artresizer (#64): helper functions, not classes

The previous method was to change self.__class__ dynamically to make __init__
instantiate different classes. This new way, which uses bare functions instead
of separate functor-like classes, instead just forwards the resize() call to
a module-global implementation based on self.method.

Additionally, the semantics of ArtResizer have changed. Clients now *always*
call resize() and proxy_url(), regardless of method. The method makes *one* of
these a no-op. This way, clients need not manually inspect which method is
being used.

beets/util/artresizer.py

@@ -18,7 +18,6 @@ public resizing proxy if neither is available.
 import urllib
 import subprocess
 import os
-import shutil
 from tempfile import NamedTemporaryFile
 import logging
 
@@ -70,65 +69,97 @@ def temp_file_for(path):
         return f.name
 
 
-class PilResizer(object):
-    def resize(self, maxwidth, path_in, path_out=None):
-        """Resize using Python Imaging Library (PIL).  Return the output path 
-        of resized image.
-        """
-        from PIL import Image
-        if not path_out:
-            path_out = temp_file_for(path_in)
-        try:
-            im = Image.open(path_in)
-            size = maxwidth, maxwidth
-            im.thumbnail(size, Image.ANTIALIAS)
-            im.save(path_out)
-            return path_out
-        except IOError:
-            log.error("Cannot create thumbnail for '%s'" % path_in)
-
-
-class ImageMagickResizer(object):
-    def resize(self, maxwidth, path_in, path_out=None):
-        """Resize using ImageMagick <http://www.imagemagick.org> command-line 
-        tool. Return the output path of resized image.
-        """
-        if not path_out:
-            path_out = temp_file_for(path_in)
-
-        # widthxheight> Shrinks images with dimension(s) larger than the 
-        # corresponding width and/or height dimension(s).
-        # "only shrink flag" is prefixed by ^ escape char for Windows compat.
-        cmd = ['convert', path_in, '-resize',
-               '{0}x^>'.format(maxwidth), path_out]
-        call(cmd)
+def pil_resize(self, maxwidth, path_in, path_out=None):
+    """Resize using Python Imaging Library (PIL).  Return the output path 
+    of resized image.
+    """
+    from PIL import Image
+    if not path_out:
+        path_out = temp_file_for(path_in)
+    try:
+        im = Image.open(path_in)
+        size = maxwidth, maxwidth
+        im.thumbnail(size, Image.ANTIALIAS)
+        im.save(path_out)
         return path_out
+    except IOError:
+        log.error("Cannot create thumbnail for '%s'" % path_in)
+
+
+def im_resize(self, maxwidth, path_in, path_out=None):
+    """Resize using ImageMagick's ``convert`` tool.
+    tool. Return the output path of resized image.
+    """
+    if not path_out:
+        path_out = temp_file_for(path_in)
+
+    # "-resize widthxheight>" shrinks images with dimension(s) larger
+    # than the corresponding width and/or height dimension(s). The >
+    # "only shrink" flag is prefixed by ^ escape char for Windows
+    # compatability.
+    cmd = ['convert', path_in,
+           '-resize', '{0}x^>'.format(maxwidth), path_out]
+    call(cmd)
+    return path_out
+
+
+BACKEND_FUNCS = {
+    PIL: pil_resize,
+    IMAGEMAGICK: im_resize,
+}
 
 
 class ArtResizer(object):
     """A singleton class that performs image resizes.
     """
-    def __init__(self):
-        """ArtResizer factory method"""
-        self.method = self.set_method()
+    def __init__(self, method=None):
+        """Create a resizer object for the given method or, if none is
+        specified, with an inferred method.
+        """
+        self.method = method or self._guess_method()
+        log.debug("ArtResizer method is {0}".format(self.method))        
+    
+    def resize(self, maxwidth, path_in, path_out=None):
+        """Manipulate an image file according to the method, returning a
+        new path. For PIL or IMAGEMAGIC methods, resizes the image to a
+        temporary file. For WEBPROXY, returns `path_in` unmodified.
+        """
+        if self.local:
+            func = BACKEND_FUNCS[self.method]
+            return func(maxwidth, path_in, path_out)
+        else:
+            return path_in
 
