How to perform type checking with the typing Python module?

Question

I am reading the typing module code and also looking into mypy to understand how it does type checking. Unfortunately for me, mypy builds a very smart tree with typed expressions that I still don't understand, and it's all based on static analysis.

I would like to implement a type check system that is dynamic (no static analysis) in Python. Assuming the function that performs type checks is called check_type, I want to accomplish the following:

>>> import typing >>> >>> check_type(1, int) True >>> check_type(1, float) False >>> check_type([1], typing.List[int]) True >>> check_type([1], typing.List[float]) False >>> check_type([1], typing.List[typing.Any]) True >>> check_type((1,), typing.Tuple[int]) True

I thought about recreating the object type from its value, such as:

>>> get_type([1]) typing.List<~T>[int]

But this doesn't work with issubclass:

>>> issubclass(typing.List[int], typing.List[typing.Any]) False

I don't see a simple way to check types in Python without assuming a lot of things about the internals of the typing stdlib module (e.g., accessing __args__ or __tuple_params__).

How can I properly implement a check_type function that work for the cases previously listed? I am using Python 2.7.

No, @user2357112. isinstance doesn't check the type completely. For example: isinstance(['string'], typing.List[int]) is True when that's not exactly what I am looking for. — Hugo Lopes Tavares
– Hugo Lopes Tavares, Commented Jun 22, 2016 at 16:42
@freakish: Check out mypy, a static type analysis tool for Python and one of the big drivers behind type hints. — user2357112
– user2357112, Commented Jun 22, 2016 at 16:48
Not really an answer but the pythonic way to check types is to use a try/except statement and treat the value you want to test as the value you expect and if a TypeError exception is raised do something else or just pass — danidee
– danidee, Commented Jun 22, 2016 at 16:49
@freakish: PEP-484 (accepted). Although it doesn't quite apply to Python 2.7. — kennytm
– kennytm, Commented Jun 22, 2016 at 17:00

max · Accepted Answer · 2017-04-17 19:34:11Z

You can easily get the very limited functionality that works correctly for the simple examples provided in your question:

import mypy.api def check_type(value, typ): program_text = 'from typing import *; v: {} = {}'.format(typ, repr(value)) normal_report, error_report, exit_code = mypy.api.run(['-c', program_text]) return exit_code == 0 int_ = 1 str_ = 'a' list_str_ = ['a'] list_int_ = [1] tuple_int_ = (1,) assert check_type(int_, 'int') assert not check_type(str_, 'int') assert check_type(list_int_, 'List[int]') assert not check_type(list_str_, 'List[int]') assert check_type(list_int_, 'List[Any]') assert check_type(tuple_int_, 'Tuple[int]')

You can even do some more advanced stuff (for example, refer to the types that correspond to classes you defined in your program) by extending this code a bit, so that mypy gets to parse your entire source code as opposed to just the current line.

Alternatively, you might want to look at enforce or typeguard.

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