def __init__(self, manager):
self.manager = manager<EOL>self._var_cache = dict((k, EnvironmentVariable(k, self))<EOL>for k in manager.parent_environ.keys())<EOL>
Creates an `EnvironmentDict`.

        Args:
            override_existing_lists (bool): If True, the first call to append
                or prepend will override the value in `environ` and effectively
                act as a setenv operation. If False, pre-existing values will
                be appended/prepended to as usual.