Make Pillow optional due to external dependencies (#326)

Fixes #325.

`Pillow` has a number of external dependencies needed for installation. Since most media files are detected correctly with `imghdr` and `Pillow` is used as a fallback we can make it optional.

`Pillow` support was added in #320 to fix #281.

elodie/media/photo.py

@@ -12,10 +12,8 @@ import os
 import re
 import time
 from datetime import datetime
-from PIL import Image
 from re import compile
 
-
 from elodie import log
 from .media import Media
 
@@ -38,6 +36,15 @@ class Photo(Media):
         # We only want to parse EXIF once so we store it here
         self.exif = None
 
+        # Optionally import Pillow - see gh-325
+        # https://github.com/jmathai/elodie/issues/325
+        self.pillow = None
+        try:
+            from PIL import Image
+            self.pillow = Image
+        except ImportError:
+            pass
+
     def get_date_taken(self):
         """Get the date which the photo was taken.
 
@@ -98,22 +105,28 @@ class Photo(Media):
         if(extension != 'heic'):
             # gh-4 This checks if the source file is an image.
             # It doesn't validate against the list of supported types.
+            # We check with imghdr and pillow.
             if(imghdr.what(source) is None):
-                # imghdr won't detect all variants of images (https://bugs.python.org/issue28591)
-                # see https://github.com/jmathai/elodie/issues/281
-                # before giving up, we use `pillow` imaging library to detect file type
-                #
-                # It is important to note that the library doesn't decode or load the
-                # raster data unless it really has to. When you open a file,
-                # the file header is read to determine the file format and extract
-                # things like mode, size, and other properties required to decode the file,
-                # but the rest of the file is not processed until later.
-                try:
-                    im = Image.open(source)
-                except IOError:
-                    return False
-
-                if(im.format is None):
+                # Pillow is used as a fallback and if it's not available we trust
+                #   what imghdr returned.
+                if(self.pillow is None):
                     return False
+                else:
+                    # imghdr won't detect all variants of images (https://bugs.python.org/issue28591)
+                    # see https://github.com/jmathai/elodie/issues/281
+                    # before giving up, we use `pillow` imaging library to detect file type
+                    #
+                    # It is important to note that the library doesn't decode or load the
+                    # raster data unless it really has to. When you open a file,
+                    # the file header is read to determine the file format and extract
+                    # things like mode, size, and other properties required to decode the file,
+                    # but the rest of the file is not processed until later.
+                    try:
+                        im = self.pillow.open(source)
+                    except IOError:
+                        return False
 
+                    if(im.format is None):
+                        return False
+        
         return extension in self.extensions

elodie/tests/media/photo_test.py

@@ -165,6 +165,11 @@ def test_is_valid_fallback_using_pillow():
 
     assert photo.is_valid()
 
+def test_pillow_not_loaded():
+    photo = Photo(helper.get_file('imghdr-error.jpg'))
+    photo.pillow = None
+
+    assert photo.is_valid() == False
 
 def test_set_album():
     temporary_folder, folder = helper.create_working_folder()