typing.get_type_hints not working with forward-declaration and decorated functions

[netbnd]

BPO	37838
Nosy	@gvanrossum, @ilevkivskyi, @miss-islington, @nanjekyejoannah, @benedwards14
PRs	bpo-37838: get_type_hints for wrapped functions with forward reference #17126 [3.8] bpo-37838: get_type_hints for wrapped functions with forward reference (GH-17126) #17324 [3.7] bpo-37838: get_type_hints for wrapped functions with forward reference (GH-17126) #17325
Files	typing_check_wrapped.zip: Sample code with working and non working cases is attached to reproduce the issue.

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-11-21.17:47:38.989>
created_at = <Date 2019-08-13.10:42:59.139>
labels = ['3.7', 'easy', 'type-bug', 'library']
title = 'typing.get_type_hints not working with forward-declaration and decorated functions'
updated_at = <Date 2019-11-21.17:47:38.988>
user = 'https://bugs.python.org/netbnd'

bugs.python.org fields:

activity = <Date 2019-11-21.17:47:38.988>
actor = 'levkivskyi'
assignee = 'none'
closed = True
closed_date = <Date 2019-11-21.17:47:38.989>
closer = 'levkivskyi'
components = ['Library (Lib)']
creation = <Date 2019-08-13.10:42:59.139>
creator = 'netbnd'
dependencies = []
files = ['48540']
hgrepos = []
issue_num = 37838
keywords = ['patch', 'newcomer friendly']
message_count = 16.0
messages = ['349542', '349565', '349580', '349581', '349594', '355477', '355501', '355502', '355585', '355643', '355747', '356390', '356470', '357184', '357187', '357188']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'levkivskyi', 'miss-islington', 'nanjekyejoannah', 'netbnd', 'benedwards14']
pr_nums = ['17126', '17324', '17325']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37838'
versions = ['Python 3.6', 'Python 3.7']

[netbnd]

When decorating a function and using a forward declaration as type hint, the typing.get_type_hints function does not work anymore, since it cannot find the forward declared name in the namespace. After debugging I think, the typing.get_type_hints function is actually using the namespace of the decorator instead of the decorated function.

When using a normal class type (no forward declaration) everything works fine and also when not using any decorator it works like expected.

As a workaround, one could pass the local namespace to typing.get_type_hints. However in normal usecases this function is used for runtime typechecking in a deep call hierarchy. So one would normally not have access to the right local namespace, only to the function object itself.

However there is another easy workaround. At least when using the functool.wraps method to create a function decorator. The decorated functions has a "__wrapped__" attribute, which references the original function. When using "typing.get_type_hints(function.__wrapped__)" instead of "typing.get_type_hints(function)", it works like expected. So maybe this could be built in into the get_type_hints method.

[gvanrossum]

Using __wrapped__ if present sounds like a good idea. Ivan, what do you think?

Netzeband, have you considered submitting a PR with the necessary changes (and tests and docs)?

[ilevkivskyi]

Using __wrapped__ if present sounds like a good idea.

Yeah, I like this idea, this will likely cover most use cases (since most people actually do use @wraps).

[gvanrossum]

Maybe we should tag this issue "newcomer friendly"? It seems a pretty open
and shut case.

[netbnd]

Thanks for your feedback. I can create a pull-request. However, I did it never before for the Python-Lib, so I first have to setup the correct environment for that and ensure, that I can run all test-cases.

Since I'm on vacation soon, it could take several weeks. If someone wants to solve the issue earlier, please don't hesitate.

I will write it in this issue ticket, as soon as I finished the PR.

[benedwards14]

Hey Netzeband, are you still working on this, or would it be ok for me to have a go?

If all you needed was help with a PR let me know, i have some experience?

[nanjekyejoannah]

@benedwards14 Feel free to open a PR. @netbnd said anyone can take on the issue since he may take long to revert with a PR.

[nanjekyejoannah]

Actually there is a patch. So please @netbnd please open a PR https://github.com/python/cpython

[netbnd]

Hello,

I'm very sorry, I was not able to find the time to finish the patch in the quality I wanted to have. So @benedwards14 please feel free to provide a pull request.

Best regards,
André

[benedwards14]

Thanks Netzeband,

Will get on this.

Ben

[netbnd]

Thanks a lot!