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Python's (future) type annotation system(s)

Description

Python is a dynamically (but strongly, for some value of "strongly") typed programming language. Notwithstanding its dynamism, checking types--or other behaviors--of variables has always been possible in Python code, and a steady stream of users have had a desire to do so. At a conceptual level, enforcing a type is a subset of enforcing an invariant on a variable, and the broader demand for design by contract has been a recurrent theme in Python discussions. PEP 316 addressed this desire (but was not accepted) a decade ago, as did the long defunct library PyDBC. Currently maintained, however, is the PyContracts library, which allows documenting and enforcing both types narrowly, and predicates of variables more broadly. I myself wrote a simple recipe for basic type checking using PEP 3107 annotations at the Python Cookbook: Type checking using Python 3.x annotations (http://code.activestate.com/recipes/578528-type-checking-using- python-3x-annotations/).

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