Please upload your patch from a Mercurial clone: * https://docs.python.org/devguide/setup.html#checkout * https://docs.python.org/devguide/patch.html Currently, if you pass a string as a second argument, you get: >>> complex(1, "1") Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: complex() second arg can't be a string So the exception message should probably be changed to include "second arg" or "second argument": >>> complex(1j, {1: 2}) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: complex() second arg must be a number, not 'dict' Also, there is already a check for second argument in line 952 so the "must be a string" part is probably not needed. We probably need to check whether these two cases can be combined. You also need to add some tests to Lib/test/test_complex.py.