Runtime Type Checking in Python 3
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This package intends to provide run-time type checking for functions annotated with argument type hints (standard library typing module in Python 3.5, or backports.typing package in Python 3.3 / 3.4).


from typing import Sequence, List
from typo import type_check

def f(x: int, s: Sequence[int]) -> List[int]:

The @type_check decorator ensures that the values passed to annotated arguments will have their types checked before the function is executed; return value can be optionally checked as well.

If the value types are not consistent with the function signature, a TypeError with a descriptive error message will be raised. For instance, calling function f from the example above with wrong argument types,

>>> f(1, (0, 2.2))

results in an exception being thrown:

TypeError: invalid item #1 of `s`: expected int, got float

Note: this is work-in-progress and not all typing primitives are supported; however all supported constructs should be covered by a good number of tests.

Here's some of the supported type hints: simple types, List, Dict, Tuple, Sequence, Set, TypeVar (with support for constraints and upper bounds).

What's not supported: Iterator and Generator (which we can't inspect due to their laziness), Callable (which we can't check without calling it), forward references (which is possible to support but requires more work), covariant and contravariant type variables (this requires more thought but isn't likely to be helpful in the runtime context).