A dict which raises a warning when some keys are looked up

deprecdict.py

import warnings
from sklearn.utils.testing import assert_warns_message, assert_no_warnings


class DeprecationDict(dict):
    """A dict which raises a warning when some keys are looked up

    Note, this does not raise a warning for __contains__ and iteration.

    It also will raise a warning even after the key has been manually set by
    the user.
    """
    def __init__(self, *args, **kwargs):
        self._deprecations = {}
        super(DeprecationDict, self).__init__(*args, **kwargs)

    def __getitem__(self, key):
        if key in self._deprecations:
            warn_args, warn_kwargs = self._deprecations[key]
            warnings.warn(*warn_args, **warn_kwargs)
        return super(DeprecationDict, self).__getitem__(key)

    def get(self, key, default=None):
        # dict does not implement it like this, hence it needs to be overridden
        try:
            return self[key]
        except KeyError:
            return default

    def add_warning(self, key, *args, **kwargs):
        """Add a warning to be triggered when the specified key is read"""
        self._deprecations[key] = (args, kwargs)


def test_deprecationdict():
    dd = DeprecationDict()
    dd.add_warning('a', 'hello')
    dd.add_warning('b', 'world', DeprecationWarning)
    assert 1 == assert_warns_message(UserWarning, 'hello', dd.get, 'a', 1)
    dd['a'] = 5
    dd['b'] = 6
    dd['c'] = 7
    assert 5 == assert_warns_message(UserWarning, 'hello', dd.__getitem__, 'a')
    assert 6 == assert_warns_message(DeprecationWarning, 'world',
                                     dd.__getitem__, 'b')
    assert 7 == assert_no_warnings(dd.get, 'c')