-        if self.method == PIL:
-            self.__class__ = PilResizer
-        elif self.method == IMAGEMAGICK:
-            self.__class__ = ImageMagickResizer
-        log.debug("ArtResizer method is %s" % self.__class__)        
+    def proxy_url(self, maxwidth, url):
+        """Modifies an image URL according the method, returning a new
+        URL. For WEBPROXY, a URL on the proxy server is returned.
+        Otherwise, the URL is returned unmodified.
+        """
+        if self.local:
+            return url
+        else:
+            return resize_url(url, maxwidth)
 
-    def set_method(self):
-        """Set the most appropriate resize method. Use PIL if present, else 
-        check if ImageMagick is installed.
-        If none is available, use a web proxy."""
+    @property
+    def local(self):
+        """A boolean indicating whether the resizing method is performed
+        locally (i.e., PIL or IMAGEMAGICK).
+        """
+        return self.method in BACKEND_FUNCS
 
+    @staticmethod
+    def _guess_method():
+        """Determine which resizing method to use. Returns PIL,
+        IMAGEMAGICK, or WEBPROXY depending on available dependencies.
+        """
+        # Try importing PIL.
         try:
             __import__('PIL', fromlist=['Image'])
             return PIL
         except ImportError:
             pass
 
+        # Try invoking ImageMagick's "convert".
         try:
             out = subprocess.check_output(['convert', '--version']).lower()
             if 'imagemagick' in out:
@@ -136,24 +167,9 @@ class ArtResizer(object):
         except subprocess.CalledProcessError:
             pass # system32/convert.exe may be interfering 
 
+        # Fall back to Web proxy method.
         return WEBPROXY
 
-    def resize(self, maxwidth, url, path_out=None):
-        """Resize using web proxy. Return the output path of resized image.
-        """
-
-        reqUrl = resize_url(url, maxwidth)
-        try:
-            fn, headers = urllib.urlretrieve(reqUrl)
-        except IOError:
-            log.debug('error fetching resized image')
-            return
-
-        if not path_out:
-            path_out = get_temp_file_out(fn)
-        shutil.copy(fn, path_out)
-        return path_out
-
 
 # Singleton instantiation.
 inst = ArtResizer()

beetsplug/embedart.py

@@ -29,7 +29,6 @@ def _embed(path, items):
     """
     if options['maxwidth']:
         path = artresizer.inst.resize(options['maxwidth'], syspath(path))
-        log.debug('Resize album art to %s before embedding' % path)
 
     data = open(syspath(path), 'rb').read()
     kindstr = imghdr.what(None, data)
@@ -64,10 +63,10 @@ class EmbedCoverArtPlugin(BeetsPlugin):
         options['maxwidth'] = \
             int(ui.config_val(config, 'embedart', 'maxwidth', '0'))
 
-        if options['maxwidth'] and artresizer.inst.method == artresizer.WEBPROXY:
+        if options['maxwidth'] and not artresizer.inst.local:
             options['maxwidth'] = 0
-            log.error("embedart: 'maxwidth' option ignored, " 
-                       "please install ImageMagick first")
+            log.error("embedart: ImageMagick or PIL not found; "
+                      "'maxwidth' option ignored")
 
     def commands(self):
         # Embed command.

beetsplug/fetchart.py

@@ -36,8 +36,8 @@ log = logging.getLogger('beets')
 def do_resize_url(func):
     def wrapper(url, maxwidth=None):
         """Returns url pointing to resized image instead of original one"""
-        if maxwidth and artresizer.inst.method == artresizer.WEBPROXY :
-            url = artresizer.resize_url(url, maxwidth)
+        if maxwidth:
+            url = artresizer.inst.proxy_url(url, maxwidth)
         return func(url)
     return wrapper
 
@@ -174,7 +174,7 @@ def art_for_album(album, path, maxwidth=None, local_only=False):
 
         out = _fetch_image(url, maxwidth)
         
-    if maxwidth and artresizer.inst.method != artresizer.WEBPROXY :
+    if maxwidth:
         artresizer.inst.resize(maxwidth, out, out)
     return out