Clean up test

skimage/io/tests/test_null.py

@@ -1,5 +1,6 @@
 import os
 import warnings
+from contextlib import contextmanager
 
 import numpy as np
 from numpy.testing import raises
@@ -8,35 +9,38 @@ from skimage import io
 from skimage import data_dir
 
 
+@contextmanager
+def warnings_as_errors():
+    # Temporarily set warnings as errors so we can test the warning is raised.
+    with warnings.catch_warnings():
+        warnings.filterwarnings('error')
+        yield
+
 @raises(Warning)
 def test_null_imread():
     path = os.path.join(data_dir, 'color.png')
-    with warnings.catch_warnings():  # Temporarily set warnings as errors.
-        warnings.filterwarnings('error')
+    with warnings_as_errors():
         io.imread(path, plugin='null')
 
 
 @raises(Warning)
 def test_null_imsave():
-    with warnings.catch_warnings():  # Temporarily set warnings as errors.
-        warnings.filterwarnings('error')
+    with warnings_as_errors():
         io.imsave('dummy.png', np.zeros((3, 3)), plugin='null')
 
 
 @raises(Warning)
 def test_null_imshow():
-    with warnings.catch_warnings():  # Temporarily set warnings as errors.
-        warnings.filterwarnings('error')
+    with warnings_as_errors():
         io.imshow(np.zeros((3, 3)), plugin='null')
 
 
 @raises(Warning)
 def test_null_imread_collection():
     # Note that the null plugin doesn't define an `imread_collection` plugin
-    # but this function dynamically added by the plugin manager.
+    # but this function is dynamically added by the plugin manager.
     path = os.path.join(data_dir, '*.png')
-    with warnings.catch_warnings():  # Temporarily set warnings as errors.
-        warnings.filterwarnings('error')
+    with warnings_as_errors():
         collection = io.imread_collection(path, plugin='null')
         collection[0]