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pythonpython-3.6backwards-compatibilitytypechecking

Providing Backwards Compatability with Python 3.6 Variable Annotations

I am trying to create a python package (not anywhere at the moment) and I want to use 3.6 python variable annotations, that is

foo: int = 5

while still providing support for Python 3.5.

Is there any way to provide these style of variable annotations inside Python 3.5, either through a 

from __future__ import variable_annotations

or similar. I know it is possible to use comment type annotations, but I would like to be able to use this style.

Solution

  • TLDR: No, you can not use variable annotations in Python 3.5 and earlier.

    First, Python 3.6.0 changelog reports variable annotations to be a new feature comparing to 3.5.

    Second, PEP 526 is defined as backward compatible. According to PEP 387:

    Unless it is going through the deprecation process below, the behavior of an API must not change between any two consecutive releases.

    And this includes:

    Syntax and behavior of these constructs as defined by the reference manual

    So PEP 526's "fully backwards compatible" just means that Python 3.5 (or to be scrupulous, pre-PEP526) syntax will be working in 3.6.0 without changes: variable annotations are not mandatory.

    At last, you mentioned "comment type annotations". So I will just give a link to PEP 484, which is available for Python 3.5 (it might be useful for some readers of this question).