Add all_warnings context manager

The context manager searches for __warningregistry__ entries that
Python leaves all over the place, clearing them, thereby ensuring
that warnings will always be raised.  This is necessary for the test
suite to detect warnings even if they were raised before under
the "once" filter.

skimage/util/__init__.py

@@ -7,6 +7,8 @@ from .arraypad import pad
 from ._regular_grid import regular_grid
 from .unique import unique_rows
 
+from ._warnings import all_warnings
+
 
 __all__ = ['img_as_float',
            'img_as_int',

skimage/util/_warnings.py

@@ -0,0 +1,60 @@
+__all__ = ['all_warnings']
+
+from contextlib import contextmanager
+import sys
+import warnings
+import inspect
+
+@contextmanager
+def all_warnings():
+    """
+    Context for use in testing to ensure that all warnings are raised.
+
+    Examples
+    --------
+    >>> import warnings
+    >>> def foo():
+    ...     warnings.warn(RuntimeWarning("bar"))
+
+    We raise the warning once, while the warning filter is set to "once".
+    Hereafter, the warning is invisible, even with custom filters:
+
+    >>> with warnings.catch_warnings():
+    ...     warnings.simplefilter('once')
+    ...     foo()
+
+    We can now run ``foo()`` without a warning being raised:
+
+    >>> from numpy.testing import assert_warns
+    >>> foo()
+
+    To catch the warning, we call in the help of ``all_warnings``:
+
+    >>> with all_warnings():
+    ...     assert_warns(RuntimeWarning, foo)
+    """
+
+    # Whenever a warning is triggered, Python adds a __warningregistry__
+    # member to the *calling* module.  The exercize here is to find
+    # and eradicate all those breadcrumbs that were left lying around.
+    #
+    # We proceed by first searching all parent calling frames and explicitly
+    # clearing their warning registries (necessary for the doctests above to
+    # pass).  Then, we search for all submodules of skimage and clear theirs
+    # as well (necessary for the skimage test suite to pass).
+
+    frame = inspect.currentframe()
+    if frame:
+        for f in inspect.getouterframes(frame):
+            f[0].f_locals['__warningregistry__'] = {}
+    del frame
+
+    for mod in sys.modules.values():
+        try:
+            mod.__warningregistry__.clear()
+        except AttributeError:
+            pass
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("always")
+        yield