inspect.signature: Consider exposing 'follow_wrapper_chains' option in public API

[1st1]

BPO	20691
Nosy	@ncoghlan, @larryhastings, @1st1
Files	sig_wrapped.patch

^{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/1st1'
closed_at = <Date 2015-05-20.18:31:21.567>
created_at = <Date 2014-02-19.22:21:01.453>
labels = ['type-feature']
title = "inspect.signature: Consider exposing 'follow_wrapper_chains' option in public API"
updated_at = <Date 2015-05-20.18:31:21.566>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2015-05-20.18:31:21.566>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2015-05-20.18:31:21.567>
closer = 'yselivanov'
components = []
creation = <Date 2014-02-19.22:21:01.453>
creator = 'yselivanov'
dependencies = []
files = ['39435']
hgrepos = []
issue_num = 20691
keywords = ['patch']
message_count = 8.0
messages = ['211668', '214992', '243556', '243558', '243627', '243629', '243682', '243683']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'larry', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20691'
versions = ['Python 3.5']

[1st1]

In the issue bpo-20684, Nick had an idea of exposing 'follow_wrapper_chains' option to the public API. This might be actually very useful.

[1st1]

So... should we expose two keyword only parameters for Signature.from_callable() and signature():

• 'follow_wrapped=True' to follow __wrapped__ chains;
• 'keep_bound_arg=False' to skip/include first bound arg?

[1st1]

Nick, what do you think of this one?

[ncoghlan]

I like the idea of the following signature:

    def signature(callable, *, follow_wrapped=True):
        ...

I'm less convinced about exposing the flag to optionally show the first positional arg for bound methods, as that's currently specific to method objects and builtins, rather than being a protocol like __wrapped__ that any callable can readily implement.

If we *did* do that, then I think we should follow the convention of naming it after an attribute we look for (in this case, __self__) rather than limiting it to a specific type, and also make it default to true for consistency.

That would give us:

    def signature(callable, *, follow_wrapped=True, omit_bound_self=True):
        ...

The "omit_bound_self" flag would then be documented along the following lines: "If 'omit_bound_self' is true, and the callable has a '__self__' attribute set to a value other than None, then the first positional argument will be hidden from the displayed signature. Setting this to false means that bound methods will be displayed the same"

However, as noted, I doubt the latter is worth it - let's just expose the flag to decide whether to resolve wrapper chains or not, and leave the inclusion of the already bound value in the reported signature solely as a legacy behaviour of getargspec and getfullargspec.

[1st1]

Nick,

However, as noted, I doubt the latter is worth it - let's just expose the flag to decide whether to resolve wrapper chains or not, and leave the inclusion of the already bound value in the reported signature solely as a legacy behaviour of getargspec and getfullargspec.

I agree. Please take a look at the attached patch.

BTW, Signature.from_function and Signature.from_builtin aren't documented. Do you think it's OK if I drop them? There is no good use case for them (and we have Signature.from_callable in 3.5).

Also, while working on the patch, I noticed that functools.wraps copies __annotations__ attribute. This is really strange. Most of the times, wrapper in python has signature akin to (*args, **kwargs). Blindly copying __annotations__ doesn't make any sense. What do you think about this?

[1st1]

Do you think it's OK if I drop them?

Dropping == DeprecationWarning

[python-dev]

New changeset 0c298f1ee3f6 by Yury Selivanov in branch 'default':
bpo-20691: Add follow_wrapped arg to inspect.signature/from_callable.
https://hg.python.org/cpython/rev/0c298f1ee3f6

[1st1]

Closing this one.