelow(60)/60
                 with_open_db(_schedule_checking, earliest_job['channel_id'], next_check_time)
                 autocheck_jobs[earliest_job_index]['next_check_time'] = next_check_time
@@ -451,7 +451,7 @@ def check_channels_if_necessary(channel_ids):
 
 
 def _get_atoma_feed(channel_id):
-    url = 'https://www.youtube.com/feeds/videos.xml?channel_id=' + channel_id
+    url = f'https://www.youtube.com/feeds/videos.xml?channel_id={channel_id}'
     try:
         return util.fetch_url(url).decode('utf-8')
     except util.FetchError as e:
@@ -485,16 +485,15 @@ def _get_channel_videos_first_page(channel_id, channel_status_name):
         return channel_info
     except util.FetchError as e:
         if e.code == '429' and settings.route_tor:
-            error_message = ('Error checking channel ' + channel_status_name
-                + ': YouTube blocked the request because the'
-                + ' Tor exit node is overutilized. Try getting a new exit node'
-                + ' by using the New Identity button in the Tor Browser.')
+            error_message = (f'Error checking channel {channel_status_name}: '
+                f'YouTube blocked the request because the Tor exit node is overutilized. '
+                f'Try getting a new exit node by using the New Identity button in the Tor Browser.')
             if e.ip:
-                error_message += ' Exit node IP address: ' + e.ip
+                error_message += f' Exit node IP address: {e.ip}'
             print(error_message)
             return None
         elif e.code == '502':
-            print('Error checking channel', channel_status_name + ':', str(e))
+            print(f'Error checking channel {channel_status_name}: {e}')
             return None
         raise
 
@@ -505,7 +504,7 @@ def _get_upstream_videos(channel_id):
     except KeyError:
         channel_status_name = channel_id
 
-    print("Checking channel: " + channel_status_name)
+    print(f"Checking channel: {channel_status_name}")
 
     tasks = (
         # channel page, need for video duration
@@ -550,15 +549,15 @@ def _get_upstream_videos(channel_id):
             times_published[video_id_element.text] = time_published
 
     except ValueError:
-        print('Failed to read atoma feed for ' + channel_status_name)
+        print(f'Failed to read atoma feed for {channel_status_name}')
         traceback.print_exc()
     except defusedxml.ElementTree.ParseError:
-        print('Failed to read atoma feed for ' + channel_status_name)
+        print(f'Failed to read atoma feed for {channel_status_name}')
 
     if channel_info is None: # there was an error
         return
     if channel_info['error']:
-        print('Error checking channel ' + channel_status_name + ': ' + channel_info['error'])
+        print(f'Error checking channel {channel_status_name}: {channel_info["error"]}')
         return
 
     videos = channel_info['items']
@@ -1023,7 +1022,7 @@ def get_subscriptions_page():
             tag = request.args.get('tag', None)
             videos, number_of_videos_in_db = _get_videos(cursor, 60, (page - 1)*60, tag)
             for video in videos:
-                video['thumbnail'] = util.URL_ORIGIN + '/data/subscription_thumbnails/' + video['id'] + '.jpg'
+                video['thumbnail'] = f'{util.URL_ORIGIN}/data/subscription_thumbnails/{video["id"]}.jpg'
                 video['type'] = 'video'
                 video['item_size'] = 'small'
                 util.add_extra_html_info(video)
@@ -1033,7 +1032,7 @@ def get_subscriptions_page():
             subscription_list = []
             for channel_name, channel_id, muted in _get_subscribed_channels(cursor):
                 subscription_list.append({
-                    'channel_url': util.URL_ORIGIN + '/channel/' + channel_id,
+                    'channel_url': f'{util.URL_ORIGIN}/channel/{channel_id}',
                     'channel_name': channel_name,
                     'channel_id': channel_id,
                     'muted': muted,
@@ -1109,17 +1108,17 @@ def serve_subscription_thumbnail(thumbnail):
     for quality in ('hq720.jpg', 'sddefault.jpg', 'hqdefault.jpg'):
         url = f"https://i.ytimg.com/vi/{video_id}/{quality}"
         try:
-            image = util.fetch_url(url, report_text="Saved thumbnail: " + video_id)
+            image = util.fetch_url(url, report_text=f"Saved thumbnail: {video_id}")
             break
         except util.FetchError as e:
             if '404' in str(e):
                 continue
-            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            print(f"Failed to download thumbnail for {video_id}: {e}")
             flask.abort(500)
         except urllib.error.HTTPError as e:
             if e.code == 404:
                 continue
-            print("Failed to download thumbnail for " + video_id + ": " + str(e))
+            print(f"Failed to download thumbnail for {video_id}: {e}")
             flask.abort(e.code)
 
