def strip_required_suffix(string, suffix):
    """
    >>> strip_required_suffix('abcdef', 'def')
    'abc'
    >>> strip_required_suffix('abcdef', '123')
    Traceback (most recent call last):
    ...
    AssertionError: String ends with 'def', not '123'
    """
    if string.endswith(suffix):
        return string[:-len(suffix)]
    raise AssertionError('String ends with %r, not %r' % (string[-len(suffix):], suffix))
>>> strip_required_suffix('abcdef', 'def')
    'abc'
    >>> strip_required_suffix('abcdef', '123')
    Traceback (most recent call last):
    ...
    AssertionError: String ends with 'def', not '123'