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[Python-ideas] Type Hinting Kick-off

Andrew Svetlov andrew.svetlov at gmail.com
Sun Dec 21 20:55:14 CET 2014 
The silly question.

I use python 3 annotations for argument checking.

My assumption is very simple, for

def f(arg: checker): pass

the checker will raise ValueError or TypeError if arg is not correct.
I do it by `checker(arg)` call.

I use this in aiozmq rpc
(http://aiozmq.readthedocs.org/en/0.5/rpc.html#signature-validation)
for example and checkers from trafaret
(https://github.com/Deepwalker/trafaret) works fine.

Will proposed change break my workflow?

On Sun, Dec 21, 2014 at 6:53 PM, Guido van Rossum <guido at python.org> wrote:
> On Sat, Dec 20, 2014 at 2:00 PM, Dennis Brakhane <brakhane at googlemail.com>
> wrote:
>>
>> Maybe I'm missing something, but wouldn't type hinting as it's defined
>> now break "virtual subclassing" of ABC?
>>
>> For example, given the following code:
>>
>>   from collections import Sequence
>>
>>   class MySequence:
>>     ...
>>
>>   Sequence.register(MySequence)
>>
>> it seems to me like the following would work:
>>
>>   def foo(bar):
>>       if not isinstance(bar, Sequence):
>>           raise RuntimeError("Foo can only work with sequences")
>>       ...
>>
>> but when rewritten for static type checking
>>
>>     def foo(bar: Sequence):
>>         ....
>>
>> it would cease to work. At least I don't see a way a static type checker
>> could handle this relaiably (the register call might occur anywhere,
>> after all)
>>
>> Is this intentional?
>>
>> Even if this might be more a mypy/implementation question, it should be
>> clear to users of typing.py if they should expect ABCs to break or not
>
>
> Well, the program will still run fine with CPython, assuming you keep the
> isinstance() check in your program. The argument declaration does not cause
> calls with non-Sequence arguments to be rejected, it just guides the static
> checker (which is a program that you must run separately, in a similar way
> as a linter -- it is not built into CPython).
>
> The static checker may request the call foo(MySequence()) if it cannot see
> that MySequence is a Sequence. You have two options there: if you can change
> the code of MySequence, you should just inherit from Sequence rather than
> using the register() call; if you cannot change the code of MySequence, you
> should use a *stub* module which declares that MySequence is a Sequence. You
> can read up on stub modules in the mypy docs:
> http://mypy.readthedocs.org/en/latest/basics.html#library-stubs
>
> --
> --Guido van Rossum (python.org/~guido)
>
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-- 
Thanks,
Andrew Svetlov
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