mypy: Raise TypeError during execution

Question

Context

mypy is great to check whether types are handled correctly.

# example.py

def test(string: str):
    return str

if __name__ == '__main__':
    test('19')
    test(19)

This will work for the first case, but not for the second one.

>> mypy example.py
example.py:6: error: Argument 1 to "test" has incompatible type "int"; expected "str"

However, just running this via

>> python example.py

does not raise any errors.

Question

Is it possible to use the optional static typing in python to raise errors when the code is actually executed?

I was hoping to use this e.g. to check variable types in unittests, where it would be much easier than having various if not isinstance(...) statements.

Answer 1

In short: Python does not check your types even if you have included type-hints.

Type-Hinting has been introduced to provide the option for static type-checking. This helps you get rid of many errors before even running your code. But you will need to use a static type-checker like mypy to do this. Python is still a dynamically typed language and therefore doesn't do any type-checking itself.

Answer 2

You can self call to mypy or use a similar approach. I put this in the head of my scripts to automate the process of checking before running the program.

import os, sys

def enforce_type_checking() -> None:
    result : int = os.system("mypy \"" + __file__ + "\""
        + " --strict --strict-equality --extra-checks"
        + " --pretty --show-error-context --show-column-numbers"
        + " --warn-return-any --warn-unreachable"
        + " --disallow-any-unimported --disallow-any-expr --disallow-any-decorated --disallow-any-explicit --disallow-any-generics --disallow-subclassing-any"
        + " --disallow-untyped-calls --disallow-untyped-defs --disallow-incomplete-defs --check-untyped-defs --disallow-untyped-decorators"
        + " --cache-dir \"" + sys.path[0] + "\" --install-types"
    )
    if result: input("Press <ENTER> to continue...")

# program #####################################################################

enforce_type_checking()

print("hello world")