     if image is None:
diff --git a/youtube/util.py b/youtube/util.py
index 5e60d1c..7901a89 100644
--- a/youtube/util.py
+++ b/youtube/util.py
@@ -72,7 +72,7 @@ class TorManager:
     def __init__(self):
         self.old_tor_connection_pool = None
         self.tor_connection_pool = urllib3.contrib.socks.SOCKSProxyManager(
-            'socks5h://127.0.0.1:' + str(settings.tor_port) + '/',
+            f'socks5h://127.0.0.1:{settings.tor_port}/',
             cert_reqs='CERT_REQUIRED')
         self.tor_pool_refresh_time = time.monotonic()
         settings.add_setting_changed_hook(
@@ -92,7 +92,7 @@ class TorManager:
         self.old_tor_connection_pool = self.tor_connection_pool
 
         self.tor_connection_pool = urllib3.contrib.socks.SOCKSProxyManager(
-            'socks5h://127.0.0.1:' + str(settings.tor_port) + '/',
+            f'socks5h://127.0.0.1:{settings.tor_port}/',
             cert_reqs='CERT_REQUIRED')
         self.tor_pool_refresh_time = time.monotonic()
 
@@ -198,9 +198,9 @@ class HTTPAsymmetricCookieProcessor(urllib.request.BaseHandler):
 class FetchError(Exception):
     def __init__(self, code, reason='', ip=None, error_message=None):
         if error_message:
-            string = code + ' ' + reason + ': ' + error_message
+            string = f"{code} {reason}: {error_message}"
         else:
-            string = 'HTTP error during request: ' + code + ' ' + reason
+            string = f"HTTP error during request: {code} {reason}"
         Exception.__init__(self, string)
         self.code = code
         self.reason = reason
@@ -294,14 +294,12 @@ def fetch_url_response(url, headers=(), timeout=15, data=None,
             exception_cause = e.__context__.__context__
             if (isinstance(exception_cause, socks.ProxyConnectionError)
                     and settings.route_tor):
-                msg = ('Failed to connect to Tor. Check that Tor is open and '
-                       'that your internet connection is working.\n\n'
-                       + str(e))
+                msg = f'Failed to connect to Tor. Check that Tor is open and that your internet connection is working.\n\n{e}'
                 raise FetchError('502', reason='Bad Gateway',
                                  error_message=msg)
             elif isinstance(e.__context__,
                             urllib3.exceptions.NewConnectionError):
-                msg = 'Failed to establish a connection.\n\n' + str(e)
+                msg = f'Failed to establish a connection.\n\n{e}'
                 raise FetchError(
                     '502', reason='Bad Gateway',
                      error_message=msg)
@@ -391,7 +389,7 @@ def fetch_url(url, headers=(), timeout=15, report_text=None, data=None,
                 if error:
                     raise FetchError(
                         '429', reason=response.reason, ip=ip,
-                        error_message='Automatic circuit change: ' + error)
+                        error_message=f'Automatic circuit change: {error}')
                 continue  # retry with new identity
 
             # Check for client errors (400, 404) - don't retry these
@@ -467,10 +465,7 @@ def head(url, use_tor=False, report_text=None, max_redirects=10):
     headers = {'User-Agent': 'Python-urllib'}
     response = pool.request('HEAD', url, headers=headers, retries=retries)
     if report_text:
-        print(
-            report_text,
-            '    Latency:',
-            round(time.monotonic() - start_time, 3))
+        print(f'{report_text}    Latency: {round(time.monotonic() - start_time, 3)}')
     return response
 
 mobile_user_agent = 'Mozilla/5.0 (Linux; Android 7.0; Redmi Note 4 Build/NRD90M) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/69.0.3497.100 Mobile Safari/537.36'
@@ -544,16 +539,16 @@ def download_thumbnail(save_directory, video_id):
     for quality in ('hq720.jpg', 'sddefault.jpg', 'hqdefault.jpg'):
         url = f'https://i.ytimg.com/vi/{video_id}/{quality}'
         try:
-            thumbnail = fetch_url(url, report_text='Saved thumbnail: ' + video_id)
+            thumbnail = fetch_url(url, report_text=f'Saved thumbnail: {video_id}')
         except FetchError as e:
             if '404' in str(e):
                 continue
-            print('Failed to download thumbnail for ' + video_id + ': ' + str(e))
+            print(f'Failed to download thumbnail for {video_id}: {e}')
             return False
         except urllib.error.HTTPError as e:
             if e.code == 404:
                 continue
-            print('Failed to download thumbnail for ' + video_id + ': ' + str(e))
+            print(f'Failed to download thumbnail for {video_id}: {e}')
             return False
         try:
             with open(save_location, 'wb') as f:
@@ -563,7 +558,7 @@ def download_thumbnail(save_directory, video_id):
             with open(save_location, 'wb') as f:
                 f.write(thumbnail)
         return True
-    print('No thumbnail available for ' + video_id)
+    print(f'No thumbnail available for {video_id}')
     return False
 
