Python compatibility clarifications

[pavel-kirienko]

Python 3.5.2 turned out to be unsupported because there was an issue in the static typing library: python/typing#308. The issue has been fixed in Python 3.5.3, so all newer versions function correctly. This is how you can reproduce the problem:

>>> import typing
>>> T = typing.TypeVar('T')
>>> G = typing.Callable[[], T]
>>> G[int]
typing.Callable[[], int]

The above is the expected behavior observable in Python 3.5.3+. In 3.5.2 you get this instead:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/typing.py", line 815, in __getitem__
    raise TypeError("This Callable type is already parameterized.")
TypeError: This Callable type is already parameterized.

An alternative solution would be to remove the offending type annotations, which you can look at here:

https://github.com/UAVCAN/pydsdl/blob/9a0bd72b4fd298e96c57f77e3c9a48b6fe1ee70a/pydsdl/_expression.py#L563-L565

However, I would rather not do that because that would require us to plaster the code with linter suppression comments and doing so for the sake of an ancient version of the interpreter (which will be EOLed next year) doesn't seem particularly sensible.

Should we accept this change, or should we add workarounds instead and retain compatibility with all v3.5.x?

[coveralls]

Pull Request Test Coverage Report for Build 232

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 225:	0.0%
Covered Lines:	2658
Relevant Lines:	2658

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💛 - Coveralls

[thirtytwobits]

Sorry I didn't comment on this sooner. I'm okay with this stipulation. Nunavut uses python 3.5.7 as it's minimal support target (i.e. our python tox container uses 3.5.7).

Have you considered enabling tox for this project?

We should also discuss standardizing the UAVCAN python tools since nunavut uses a different set of them than pydsdl. This is fine for now but at some point we'll tire of supporting so many external dependencies.