fetchart complains if no imaging backend available

The `enforce_ratio` and `minwidth` options depend on PIL or ImageMagick.
Previously it silently fails. Now it will log a warning, and accept the
image.

Tests concerning these options are skipped when no imaging backend is available.

Fix #1460

beetsplug/fetchart.py

@@ -409,7 +409,16 @@ def _is_valid_image_candidate(self, candidate):
         if not (self.enforce_ratio or self.minwidth):
             return True
 
+        # get_size returns None if no local imaging backend is available
         size = ArtResizer.shared.get_size(candidate)
+
+        if not size:
+            self._log.warning(u'could not verify size of image: please see '
+                              u'documentation for dependencies. '
+                              u'The configuration options `minwidth` and '
+                              u'`enforce_ratio` may be violated.')
+            return True
+
         return size and size[0] >= self.minwidth and \
             (not self.enforce_ratio or size[0] == size[1])
 
@@ -446,6 +455,7 @@ def art_for_album(self, album, paths, local_only=False):
 
         if self.maxwidth and out:
             out = ArtResizer.shared.resize(self.maxwidth, out)
+
         return out
 
     def batch_fetch_art(self, lib, albums, force):

docs/changelog.rst

@@ -11,6 +11,8 @@ New features:
   or alternatively any list of attributes that should be favored.
 * The :doc:`/plugins/metasync` plugin now lets you get metadata from iTunes.
   This plugin is still in an experimental phase. :bug:`1450`
+* The :doc:`/plugins/fetchart` plugin will now complain for the `enforce_ratio`
+  and `min_width` options if no local imaging backend is available. :bug:`1460`
 
 
 Fixes:

docs/plugins/fetchart.rst

@@ -54,6 +54,12 @@ file. The available options are:
   Default: ``coverart itunes albumart amazon google wikipedia``, i.e.,
   all sources.
 
+Note: ``minwidth`` and ``enforce_ratio`` options require either `ImageMagick`_
+or `PIL`_.
+
+.. _PIL: http://www.pythonware.com/products/pil/
+.. _ImageMagick: http://www.imagemagick.org/
+
 Here's an example that makes plugin select only images that contain *front* or
 *back* keywords in their filenames and prioritizes the iTunes source over
 others::

test/test_art.py

@@ -30,11 +30,15 @@
 from beets import importer
 from beets import config
 from beets import logging
+from beets.util.artresizer import ArtResizer, WEBPROXY
 
 
 logger = logging.getLogger('beets.test_art')
 
 
+ARTRESIZER_USES_FALLBACK_BACKEND = ArtResizer.shared.method[0] == WEBPROXY
+
+
 class UseThePlugin(_common.TestCase):
     def setUp(self):
         super(UseThePlugin, self).setUp()
@@ -408,11 +412,15 @@ def _assertImageIsValidArt(self, image_file, should_exist):
         else:
             self.assertIsNone(local_artpath)
 
+    @unittest.skipIf(ARTRESIZER_USES_FALLBACK_BACKEND,
+                     'ArtResizer has no local imaging backend available')
     def test_respect_minwidth(self):
         self.plugin.minwidth = 300
         self._assertImageIsValidArt(self.IMG_225x225, False)
         self._assertImageIsValidArt(self.IMG_348x348, True)
 
+    @unittest.skipIf(ARTRESIZER_USES_FALLBACK_BACKEND,
+                     'ArtResizer has no local imaging backend available')
     def test_respect_enforce_ratio_yes(self):
         self.plugin.enforce_ratio = True
         self._assertImageIsValidArt(self.IMG_500x490, False)