Edit

[EDIT] FILE: shouldwarn.py

import warnings
from typing import Union, Type

from testfixtures import Comparison as C, compare

WarningOrType = Union[Warning, Type[Warning]]

class ShouldWarn(warnings.catch_warnings):
    """
    This context manager is used to assert that warnings are issued
    within the context it is managing.

:param expected: This should be a sequence made up of one or more elements,
                     each of one of the following types:

* A warning class, indicating that the type
                       of the warnings is important but not the
                       parameters it is created with.

* A warning instance, indicating that a
                       warning exactly matching the one supplied
                       should have been issued.

If no expected warnings are passed, you will need to inspect
                     the contents of the list returned by the context manager.

:param filters:
      If passed, these are used to create a filter such that only warnings you
      are interested in will be considered by this :class:`ShouldWarn`
      instance. The names and meanings are the same as the parameters for
      :func:`warnings.filterwarnings`.

"""

_empty_okay = False

def __init__(self, *expected: WarningOrType, **filters):
        super(ShouldWarn, self).__init__(record=True)
        self.expected = [C(e) for e in expected]
        self.filters = filters

def __enter__(self):
        self.recorded = super(ShouldWarn, self).__enter__()
        warnings.filterwarnings("always", **self.filters)
        return self.recorded

def __exit__(self, exc_type, exc_val, exc_tb):
        super(ShouldWarn, self).__exit__(exc_type, exc_val, exc_tb)
        if not self.recorded and self._empty_okay:
            return
        if not self.expected and self.recorded and not self._empty_okay:
            return
        compare(self.expected, actual=[wm.message for wm in self.recorded])

class ShouldNotWarn(ShouldWarn):
    """
    This context manager is used to assert that no warnings are issued
    within the context it is managing.
    """

_empty_okay = True

def __init__(self):
        super(ShouldNotWarn, self).__init__()

CANCEL