Update docstring and fix pyamg warning check

skimage/_shared/_warnings.py

@@ -84,9 +84,16 @@ def expected_warnings(matching):
     -----
     Uses `all_warnings` to ensure all warnings are raised.
     Upon exiting, it checks the recorded warnings for the desired matching
-    string.  Raises a warning if the match was not found or an Unexpected
-    warning is found.  You can pass an empty regex as on of the matches
-    to allow for no matches: "\A\Z".
+    pattern(s).  
+    Raises a ValueError if any match was not found or an unexpected
+    warning was raised.  
+    Allows for three types of behaviors: "and", "or", and "optional" matches. 
+    This is done to accomodate different build enviroments or loop conditions
+    that may produce different warnings.  The behaviors can be combined.
+    If you pass multiple patterns, you get an orderless "and", where all of the
+    warnings must be raised.
+    If you use the "|" operator in a pattern, you can catch one of several warnings.
+    Finally, you can use "|\A\Z" in a pattern to signify it as optional.
 
     """
     with all_warnings() as w:

skimage/segmentation/tests/test_random_walker.py

@@ -2,13 +2,8 @@ import numpy as np
 from skimage.segmentation import random_walker
 from skimage.transform import resize
 from skimage._shared._warnings import expected_warnings
-from skimage._shared.version_requirements import is_installed
 
-
-if is_installed('pyamg'):
-    PYAMG_EXPECTED_WARNING = []
-else:
-    PYAMG_EXPECTED_WARNING = ['pyamg']
+PYAMG_EXPECTED_WARNING = 'pyamg|\A\Z'
 
 
 def make_2d_syntheticdata(lx, ly=None):
@@ -99,11 +94,11 @@ def test_2d_cg_mg():
     lx = 70
     ly = 100
     data, labels = make_2d_syntheticdata(lx, ly)
-    with expected_warnings('%s|\A\Z' % PYAMG_EXPECTED_WARNING):
+    with expected_warnings([PYAMG_EXPECTED_WARNING]):
         labels_cg_mg = random_walker(data, labels, beta=90, mode='cg_mg')
     assert (labels_cg_mg[25:45, 40:60] == 2).all()
     assert data.shape == labels.shape
-    with expected_warnings(PYAMG_EXPECTED_WARNING):
+    with expected_warnings([PYAMG_EXPECTED_WARNING]):
         full_prob = random_walker(data, labels, beta=90, mode='cg_mg',
                               return_full_prob=True)
     assert (full_prob[1, 25:45, 40:60] >=
@@ -118,7 +113,7 @@ def test_types():
     data, labels = make_2d_syntheticdata(lx, ly)
     data = 255 * (data - data.min()) // (data.max() - data.min())
     data = data.astype(np.uint8)
-    with expected_warnings(PYAMG_EXPECTED_WARNING):
+    with expected_warnings([PYAMG_EXPECTED_WARNING]):
         labels_cg_mg = random_walker(data, labels, beta=90, mode='cg_mg')
     assert (labels_cg_mg[25:45, 40:60] == 2).all()
     assert data.shape == labels.shape