beets.util.artresizer offers has_PIL() and has_IM()

has_PIL() determines Python Imaging Library version (or None) and
has_IM() does the same for Image Magick.

ArtResizer._check_method() relies on those functions. It also does not
accept a "method" parameter anymore, and neither does ArtResizer(). It
was unused.

beets/util/artresizer.py

@@ -159,11 +159,10 @@ class ArtResizer(object):
     """
     __metaclass__ = Shareable
 
-    def __init__(self, method=None):
-        """Create a resizer object for the given method or, if none is
-        specified, with an inferred method.
+    def __init__(self):
+        """Create a resizer object with an inferred method.
         """
-        self.method = self._check_method(method)
+        self.method = self._check_method()
         log.debug(u"artresizer: method is {0}", self.method)
         self.can_compare = self._can_compare()
 
@@ -211,41 +210,43 @@ class ArtResizer(object):
         return self.method[0] == IMAGEMAGICK and self.method[1] > (6, 8, 7)
 
     @staticmethod
-    def _check_method(method=None):
-        """A tuple indicating whether current method is available and its
-        version. If no method is given, it returns a supported one.
-        """
-        # Guess available method
-        if not method:
-            for m in [IMAGEMAGICK, PIL]:
-                _, version = ArtResizer._check_method(m)
-                if version:
-                    return (m, version)
-            return (WEBPROXY, (0))
+    def _check_method():
+        """Return a tuple indicating an available method and its version."""
+        version = has_IM()
+        if version:
+            return IMAGEMAGICK, version
 
-        if method == IMAGEMAGICK:
+        version = has_PIL()
+        if version:
+            return PIL, version
 
-            # Try invoking ImageMagick's "convert".
-            try:
-                out = util.command_output(['identify', '--version'])
+        return WEBPROXY, (0)
 
-                if 'imagemagick' in out.lower():
-                    pattern = r".+ (\d+)\.(\d+)\.(\d+).*"
-                    match = re.search(pattern, out)
-                    if match:
-                        return (IMAGEMAGICK,
-                                (int(match.group(1)),
-                                 int(match.group(2)),
-                                 int(match.group(3))))
-                    return (IMAGEMAGICK, (0))
 
-            except (subprocess.CalledProcessError, OSError):
-                return (IMAGEMAGICK, None)
+def has_IM():
+    """Return Image Magick version or None if it is unavailable
+    Try invoking ImageMagick's "convert"."""
+    try:
+        out = util.command_output(['identify', '--version'])
 
-        if method == PIL:
-            # Try importing PIL.
-            try:
-                __import__('PIL', fromlist=['Image'])
-                return (PIL, (0))
-            except ImportError:
-                return (PIL, None)
+        if 'imagemagick' in out.lower():
+            pattern = r".+ (\d+)\.(\d+)\.(\d+).*"
+            match = re.search(pattern, out)
+            if match:
+                return (int(match.group(1)),
+                        int(match.group(2)),
+                        int(match.group(3)))
+            return (0,)
+
+    except (subprocess.CalledProcessError, OSError):
+        return None
+
+
+def has_PIL():
+    """Return Image Magick version or None if it is unavailable
+    Try importing PIL."""
+    try:
+        __import__('PIL', fromlist=['Image'])
+        return (0,)
+    except ImportError:
+        return None