From the mypy docs linked in the article (http://mypy.readthedocs.io/en/latest/duck_type_compatibility.html http://mypy.readthedocs.io/en/latest/duck_type_compa

From the mypy docs linked in the article (http://mypy.readthedocs.io/en/latest/duck_type_compatibility.html http://mypy.readthedocs.io/en/latest/duck_type_compatibility...), it seems it supports a small number of built-in duck types currently, and has support for some protocols (http://mypy.readthedocs.io/en/latest/cheat_sheet.html#standard-duck-types http://mypy.readthedocs.io/en/latest/cheat_sheet.html#standa...) > mypy considers an int object to be valid whenever a float object is expected. > Use Iterable for generic iterables (anything usable in `for`) Thus you should be able to get a more broadly correct (though still not 100% if you have your own types implementing __add__ and __radd__) like this: from typing import Iterator def sum_and_stringify(nums: Iterator[float]) -> str: return str(sum(nums)) It also looks like implementing an abstract base class SupportsSum (to check for __add__ and __radd__) may not take a whole lot of work, see `SupportsInt`: https://github.com/python/mypy/blob/d7f6ea617317103534d8668658d176d7a8972cb2/lib-typing/3.2/typing.py#L1370 https://github.com/python/mypy/blob/d7f6ea617317103534d86686... http://mypy.readthedocs.io/en/latest/class_basics.html?highlight=protocol#abstract-base-classes-and-multiple-inheritance http://mypy.readthedocs.io/en/latest/class_basics.html?highl...