 
@@ -698,7 +693,7 @@ def prefix_urls(item):
 
 def add_extra_html_info(item):
     if item['type'] == 'video':
-        item['url'] = (URL_ORIGIN + '/watch?v=' + item['id']) if item.get('id') else None
+        item['url'] = f'{URL_ORIGIN}/watch?v={item["id"]}' if item.get('id') else None
 
         video_info = {}
         for key in ('id', 'title', 'author', 'duration', 'author_id'):
@@ -721,7 +716,7 @@ def add_extra_html_info(item):
         item['url'] = concat_or_none(URL_ORIGIN, "/channel/", item['id'])
 
     if item.get('author_id') and 'author_url' not in item:
-        item['author_url'] = URL_ORIGIN + '/channel/' + item['author_id']
+        item['author_url'] = f'{URL_ORIGIN}/channel/{item["author_id"]}'
 
 
 def check_gevent_exceptions(*tasks):
@@ -967,7 +962,7 @@ def call_youtube_api(client, api, data):
     user_agent = context['client'].get('userAgent') or mobile_user_agent
     visitor_data = get_visitor_data()
 
-    url = 'https://' + host + '/youtubei/v1/' + api + '?key=' + key
+    url = f'https://{host}/youtubei/v1/{api}?key={key}'
     if visitor_data:
         context['client'].update({'visitorData': visitor_data})
     data['context'] = context
@@ -978,8 +973,8 @@ def call_youtube_api(client, api, data):
         headers = ( *headers, ('X-Goog-Visitor-Id', visitor_data ))
     response = fetch_url(
         url, data=data, headers=headers,
-        debug_name='youtubei_' + api + '_' + client,
-        report_text='Fetched ' + client + ' youtubei ' + api
+        debug_name=f'youtubei_{api}_{client}',
+        report_text=f'Fetched {client} youtubei {api}'
     ).decode('utf-8')
     return response
 
diff --git a/youtube/watch.py b/youtube/watch.py
index 7f87215..9d1e442 100644
--- a/youtube/watch.py
+++ b/youtube/watch.py
@@ -53,7 +53,7 @@ def get_video_sources(info, target_resolution):
         if fmt['acodec'] and fmt['vcodec']:
             if fmt.get('audio_track_is_default', True) is False:
                 continue
-            source = {'type': 'video/' + fmt['ext'],
+            source = {'type': f"video/{fmt['ext']}",
                       'quality_string': short_video_quality_string(fmt)}
             source['quality_string'] += ' (integrated)'
             source.update(fmt)
@@ -70,10 +70,10 @@ def get_video_sources(info, target_resolution):
         if fmt['acodec'] and not fmt['vcodec'] and (fmt['audio_bitrate'] or fmt['bitrate']):
             if fmt['bitrate']:
                 fmt['audio_bitrate'] = int(fmt['bitrate']/1000)
-            source = {'type': 'audio/' + fmt['ext'],
+            source = {'type': f"audio/{fmt['ext']}",
                       'quality_string': audio_quality_string(fmt)}
             source.update(fmt)
-            source['mime_codec'] = source['type'] + '; codecs="' + source['acodec'] + '"'
+            source['mime_codec'] = f"{source['type']}; codecs=\"{source['acodec']}\""
             tid = fmt.get('audio_track_id') or 'default'
             if tid not in audio_by_track:
                 audio_by_track[tid] = {
@@ -85,11 +85,11 @@ def get_video_sources(info, target_resolution):
         elif all(fmt[attr] for attr in ('vcodec', 'quality', 'width', 'fps', 'file_size')):
             if codec_name(fmt['vcodec']) == 'unknown':
                 continue
-            source = {'type': 'video/' + fmt['ext'],
+            source = {'type': f"video/{fmt['ext']}",
                       'quality_string': short_video_quality_string(fmt)}
             source.update(fmt)
-            source['mime_codec'] = source['type'] + '; codecs="' + source['vcodec'] + '"'
-            quality = str(fmt['quality']) + 'p' + str(fmt['fps'])
+            source['mime_codec'] = f"{source['type']}; codecs=\"{source['vcodec']}\""
+            quality = f"{fmt['quality']}p{fmt['fps']}"
             video_only_sources.setdefault(quality, []).append(source)
 
     audio_tracks = []
@@ -141,7 +141,7 @@ def get_video_sources(info, target_resolution):
 
         def video_rank(src):
             ''' Sort by settings preference. Use file size as tiebreaker '''
-            setting_name = 'codec_rank_' + codec_name(src['vcodec'])
+            setting_name = f'codec_rank_{codec_name(src["vcodec"])}'
             return (settings.current_settings_dict[setting_name],
                     src['file_size'])
         pair_info['videos'].sort(key=video_rank)
@@ -183,7 +183,7 @@ def make_caption_src(info, lang, auto=False, trans_lang=None):
     if auto:
         label += ' (Automatic)'
     if trans_lang:
-        label += ' -> ' + trans_lang
+        label += f' -> {trans_lang}'
 
