Undeprecate inspect.getfullargspec()

[ryanpetrello]

BPO	27172
Nosy	@brettcannon, @ncoghlan, @larryhastings, @ned-deily, @bitdancer, @ericsnowcurrently, @serhiy-storchaka, @1st1, @timgraham
Files	signature-from-callable-skip-bound-arg.patch signature-from-callable-skip-bound-arg.patch signature-from-callable-skip-bound-arg.patch signature-from-callable-skip-bound-arg.patch signature-from-callable-skip-bound-arg.patch issue27172_undeprecate_getfullargspec.diff issue27172_undeprecate_getfullargspec_v2.diff

^{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 2019-05-11.04:32:59.790>
created_at = <Date 2016-06-01.04:04:50.799>
labels = ['type-bug', 'docs']
title = 'Undeprecate inspect.getfullargspec()'
updated_at = <Date 2019-05-11.04:32:59.789>
user = 'https://bugs.python.org/ryanpetrello'

bugs.python.org fields:

activity = <Date 2019-05-11.04:32:59.789>
actor = 'ncoghlan'
assignee = 'none'
closed = True
closed_date = <Date 2019-05-11.04:32:59.790>
closer = 'ncoghlan'
components = ['Documentation']
creation = <Date 2016-06-01.04:04:50.799>
creator = 'ryan.petrello'
dependencies = []
files = ['43078', '43081', '43087', '43089', '43090', '45663', '45723']
hgrepos = []
issue_num = 27172
keywords = ['patch']
message_count = 37.0
messages = ['266807', '266813', '266832', '266972', '268932', '269016', '269071', '269081', '269082', '269085', '269129', '269130', '270121', '270122', '281603', '281604', '281605', '281825', '281884', '282174', '282234', '282235', '282256', '282268', '292386', '292408', '292410', '303936', '303960', '303983', '304017', '304018', '307253', '307302', '307303', '307337', '307363']
nosy_count = 12.0
nosy_names = ['brett.cannon', 'ncoghlan', 'larry', 'ned.deily', 'r.david.murray', 'SilentGhost', 'python-dev', 'eric.snow', 'serhiy.storchaka', 'yselivanov', 'Tim.Graham', 'ryan.petrello']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27172'
versions = ['Python 3.6']

[SilentGhost]

This could only go in 3.6, but it needs proper documentation and test(s).

[ryanpetrello]

Thanks, I'll upload a new patch shortly that addresses these.

[1st1]

Please don't commit this without my review.

[1st1]

Nick, what do you think about this one?

I'm +1 to add this, but I'm not sure about the name for the argument. Do you have better ideas, or "skip_bound_arg" is good enough?

[ryanpetrello]

Yury/Nick,

Any word on this? I know it's minor, but I'd love to see this make it into Python3.6.

[ncoghlan]

Hi Ryan, sorry for the delayed response.

My own concern with this proposal is that I don't understand the use case for it. We changed the inspect.signature behaviour away from that of inspect.getfullargspec because we considered the latter behaviour to be *wrong*: it reported a parameter the already bound method didn't actually accept when called. The "skip_bound_arg" functionality then remains *within* the inspect module for the sake of providing backwards compatible implementations of getargspec and getfullargspec without duplicating a lot of other callable introspection logic.

If you can provide more information on the motivating use case, we can better determine if exposing this option directly is a suitable design response, or if there may be better alternatives available.

[ryanpetrello]

Nick,

My use case is an issue of backwards compatibility and multiple Python version support for a library that makes prolific use of the legacy argspec (args, varargs, varkw, defaults) namedtuple, *including* the bound self argument behavior. argspec and signature are quite different, and supporting a Py26-Py36 codebase that handles both simultaneously seemed like quite a burden, so I opted to write a compatibility shim that returned an Argspec-like object for all versions of Python; this seemed the simplest approach to bridge the gap in a codebase that supports Python 2.6 through 3.6, and it's the approach that I've seen other major libraries (like Django) take:

pecan/pecan#61

I'm using a similar approach to Python 3.5's getfullargspec() implementation:

https://github.com/python/cpython/blob/3.5/Lib/inspect.py#L1069

...but I don't have a long-term public API for doing it (so I have to rely on the private inspect._signature_from_callable call, which seems icky).

[ncoghlan]

Are you able to use inspect.getfullargspec() on Python 3 rather than inspect.getargspec()?

While both are technically deprecated, only inspect.getargspec() actually emits a deprecation warning - inspect.getfullargspec() still works without complaint and is in no danger of being removed.

[ryanpetrello]

Nick,

My main reasoning for not using it is that it's marked as deprecated in the docstring, and I want to avoid relying on it if disappears in the future :)

Warnings or not, the shim that I wrote doesn't use any deprecated code, so that's why I took that approach.

[ncoghlan]

OK, so maybe the right answer here is to officially undeprecate inspect.getfullargspec(), as we definitely *don't* want people feeling obliged to write their own version of that, and potentially introducing inconsistencies between different tools.

Then the deprecation warning on inspect.getargspec() can be updated to point people towards "inspect.signature() or inspect.getfullargspec()", while the docs for getfullargspec() itself can be updated to say it isn't recommended for use in new code, rather than that it's deprecated.

[ryanpetrello]

Nick,

That seems reasonable to me :) I've updated my library to just use inspect.getfullargspec for Py3. Thanks for taking the time to walk through this with me!

[ncoghlan]

OK, as per the above discussion, I've changed the title of this issue to be to undeprecate inspect.getfullargspec().

That change involves two pieces:

• change the documented deprecation of inspect.getfullargspec() to instead be a recommendation to avoid using it in new code, and instead use inspect.signature()
• update the inspect.getargspec() documentation and programmatic deprecation warning to point to both inspect.signature() and inspect.getfullargspec() as potential replacements

That gives folks already using inspect.getfullargspec assurance that it isn't going away anytime soon (if ever), while folks using inspect.getargspec get a lower impact migration path to a more Python 3 friendly version of the callable introspection API.