Add inspect.Signature.from_text().

[ericsnowcurrently]

BPO	37497
Nosy	@ericsnowcurrently, @1st1
PRs	gh-81678: Add inspect.Signature.from_text() #14579

^{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 = 'https://github.com/ericsnowcurrently'
closed_at = None
created_at = <Date 2019-07-04.00:11:26.728>
labels = ['type-feature', 'library', '3.9']
title = 'Add inspect.Signature.from_text().'
updated_at = <Date 2020-03-06.20:46:18.865>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2020-03-06.20:46:18.865>
actor = 'yselivanov'
assignee = 'eric.snow'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-07-04.00:11:26.728>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 37497
keywords = []
message_count = 5.0
messages = ['347250', '347251', '362765', '363535', '363551']
nosy_count = 2.0
nosy_names = ['eric.snow', 'yselivanov']
pr_nums = ['14579']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37497'
versions = ['Python 3.9']

[ericsnowcurrently]

In early 2014 (3.3), when argument clinic was added, we added support for turning func.__text_signature__ into an inspect.Signature object. However, the functionality to convert a string into a Signature was never exposed publicly. Here's a patch to do so.

Note that inspect.signature() will also support taking a string.

[ericsnowcurrently]

Note that there has been a little discussion of this in the past, particularly around the time that argument clinic was introduced: https://bugs.python.org/issue23967#msg241140.

[1st1]

What's the actual use case for exposing this functionality?

[ericsnowcurrently]

Honestly, I don't recall exactly the concrete use case for which I opened this. :) I *think* it was related to applying a more restrictive signature onto an existing function (e.g. with a decorator).

[1st1]

I'd be fine with Signature.from_text(), but not with Signature constructor / signature() function accepting both callable and string arguments. Overall, I think that we ought to have a real need to add this new API, so unless there's a good (or any, really) use case I'd say we shouldn't merge this now.

[tungol]

This came up for me recently. I wanted to create a better signature for a built-in class's __init__ method by converting the signature from the underlying class. The text of the signature was easy to manipulate, but then I had no way to convert it to a signature object.

I worked around it by creating the signature object for the underlying class first, and then modifying it with replace(). That's not a bad work-around, but there were a couple extra considerations that I thought were slightly easier to handle when generating the text of the signature I wanted directly.

I agree that that's no need for inspect.signature() to understand the text format itself.