     # Try to use Android caption URL directly (no PO Token needed)
     caption_url = None
@@ -204,7 +204,7 @@ def make_caption_src(info, lang, auto=False, trans_lang=None):
         else:
             caption_url += '&fmt=vtt'
         if trans_lang:
-            caption_url += '&tlang=' + trans_lang
+            caption_url += f'&tlang={trans_lang}'
         url = util.prefix_url(caption_url)
     else:
         # Fallback to old method
@@ -357,10 +357,10 @@ def decrypt_signatures(info, video_id):
 
     player_name = info['player_name']
     if player_name in decrypt_cache:
-        print('Using cached decryption function for: ' + player_name)
+        print(f'Using cached decryption function for: {player_name}')
         info['decryption_function'] = decrypt_cache[player_name]
     else:
-        base_js = util.fetch_url(info['base_js'], debug_name='base.js', report_text='Fetched player ' + player_name)
+        base_js = util.fetch_url(info['base_js'], debug_name='base.js', report_text=f'Fetched player {player_name}')
         base_js = base_js.decode('utf-8')
         err = yt_data_extract.extract_decryption_function(info, base_js)
         if err:
@@ -387,11 +387,11 @@ def fetch_player_response(client, video_id):
 def fetch_watch_page_info(video_id, playlist_id, index):
     # bpctr=9999999999 will bypass are-you-sure dialogs for controversial
     # videos
-    url = 'https://m.youtube.com/embed/' + video_id + '?bpctr=9999999999'
+    url = f'https://m.youtube.com/embed/{video_id}?bpctr=9999999999'
     if playlist_id:
-        url += '&list=' + playlist_id
+        url += f'&list={playlist_id}'
     if index:
-        url += '&index=' + index
+        url += f'&index={index}'
 
     headers = (
         ('Accept', '*/*'),
@@ -493,7 +493,7 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
         # Register HLS audio tracks for proxy access
         added = 0
         for lang, track in info['hls_audio_tracks'].items():
-            ck = video_id + '_' + lang
+            ck = f"{video_id}_{lang}"
             from youtube.hls_cache import register_track
             register_track(ck, track['hls_url'],
                            video_id=video_id, track_id=lang)
@@ -502,7 +502,7 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
                 'audio_track_id': lang,
                 'audio_track_name': track['name'],
                 'audio_track_is_default': track['is_default'],
-                'itag': 'hls_' + lang,
+                'itag': f'hls_{lang}',
                 'ext': 'mp4',
                 'audio_bitrate': 128,
                 'bitrate': 128000,
@@ -516,7 +516,7 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
                 'fps': None,
                 'init_range': {'start': 0, 'end': 0},
                 'index_range': {'start': 0, 'end': 0},
-                'url': '/ytl-api/audio-track?id=' + urllib.parse.quote(ck),
+                'url': f'/ytl-api/audio-track?id={urllib.parse.quote(ck)}',
                 's': None,
                 'sp': None,
                 'quality': None,
@@ -538,11 +538,11 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
 
     # Register HLS manifest for proxying
     if info['hls_manifest_url']:
-        ck = video_id + '_video'
+        ck = f"{video_id}_video"
         from youtube.hls_cache import register_track
         register_track(ck, info['hls_manifest_url'], video_id=video_id, track_id='video')
         # Use proxy URL instead of direct Google Video URL
-        info['hls_manifest_url'] = '/ytl-api/hls-manifest?id=' + urllib.parse.quote(ck)
+        info['hls_manifest_url'] = f'/ytl-api/hls-manifest?id={urllib.parse.quote(ck)}'
 
     # Fallback to 'ios' if no valid URLs are found
     if not info.get('formats') or info.get('player_urls_missing'):
@@ -566,7 +566,7 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
     if info.get('formats'):
         decryption_error = decrypt_signatures(info, video_id)
         if decryption_error:
-            info['playability_error'] = 'Error decrypting url signatures: ' + decryption_error
+            info['playability_error'] = f'Error decrypting url signatures: {decryption_error}'
 
