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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def assert_is_valid_key(key):
 """ Raise KeyError if a given config key violates any requirements. The requirements are the following and can be individually deactivated in ``sacred.SETTINGS.CONFIG_KEYS``: * ENFORCE_MONGO_COMPATIBLE (default: True):
 make sure the keys don't contain a '.' or start with a '$' * ENFORCE_JSONPICKLE_COMPATIBLE (default: True):
 make sure the keys do not contain any reserved jsonpickle tags This is very important. Only deactivate if you know what you are doing. * ENFORCE_STRING (default: False):
 make sure all keys are string. * ENFORCE_VALID_PYTHON_IDENTIFIER (default: False):
 make sure all keys are valid python identifiers. Parameters key: The key that should be checked Raises ------ KeyError: if the key violates any requirements """

    if SETTINGS.CONFIG.ENFORCE_KEYS_MONGO_COMPATIBLE and (
            isinstance(key, basestring) and ('.' in key or key[0] == '$')):
        raise KeyError('Invalid key "{}". Config-keys cannot '
                       'contain "." or start with "$"'.format(key))

    if SETTINGS.CONFIG.ENFORCE_KEYS_JSONPICKLE_COMPATIBLE and \
            isinstance(key, basestring) and (
            key in jsonpickle.tags.RESERVED or key.startswith('json://')):
        raise KeyError('Invalid key "{}". Config-keys cannot be one of the'
                       'reserved jsonpickle tags: {}'
                       .format(key, jsonpickle.tags.RESERVED))

    if SETTINGS.CONFIG.ENFORCE_STRING_KEYS and (
            not isinstance(key, basestring)):
        raise KeyError('Invalid key "{}". Config-keys have to be strings, '
                       'but was {}'.format(key, type(key)))

    if SETTINGS.CONFIG.ENFORCE_VALID_PYTHON_IDENTIFIER_KEYS and (
            isinstance(key, basestring) and not PYTHON_IDENTIFIER.match(key)):
        raise KeyError('Key "{}" is not a valid python identifier'
                       .format(key))

    if SETTINGS.CONFIG.ENFORCE_KEYS_NO_EQUALS and (
            isinstance(key, basestring) and '=' in key):
        raise KeyError('Invalid key "{}". Config keys may not contain an'
                       'equals sign ("=").'.format('='))