scikit-image/skimage/_shared/_warnings.py

from contextlib import contextmanager
import sys
import warnings
import inspect
import re

__all__ = ['all_warnings', 'expected_warnings', 'warn']


def warn(message, category=None, stacklevel=2):
    """A version of `warnings.warn` with a default stacklevel of 2.
    """
    if category is not None:
        warnings.warn(message, category=category, stacklevel=stacklevel)
    else:
        warnings.warn(message, stacklevel=stacklevel)


@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_name, mod in list(sys.modules.items()):
        if 'six.moves' in mod_name:
            continue
        try:
            mod.__warningregistry__.clear()
        except AttributeError:
            pass

    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        yield w


@contextmanager
def expected_warnings(matching):
    """Context for use in testing to catch known warnings matching regexes

    Parameters
    ----------
    matching : list of strings or compiled regexes
        Regexes for the desired warning to catch

    Examples
    --------
    >>> from skimage import data, img_as_ubyte, img_as_float
    >>> with expected_warnings(['precision loss']):
    ...     d = img_as_ubyte(img_as_float(data.coins()))

    Notes
    -----
    Uses `all_warnings` to ensure all warnings are raised.
    Upon exiting, it checks the recorded warnings for the desired matching
    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:
        # enter context
        yield w
        # exited user context, check the recorded warnings
        remaining = [m for m in matching if '\A\Z' not in m.split('|')]
        for warn in w:
            found = False
            for match in matching:
                if re.search(match, str(warn.message)) is not None:
                    found = True
                    if match in remaining:
                        remaining.remove(match)
            if not found:
                raise ValueError('Unexpected warning: %s' % str(warn.message))
        if len(remaining) > 0:
            msg = 'No warning raised matching:\n%s' % '\n'.join(remaining)
            raise ValueError(msg)