     # check if urls ready (non-live format) in former livestream
     # urls not ready if all of them have no filesize
@@ -623,9 +623,9 @@ def extract_info(video_id, use_invidious, playlist_id=None, index=None):
 
 def video_quality_string(format):
     if format['vcodec']:
-        result = str(format['width'] or '?') + 'x' + str(format['height'] or '?')
+        result = f"{format['width'] or '?'}x{format['height'] or '?'}"
         if format['fps']:
-            result += ' ' + str(format['fps']) + 'fps'
+            result += f" {format['fps']}fps"
         return result
     elif format['acodec']:
         return 'audio only'
@@ -634,7 +634,7 @@ def video_quality_string(format):
 
 
 def short_video_quality_string(fmt):
-    result = str(fmt['quality'] or '?') + 'p'
+    result = f"{fmt['quality'] or '?'}p"
     if fmt['fps']:
         result += str(fmt['fps'])
     if fmt['vcodec'].startswith('av01'):
@@ -642,18 +642,18 @@ def short_video_quality_string(fmt):
     elif fmt['vcodec'].startswith('avc'):
         result += ' h264'
     else:
-        result += ' ' + fmt['vcodec']
+        result += f" {fmt['vcodec']}"
     return result
 
 
 def audio_quality_string(fmt):
     if fmt['acodec']:
         if fmt['audio_bitrate']:
-            result = '%d' % fmt['audio_bitrate'] + 'k'
+            result = f"{fmt['audio_bitrate']}k"
         else:
             result = '?k'
         if fmt['audio_sample_rate']:
-            result += ' ' + '%.3G' % (fmt['audio_sample_rate']/1000) + 'kHz'
+            result += f" {'%.3G' % (fmt['audio_sample_rate']/1000)}kHz"
         return result
     elif fmt['vcodec']:
         return 'video only'
@@ -737,9 +737,9 @@ def get_audio_track():
                 seg = line if line.startswith('http') else urljoin(playlist_base, line)
                 # Always use &seg= parameter, never &url= for segments
                 playlist_lines.append(
-                    base_url + '/ytl-api/audio-track?id='
-                    + urllib.parse.quote(cache_key)
-                    + '&seg=' + urllib.parse.quote(seg, safe='')
+                    f'{base_url}/ytl-api/audio-track?id='
+                    f'{urllib.parse.quote(cache_key)}'
+                    f'&seg={urllib.parse.quote(seg, safe="")}'
                 )
 
             playlist = '\n'.join(playlist_lines)
@@ -797,9 +797,7 @@ def get_audio_track():
                             return url
                         if not url.startswith('http://') and not url.startswith('https://'):
                             url = urljoin(playlist_base, url)
-                        return (base_url + '/ytl-api/audio-track?id='
-                                + urllib.parse.quote(cache_key)
-                                + '&seg=' + urllib.parse.quote(url, safe=''))
+                        return f'{base_url}/ytl-api/audio-track?id={urllib.parse.quote(cache_key)}&seg={urllib.parse.quote(url, safe="")}'
 
                     playlist_lines = []
                     for line in playlist.split('\n'):
@@ -812,7 +810,7 @@ def get_audio_track():
                         if line.startswith('#') and 'URI=' in line:
                             def rewrite_uri_attr(match):
                                 uri = match.group(1)
-                                return 'URI="' + proxy_url(uri) + '"'
+                                return f'URI="{proxy_url(uri)}"'
                             line = _re.sub(r'URI="([^"]+)"', rewrite_uri_attr, line)
                             playlist_lines.append(line)
                         elif line.startswith('#'):
@@ -883,9 +881,7 @@ def get_audio_track():
             if segment_url.startswith('/ytl-api/audio-track'):
                 return segment_url
             base_url = request.url_root.rstrip('/')
-            return (base_url + '/ytl-api/audio-track?id='
-                    + urllib.parse.quote(cache_key)
-                    + '&seg=' + urllib.parse.quote(segment_url))
+            return f'{base_url}/ytl-api/audio-track?id={urllib.parse.quote(cache_key)}&seg={urllib.parse.quote(segment_url)}'
 
         playlist_lines = []
         for line in playlist.split('\n'):
@@ -949,14 +945,10 @@ def get_hls_manifest():
 
             if is_audio_track:
                 # Audio track playlist - proxy through audio-track endpoint
-                return (base_url + '/ytl-api/audio-track?id='
-                        + urllib.parse.quote(cache_key)
-                        + '&url=' + urllib.parse.quote(url, safe=''))
+                return f'{base_url}/ytl-api/audio-track?id={urllib.parse.quote(cache_key)}&url={urllib.parse.quote(url, safe="")}'
             else:
                 # Video segment or variant playlist - proxy through audio-track endpoint
-                return (base_url + '/ytl-api/audio-track?id='
-                        + urllib.parse.quote(cache_key)
-                        + '&seg=' + urllib.parse.quote(url, safe=''))
+                return f'{base_url}/ytl-api/audio-track?id={urllib.parse.quote(cache_key)}&seg={urllib.parse.quote(url, safe="")}'
 
