@fixture<EOL>def monkeypatch():
mpatch = MonkeyPatch()<EOL>yield mpatch<EOL>mpatch.undo()<EOL>
The returned ``monkeypatch`` fixture provides these
    helper methods to modify objects, dictionaries or os.environ::

        monkeypatch.setattr(obj, name, value, raising=True)
        monkeypatch.delattr(obj, name, raising=True)
        monkeypatch.setitem(mapping, name, value)
        monkeypatch.delitem(obj, name, raising=True)
        monkeypatch.setenv(name, value, prepend=False)
        monkeypatch.delenv(name, raising=True)
        monkeypatch.syspath_prepend(path)
        monkeypatch.chdir(path)

    All modifications will be undone after the requesting
    test function or fixture has finished. The ``raising``
    parameter determines if a KeyError or AttributeError
    will be raised if the set/deletion operation has no target.