Run-time type checker for Python

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This library provides run-time type checking for functions defined with argument type annotations.

The typing module introduced in Python 3.5 (and available on PyPI for older versions of Python 3) is supported. See below for details.

There are two principal ways to use type checking, each with its pros and cons:

  1. calling check_argument_types() from within the function body:

    debugger friendly but cannot check the type of the return value

  2. decorating the function with @typechecked:

    can check the type of the return value but adds an extra frame to the call stack for every call to a decorated function

If a function is called with incompatible argument types or a @typechecked decorated function returns a value incompatible with the declared type, a descriptive TypeError exception is raised.

Type checks can be fairly expensive so it is recommended to run Python in “optimized” mode (python -O or setting the PYTHONOPTIMIZE environment variable) when running code containing type checks in production. The optimized mode will disable the type checks, by virtue of removing all assert statements and setting the __debug__ constant to False.

Using check_argument_types():

from typeguard import check_argument_types

def some_function(a: int, b: float, c: str, *args: str):
    assert check_argument_types()
    ...

Using @typechecked:

from typeguard import typechecked

@typechecked
def some_function(a: int, b: float, c: str, *args: str) -> bool:
    ...

To enable type checks even in optimized mode:

@typechecked(always=True)
def foo(a: str, b: int, c: Union[str, int]) -> bool:
   ...

The following types from the typing package have specialized support:

Type

Notes

Dict

Keys and values are typechecked

List

Contents are typechecked

Set

Contents are typechecked

Tuple

Contents are typechecked

Callable

Argument count is checked but types are not (yet)

TypeVar

Constraints, bound types and co/contravariance are supported but custom generic types are not (due to type erasure)

Union

Supported by

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