Python is_format_string() predicate

is_format_string.py

def is_format_string(s):
    """Test if the given string is a format string.

    Args:
        s (str): String to test.

    Returns:
        bool: True if `s` is a format string, False otherwise.

    Bugs:
        The string '{}{' is considered a format string since the IndexError is
        raised before the ValueError. The same applies to '{}}'.

    Examples:
        >>> is_format_string()
        Traceback (most recent call last):
        TypeError: is_format_string() takes exactly 1 argument (0 given)
        >>> is_format_string(None)
        False
        >>> is_format_string('')
        False
        >>> is_format_string('foo')
        False
        >>> is_format_string('{')
        False
        >>> is_format_string('{{')
        False
        >>> is_format_string('}')
        False
        >>> is_format_string('}}')
        False
        >>> is_format_string('{{}}')
        False
        >>> is_format_string('{}')
        True
        >>> is_format_string('{foo}')
        True

    """
    try:
        str.format(s)
    except (IndexError, KeyError):
        # s = '{}':
        #    IndexError: tuple index out of range
        # s = '{foo}':
        #    KeyError: 'foo'
        return True
    except (TypeError, ValueError):
        # s = None:
        #     TypeError: descriptor 'format' requires a 'str' object but
        #     received a 'NoneType'
        # s = '{'
        #     ValueError: Single '{' encountered in format string
        return False
    else:
        return False