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This library provides run-time type checking for functions defined with PEP 484 argument (and return) type annotations, and any arbitrary objects. It can be used together with static type checkers as an additional layer of type safety, to catch type violations that could only be detected at run time.

Two principal ways to do type checking are provided:

  1. The check_type function:

    • like isinstance(), but supports arbitrary type annotations (within limits)

    • can be used as a cast() replacement, but with actual checking of the value

  2. Code instrumentation:

    • entire modules, or individual functions (via @typechecked) are recompiled, with type checking code injected into them

    • automatically checks function arguments, return values and assignments to annotated local variables

    • for generator functions (regular and async), checks yield and send values

    • requires the original source code of the instrumented module(s) to be accessible

Two options are provided for code instrumentation:

  1. the @typechecked function:

    • can be applied to functions individually

  2. the import hook (typeguard.importhook.install_import_hook()):

    • automatically instruments targeted modules on import

    • no manual code changes required in the target modules

    • requires the import hook to be installed before the targeted modules are imported

    • may clash with other import hooks