Python >=3.5: Checking type annotation at runtime

Question

Does the typing module (or any other module) exhibit an API to typecheck a variable at runtime, similar to isinstance() but understanding the type classes defined in typing?

I’d like to be to run something akin to:

from typing import List
assert isinstance([1, 'bob'], List[int]), 'Wrong type'

Asked By: Bertrand Caron

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Source

Answer 1

There is no such function in the typing module, and most likely there won’t ever be.

Checking whether an object is an instance of a class – which only means “this object was created by the class’ constructor” – is a simple matter of testing some tagging.

However, checking whether an object is an “instance” of a type is not necessarily decidable:

assert isinstance(foo, Callable[[int], str]), 'Wrong type'

Although it is easy to inspect the typing annotations of foo (assuming it’s not a lambda), checking whether it complies to them is generally undecidable, by Rice’s theorem.

Even with simpler types, such as List[int] the test will easily become far too inefficient to be used for anything but the smallest toy examples.

xs = set(range(10000))
xs.add("a")
xs.pop()
assert isinstance(xs, Set[int]), 'Wrong type'

The trick that allows type checker to perform this operation in a relatively efficient way, is to be conservative: the type checker tries to prove that foo always return int. If it fails, it rejects the program, even though the program may be valid, i.e. this function is likely to be rejected, although it is perfectly safe:

def foo() -> int:
    if "a".startswith("a"):
        return 1
    return "x"

Answered By: Elazar

Answer 2

I was looking for something similar and found the library typeguard. This can automatically do runtime type checks wherever you want. Checking types directly as in the question is also supported. From the docs,

from typeguard import check_type

# Raises TypeError if there's a problem
check_type('variablename', [1234], List[int])

Answered By: aravindsagar

Answer 3

This is what I have discovered recently, basically this decorator does type checking at runtime raising exception if some type definition did not match. It can also do type checking for nested types (dict of strings, etc)

https://github.com/FelixTheC/strongtyping

Example:

from strongtyping.strong_typing import match_typing

@match_typing
def func_a(a: str, b: int, c: list):
   ...

func_a('1', 2, [i for i in range(5)])
# >>> True

func_a(1, 2, [i for i in range(5)])
# >>> will raise a TypeMismatch Exception

Answered By: Alex Paramonov

Python >=3.5: Checking type annotation at runtime

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