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Asked: 2026-05-15T05:45:54+00:00

Function Annotations: PEP-3107 I ran across a snippet of code demonstrating Python3’s function annotations.

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Function Annotations: PEP-3107

I ran across a snippet of code demonstrating Python3’s function annotations. The concept is simple but I can’t think of why these were implemented in Python3 or any good uses for them. Perhaps SO can enlighten me?

How it works:

def foo(a: 'x', b: 5 + 6, c: list) -> max(2, 9):
    ... function body ...

Everything following the colon after an argument is an ‘annotation’, and the information following the -> is an annotation for the function’s return value.

foo.func_annotations would return a dictionary:

{'a': 'x',
 'b': 11,
 'c': list,
 'return': 9}

What’s the significance of having this available?

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1 Answer

  1. Editorial Team

    I think this is actually great.

    Coming from an academic background, I can tell you that annotations have proved themselves invaluable for enabling smart static analyzers for languages like Java. For instance, you could define semantics like state restrictions, threads that are allowed to access, architecture limitations, etc., and there are quite a few tools that can then read these and process them to provide assurances beyond what you get from the compilers. You could even write things that check preconditions/postconditions.

    I feel something like this is especially needed in Python because of its weaker typing, but there were really no constructs that made this straightforward and part of the official syntax.

    There are other uses for annotations beyond assurance. I can see how I could apply my Java-based tools to Python. For instance, I have a tool that lets you assign special warnings to methods, and gives you indications when you call them that you should read their documentation (E.g., imagine you have a method that must not be invoked with a negative value, but it’s not intuitive from the name). With annotations, I could technically write something like this for Python. Similarly, a tool that organizes methods in a large class based on tags can be written if there is an official syntax.

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