NF - check and ignore warnings context managers

Author: matthew-brett

nibabel/checkwarns.py

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+# emacs: -*- mode: python-mode; py-indent-offset: 4; indent-tabs-mode: nil -*-
+# vi: set ft=python sts=4 ts=4 sw=4 et:
+### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
+#
+#   See COPYING file distributed along with the NiBabel package for the
+#   copyright and license terms.
+#
+### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
+''' Contexts for *with* statement allowing checks for warnings
+
+When we give up 2.5 compatibility we can use python's own
+``tests.test_support.check_warnings``
+
+'''
+from __future__ import with_statement
+
+import warnings
+
+
+class ErrorWarnings(object):
+    """ Context manager to check for warnings as errors.  Usually used with
+    ``assert_raises`` in the with block
+
+    Examples
+    --------
+    >>> with ErrorWarnings():
+    ...     try:
+    ...         warnings.warn('Message', UserWarning)
+    ...     except UserWarning:
+    ...         print 'I consider myself warned'
+    I consider myself warned
+
+    Notes
+    -----
+    The manager will raise a RuntimeError if another warning filter gets put on
+    top of the one it has just added.
+    """
+    def __init__(self):
+        self.added = None
+
+    def __enter__(self):
+        warnings.simplefilter('error')
+        self.added = warnings.filters[0]
+
+    def __exit__(self, exc, value, tb):
+        if warnings.filters[0] != self.added:
+            raise RuntimeError('Somone has done something to the filters')
+        warnings.filters.pop(0)
+        return False # allow any exceptions to propagate
+
+
+class IgnoreWarnings(ErrorWarnings):
+    """ Context manager to ignore warnings
+
+    Examples
+    --------
+    >>> with IgnoreWarnings():
+    ...     warnings.warn('Message', UserWarning)
+
+    (and you get no warning)
+
+    Notes
+    -----
+    The manager will raise a RuntimeError if another warning filter gets put on
+    top of the one it has just added.
+    """
+
+    def __enter__(self):
+        warnings.simplefilter('ignore')
+        self.added = warnings.filters[0]

nibabel/tests/test_checkwarns.py

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+""" Tests for warnings context managers
+"""
+
+from __future__ import with_statement
+
+from warnings import warn, simplefilter, filters
+
+from ..checkwarns import ErrorWarnings, IgnoreWarnings
+
+from nose.tools import assert_true, assert_equal, assert_raises
+
+
+def test_warn_error():
+    # Check warning error context manager
+    n_warns = len(filters)
+    with ErrorWarnings():
+        assert_raises(UserWarning, warn, 'A test')
+    assert_equal(n_warns, len(filters))
+    def f():
+        with ErrorWarnings():
+            simplefilter('ignore')
+    try:
+        assert_raises(RuntimeError, f)
+        assert_equal(n_warns+2, len(filters))
+    finally:
+        filters.pop(0)
+        filters.pop(0)
+    # Check other errors are propagated
+    def f():
+        with ErrorWarnings():
+            raise ValueError('An error')
+    assert_raises(ValueError, f)
+
+
+def test_warn_ignore():
+    # Check warning ignore context manager
+    n_warns = len(filters)
+    with IgnoreWarnings():
+        warn('Here is a warning, you will not see it')
+        warn('Nor this one', DeprecationWarning)
+    assert_equal(n_warns, len(filters))
+    def f():
+        with IgnoreWarnings():
+            simplefilter('error')
+    try:
+        assert_raises(RuntimeError, f)
+        assert_equal(n_warns+2, len(filters))
+    finally:
+        filters.pop(0)
+        filters.pop(0)
+    # Check other errors are propagated
+    def f():
+        with IgnoreWarnings():
+            raise ValueError('An error')
+    assert_raises(ValueError, f)