         # Parse and rewrite the manifest
         manifest_lines = []
@@ -974,7 +966,7 @@ def get_hls_manifest():
                     nonlocal rewritten_count
                     uri = match.group(1)
                     rewritten_count += 1
-                    return 'URI="' + rewrite_url(uri, is_audio_track=True) + '"'
+                    return f'URI="{rewrite_url(uri, is_audio_track=True)}"'
                 line = _re.sub(r'URI="([^"]+)"', rewrite_media_uri, line)
                 manifest_lines.append(line)
             elif line.startswith('#'):
@@ -1053,7 +1045,7 @@ def get_storyboard_vtt():
     ts = 0  # current timestamp
 
     for i in range(storyboard.storyboard_count):
-        url = '/' + storyboard.url.replace("$M", str(i))
+        url = f'/{storyboard.url.replace("$M", str(i))}'
         interval = storyboard.interval
         w, h = storyboard.width, storyboard.height
         w_cnt, h_cnt = storyboard.width_cnt, storyboard.height_cnt
@@ -1078,7 +1070,7 @@ def get_watch_page(video_id=None):
     if not video_id:
         return flask.render_template('error.html', error_message='Missing video id'), 404
     if len(video_id) < 11:
-        return flask.render_template('error.html', error_message='Incomplete video id (too short): ' + video_id), 404
+        return flask.render_template('error.html', error_message=f'Incomplete video id (too short): {video_id}'), 404
 
     time_start_str = request.args.get('t', '0s')
     time_start = 0
@@ -1141,9 +1133,9 @@ def get_watch_page(video_id=None):
             util.prefix_urls(item)
             util.add_extra_html_info(item)
             if playlist_id:
-                item['url'] += '&list=' + playlist_id
+                item['url'] += f'&list={playlist_id}'
             if item['index']:
-                item['url'] += '&index=' + str(item['index'])
+                item['url'] += f'&index={item["index"]}'
         info['playlist']['author_url'] = util.prefix_url(
             info['playlist']['author_url'])
     if settings.img_prefix:
@@ -1159,16 +1151,16 @@ def get_watch_page(video_id=None):
         filename = title
         ext = fmt.get('ext')
         if ext:
-            filename += '.' + ext
+            filename += f'.{ext}'
         fmt['url'] = fmt['url'].replace(
             '/videoplayback',
-            '/videoplayback/name/' + filename)
+            f'/videoplayback/name/{filename}')
 
     download_formats = []
 
     for format in (info['formats'] + info['hls_formats']):
         if format['acodec'] and format['vcodec']:
-            codecs_string = format['acodec'] + ', ' + format['vcodec']
+            codecs_string = f"{format['acodec']}, {format['vcodec']}"
         else:
             codecs_string = format['acodec'] or format['vcodec'] or '?'
         download_formats.append({
@@ -1247,12 +1239,9 @@ def get_watch_page(video_id=None):
     for source in subtitle_sources:
         best_caption_parse = urllib.parse.urlparse(
             source['url'].lstrip('/'))
-        transcript_url = (util.URL_ORIGIN
-            + '/watch/transcript'
-            + best_caption_parse.path
-            + '?' + best_caption_parse.query)
+        transcript_url = f'{util.URL_ORIGIN}/watch/transcript{best_caption_parse.path}?{best_caption_parse.query}'
         other_downloads.append({
-            'label': 'Video Transcript: ' + source['label'],
+            'label': f'Video Transcript: {source["label"]}',
             'ext': 'txt',
             'url': transcript_url
         })
@@ -1263,7 +1252,7 @@ def get_watch_page(video_id=None):
         template_name = 'watch.html'
     return flask.render_template(template_name,
         header_playlist_names   = local_playlist.get_playlist_names(),
-        uploader_channel_url    = ('/' + info['author_url']) if info['author_url'] else '',
+        uploader_channel_url    = f'/{info["author_url"]}' if info['author_url'] else '',
         time_published             = info['time_published'],
         view_count    = (lambda x: '{:,}'.format(x) if x is not None else "")(info.get("view_count", None)),
         like_count    = (lambda x: '{:,}'.format(x) if x is not None else "")(info.get("like_count", None)),
@@ -1305,10 +1294,10 @@ def get_watch_page(video_id=None):
         ip_address   = info['ip_address'] if settings.route_tor else None,
         invidious_used    = info['invidious_used'],
         invidious_reload_button = info['invidious_reload_button'],
-        video_url = util.URL_ORIGIN + '/watch?v=' + video_id,
+        video_url = f'{util.URL_ORIGIN}/watch?v={video_id}',
         video_id = video_id,
-        storyboard_url = (util.URL_ORIGIN + '/ytl-api/storyboard.vtt?' +
-            urlencode([('spec_url', info['storyboard_spec_url'])])
+        storyboard_url = (f'{util.URL_ORIGIN}/ytl-api/storyboard.vtt?'
+            f'{urlencode([("spec_url", info["storyboard_spec_url"])])}'
             if info['storyboard_spec_url'] else None),
 
