Cleaned up EXIF view

The last update made the assumption that EXIF metadata is in some way
consistent between camera models, images, manufacturers. This update
takes into account that nothing is certain whenever EXIF is involved.

mediagoblin/db/mixin.py

@@ -246,7 +246,8 @@ class MediaEntryMixin(GenerateSlugMixin):
 
         exif_all = self.media_data.get("exif_all")
 
-        taken = None
+        exif_short = {}
+
         if 'Image DateTimeOriginal' in exif_all:
             # format date taken
             takendate = datetime.datetime.strptime(
@@ -254,6 +255,8 @@ class MediaEntryMixin(GenerateSlugMixin):
                 '%Y:%m:%d %H:%M:%S').date()
             taken = takendate.strftime('%B %d %Y')
 
+            exif_short.update({'Date Taken': taken})
+
         aperture = None
         if 'EXIF FNumber' in exif_all:
             fnum = str(exif_all['EXIF FNumber']['printable']).split('/')
@@ -264,14 +267,25 @@ class MediaEntryMixin(GenerateSlugMixin):
             elif fnum[0] != 'None':
                 aperture = "f/%s" % (fnum[0])
 
-        return {
-            "Camera" : exif_all['Image Model']['printable'],
-            "Exposure" : '%s sec' % exif_all['EXIF ExposureTime']['printable'],
-            "Aperture" : aperture,
-            "ISO" : exif_all['EXIF ISOSpeedRatings']['printable'],
-            "Focal Length" : '%s mm' % exif_all['EXIF FocalLength']['printable'],
-            "Date Taken" : taken,
-        }
+        if aperture:
+            exif_short.update({'Aperture': aperture})
+
+        short_keys = [
+            ('Camera', 'Image Model', None),
+            ('Exposure', 'EXIF ExposureTime', lambda x: '%s sec' % x),
+            ('ISO Speed', 'EXIF ISOSpeedRatings', None),
+            ('Focal Length', 'EXIF FocalLength', lambda x: '%s mm' % x)]
+
+        for label, key, fmt_func in short_keys:
+            try:
+                val = fmt_func(exif_all[key]['printable']) if fmt_func \
+                        else exif_all[key]['printable']
+                exif_short.update({label: val})
+            except KeyError:
+                pass
+
+        return exif_short
+
 
 class MediaCommentMixin(object):
     @property