Add test for loading preferred plugins.

skimage/io/_plugins/plugin.py

@@ -24,6 +24,8 @@ plugin_provides = {}
 plugin_module_name = {}
 plugin_meta_data = {}
 
+preferred_plugins = ['matplotlib', 'pil', 'qt', 'freeimage', 'null']
+
 
 def _clear_plugins():
     """Clear the plugin state to the default, i.e., where no plugins are loaded
@@ -41,7 +43,6 @@ _clear_plugins()
 def _load_preferred_plugins():
     # Load preferred plugin for each io function.
     io_funcs = ['imsave', 'imshow', 'imread_collection', 'imread']
-    preferred_plugins = ['matplotlib', 'pil', 'qt', 'freeimage', 'null']
     for func in io_funcs:
         for plugin in preferred_plugins:
             if plugin not in available_plugins:

skimage/io/tests/test_plugin.py

@@ -30,23 +30,29 @@ def teardown_module():
 def test_read():
     io.imread('test.png', as_grey=True, dtype='i4', plugin='test')
 
+
 def test_save():
     io.imsave('test.png', [1, 2, 3], plugin='test')
 
+
 def test_show():
     io.imshow([1, 2, 3], plugin_arg=(1, 2), plugin='test')
 
+
 def test_collection():
     io.imread_collection('*.png', conserve_memory=False, plugin='test')
 
+
 def test_use():
     plugin.use_plugin('test')
     plugin.use_plugin('test', 'imshow')
 
+
 @raises(ValueError)
 def test_failed_use():
     plugin.use_plugin('asd')
 
+
 @skipif(not PIL_available and not FI_available)
 def test_use_priority():
     plugin.use_plugin(priority_plugin)
@@ -57,6 +63,7 @@ def test_use_priority():
     plug, func = plugin.plugin_store['imread'][0]
     assert_equal(plug, 'test')
 
+
 @skipif(not PIL_available)
 def test_use_priority_with_func():
     plugin.use_plugin('pil')
@@ -74,16 +81,26 @@ def test_use_priority_with_func():
     plug, func = plugin.plugin_store['imsave'][0]
     assert_equal(plug, 'test')
 
+
 def test_plugin_order():
     p = io.plugin_order()
     assert 'imread' in p
     assert 'test' in p['imread']
 
+
 def test_available():
     assert 'qt' in io.available_plugins
     assert 'test' in io.find_available_plugins(loaded=True)
 
 
+def test_load_preferred_plugins():
+    from skimage.io._plugins import null_plugin
+    plugin.preferred_plugins = ['null']
+    plugin.reset_plugins()
+    plug, func = plugin.plugin_store['imshow'][0]
+    assert func == null_plugin.imshow
+
+
 if __name__ == "__main__":
     from numpy.testing import run_module_suite
     run_module_suite()