         js_data = {
@@ -1335,7 +1324,7 @@ def get_watch_page(video_id=None):
 
 @yt_app.route('/api/<path:dummy>')
 def get_captions(dummy):
-    url = 'https://www.youtube.com' + request.full_path
+    url = f'https://www.youtube.com{request.full_path}'
     try:
         result = util.fetch_url(url, headers=util.mobile_ua)
         result = result.replace(b"align:start position:0%", b"")
@@ -1350,12 +1339,9 @@ inner_timestamp_removal_reg = re.compile(r'<[^>]+>')
 @yt_app.route('/watch/transcript/<path:caption_path>')
 def get_transcript(caption_path):
     try:
-        captions = util.fetch_url('https://www.youtube.com/'
-            + caption_path
-            + '?' + request.environ['QUERY_STRING']).decode('utf-8')
+        captions = util.fetch_url(f'https://www.youtube.com/{caption_path}?{request.environ["QUERY_STRING"]}').decode('utf-8')
     except util.FetchError as e:
-        msg = ('Error retrieving captions: ' + str(e) + '\n\n'
-            + 'The caption url may have expired.')
+        msg = f'Error retrieving captions: {e}\n\nThe caption url may have expired.'
         print(msg)
         return flask.Response(
             msg,
@@ -1403,7 +1389,7 @@ def get_transcript(caption_path):
     result = ''
     for seg in segments:
         if seg['text'] != ' ':
-            result += seg['begin'] + ' ' + seg['text'] + '\r\n'
+            result += f"{seg['begin']} {seg['text']}\r\n"
 
     return flask.Response(result.encode('utf-8'),
                           mimetype='text/plain;charset=UTF-8')
diff --git a/youtube/yt_data_extract/common.py b/youtube/yt_data_extract/common.py
index dce1d30..f91a467 100644
--- a/youtube/yt_data_extract/common.py
+++ b/youtube/yt_data_extract/common.py
@@ -212,7 +212,7 @@ def extract_date(date_text):
         month, day, year = parts[-3:]
         month = MONTH_ABBREVIATIONS.get(month[0:3]) # slicing in case they start writing out the full month name
         if month and (re.fullmatch(r'\d\d?', day) is not None) and (re.fullmatch(r'\d{4}', year) is not None):
-            return year + '-' + month + '-' + day
+            return f'{year}-{month}-{day}'
     return None
 
 def check_missing_keys(object, *key_sequences):
@@ -222,7 +222,7 @@ def check_missing_keys(object, *key_sequences):
             for key in key_sequence:
                 _object = _object[key]
         except (KeyError, IndexError, TypeError):
-            return 'Could not find ' + key
+            return f'Could not find {key}'
 
     return None
 
@@ -467,7 +467,7 @@ def extract_item_info(item, additional_info={}):
             ['shortBylineText', 'runs', 0, 'navigationEndpoint', 'browseEndpoint', 'browseId'],
             ['ownerText', 'runs', 0, 'navigationEndpoint', 'browseEndpoint', 'browseId']
         ))
-        info['author_url'] = ('https://www.youtube.com/channel/' + info['author_id']) if info['author_id'] else None
+        info['author_url'] = f'https://www.youtube.com/channel/{info["author_id"]}' if info['author_id'] else None
     info['description'] = extract_formatted_text(multi_deep_get(
         item,
         ['descriptionText'], ['descriptionSnippet'],
diff --git a/youtube/yt_data_extract/everything_else.py b/youtube/yt_data_extract/everything_else.py
index 5930111..b7379a5 100644
--- a/youtube/yt_data_extract/everything_else.py
+++ b/youtube/yt_data_extract/everything_else.py
@@ -305,7 +305,7 @@ def extract_playlist_metadata(polymer_json):
                 metadata['description'] = desc
 
