Fix: check all io functions when loading of default plugins.

Previously, the first available plugin was loaded and the plugin search quit---even if that plugin didn't provide all io functions. Loop over functions instead to ensure all io funcs have a plugin (if available).

skimage/io/__init__.py

@@ -8,22 +8,6 @@ from ._plugins import use as use_plugin
 from ._plugins import available as plugins
 from ._plugins import info as plugin_info
 from ._plugins import configuration as plugin_order
-available_plugins = plugins()
-
-for preferred_plugin in ['matplotlib', 'pil', 'qt', 'freeimage', 'null']:
-    if preferred_plugin in available_plugins:
-        try:
-            use_plugin(preferred_plugin)
-            break
-        except ImportError:
-            pass
-
-# Use PIL as the default imread plugin, since matplotlib (1.2.x)
-# is buggy (flips PNGs around, returns bytes as floats, etc.)
-try:
-    use_plugin('pil', 'imread')
-except ImportError:
-    pass
 
 from .sift import *
 from .collection import *
@@ -32,6 +16,31 @@ from ._io import *
 from .video import *
 
 
+available_plugins = plugins()
+
+
+def _load_preferred_plugins():
+    # Load preferred plugin for each io function.
+    # ('imread' must be last because the list gets modified on last iteration.)
+    io_funcs = ['imsave', 'imshow', 'imread_collection', 'imread']
+    preferred_plugins = ['matplotlib', 'pil', 'qt', 'freeimage', 'null']
+    for func in io_funcs:
+        if func == 'imread':
+            # Use PIL as the default imread plugin, since matplotlib (1.2.x)
+            # is buggy (flips PNGs around, returns bytes as floats, etc.)
+            preferred_plugins.remove('pil')
+            preferred_plugins.insert(0, 'pil')
+        for plugin in preferred_plugins:
+            if plugin not in available_plugins:
+                continue
+            try:
+                use_plugin(plugin, kind=func)
+                break
+            except (ImportError, RuntimeError):
+                pass
+_load_preferred_plugins()
+
+
 def _update_doc(doc):
     """Add a list of plugins to the module docstring, formatted as
     a ReStructuredText table.