     if metadata['author_id']:
-        metadata['author_url'] = 'https://www.youtube.com/channel/' + metadata['author_id']
+        metadata['author_url'] = f'https://www.youtube.com/channel/{metadata["author_id"]}'
 
     if metadata['first_video_id'] is None:
         metadata['thumbnail'] = None
diff --git a/youtube/yt_data_extract/watch_extraction.py b/youtube/yt_data_extract/watch_extraction.py
index de87a6a..2a60741 100644
--- a/youtube/yt_data_extract/watch_extraction.py
+++ b/youtube/yt_data_extract/watch_extraction.py
@@ -650,9 +650,9 @@ def _extract_playability_error(info, player_response, error_prefix=''):
     )
 
     if playability_status not in (None, 'OK'):
-        info['playability_error'] = error_prefix + playability_reason
+        info['playability_error'] = f'{error_prefix}{playability_reason}'
     elif not info['playability_error']: # do not override
-        info['playability_error'] = error_prefix + 'Unknown playability error'
+        info['playability_error'] = f'{error_prefix}Unknown playability error'
 
 SUBTITLE_FORMATS = ('srv1', 'srv2', 'srv3', 'ttml', 'vtt')
 def extract_watch_info(polymer_json):
@@ -726,7 +726,7 @@ def extract_watch_info(polymer_json):
         # Store the full URL from the player response (includes valid tokens)
         if base_url:
             normalized = normalize_url(base_url) if base_url.startswith('/') or not base_url.startswith('http') else base_url
-            info['_caption_track_urls'][lang_code + ('_asr' if caption_track.get('kind') == 'asr' else '')] = normalized
+            info['_caption_track_urls'][f'{lang_code}_{"asr" if caption_track.get("kind") == "asr" else ""}'] = normalized
         lang_name = deep_get(urllib.parse.parse_qs(urllib.parse.urlparse(base_url).query), 'name', 0)
         if lang_name:
             info['_manual_caption_language_names'][lang_code] = lang_name
@@ -806,7 +806,7 @@ def extract_watch_info(polymer_json):
     info['allowed_countries'] = mf.get('availableCountries', [])
 
     # other stuff
-    info['author_url'] = 'https://www.youtube.com/channel/' + info['author_id'] if info['author_id'] else None
+    info['author_url'] = f'https://www.youtube.com/channel/{info["author_id"]}' if info['author_id'] else None
     info['storyboard_spec_url'] = deep_get(player_response, 'storyboards', 'playerStoryboardSpecRenderer', 'spec')
 
     return info
@@ -912,12 +912,12 @@ def get_caption_url(info, language, format, automatic=False, translation_languag
     url = info['_captions_base_url']
     if not url:
         return None
-    url += '&lang=' + language
-    url += '&fmt=' + format
+    url += f'&lang={language}'
+    url += f'&fmt={format}'
     if automatic:
         url += '&kind=asr'
     elif language in info['_manual_caption_language_names']:
-        url += '&name=' + urllib.parse.quote(info['_manual_caption_language_names'][language], safe='')
+        url += f'&name={urllib.parse.quote(info["_manual_caption_language_names"][language], safe="")}'
 
     if translation_language:
         url += '&tlang=' + translation_language
@@ -964,7 +964,7 @@ def extract_decryption_function(info, base_js):
         return 'Could not find var_name'
 
     var_name = var_with_operation_match.group(1)
-    var_body_match = re.search(r'var ' + re.escape(var_name) + r'=\{(.*?)\};', base_js, flags=re.DOTALL)
+    var_body_match = re.search(rf'var {re.escape(var_name)}=\{{(.*?)\}};', base_js, flags=re.DOTALL)
     if var_body_match is None:
         return 'Could not find var_body'
 
@@ -988,7 +988,7 @@ def extract_decryption_function(info, base_js):
         elif op_body.startswith('var c=a[0]'):
             operation_definitions[op_name] = 2
         else:
-            return 'Unknown op_body: ' + op_body
+            return f'Unknown op_body: {op_body}'
 
     decryption_function = []
     for op_with_arg in function_body:
@@ -997,7 +997,7 @@ def extract_decryption_function(info, base_js):
             return 'Could not parse operation with arg'
         op_name = match.group(2).strip('[].')
         if op_name not in operation_definitions:
-            return 'Unknown op_name: ' + str(op_name)
+            return f'Unknown op_name: {op_name}'
         op_argument = match.group(3)
         decryption_function.append([operation_definitions[op_name], int(op_argument)])
 
@@ -1028,5 +1028,5 @@ def decrypt_signatures(info):
                 _operation_2(a, argument)
 
         signature = ''.join(a)
-        format['url'] += '&' + format['sp'] + '=' + signature
+        format['url'] += f'&{format["sp"]}={signature}'
     return False