BoundArguments facility to inject defaults

[pitrou]

BPO	24190
Nosy	@brettcannon, @pitrou, @larryhastings, @bitdancer, @1st1
Files	sig_ba_default.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 2015-05-16.17:45:53.125>
created_at = <Date 2015-05-14.10:20:00.056>
labels = ['type-feature', 'library']
title = 'BoundArguments facility to inject defaults'
updated_at = <Date 2015-05-16.17:45:53.124>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2015-05-16.17:45:53.124>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2015-05-16.17:45:53.125>
closer = 'yselivanov'
components = ['Library (Lib)']
creation = <Date 2015-05-14.10:20:00.056>
creator = 'pitrou'
dependencies = []
files = ['39372']
hgrepos = []
issue_num = 24190
keywords = ['patch']
message_count = 10.0
messages = ['243167', '243170', '243177', '243200', '243202', '243204', '243205', '243210', '243341', '243342']
nosy_count = 6.0
nosy_names = ['brett.cannon', 'pitrou', 'larry', 'r.david.murray', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24190'
versions = ['Python 3.5']

[pitrou]

The recipe to inject default values in a BoundArguments instance is given in the doc, but it's not trivial. Furthermore, it's actually incomplete: it doesn't handle any star-arguments, e.g.:

>>> sig = inspect.signature(f)
>>> ba = sig.bind(2, d=4)
>>> for param in sig.parameters.values():
...     if (param.name not in ba.arguments
...             and param.default is not param.empty):
...         ba.arguments[param.name] = param.default
... 
>>> ba.arguments
OrderedDict([('a', 2), ('d', 4), ('b', 5)])

ba['c'] would ideally contain the empty tuple.

[pitrou]

My example forgets the function declaration, which is:

>>> def f(a, b=5, *c, d=5): pass
...

[bitdancer]

See bpo-22998. The more complete and thus more complex example in the last message makes it look like including this in the library might be a good idea.

[1st1]

Well, the docs example only binds explicit defaults in function signature. Implicit defaults for *args and **kwargs (() and {}) aren't usually useful (in my opinion).

Do you guys have any good use case for such method? I use the Signature API extensively for argument types validation and for serialization of RPC calls, but I never needed this functionality from BoundArguments.

[pitrou]

Le 14/05/2015 17:45, Yury Selivanov a écrit :

Well, the docs example only binds explicit defaults in function
signature. Implicit defaults for *args and **kwargs (() and {})
aren't usually useful (in my opinion).

When the defaults are filled I expect ba.arguments to be "complete",
that is have a value for every signature parameter. Otherwise I have to
special-case *args and **kwargs.

Do you guys have any good use case for such method?

A use case was given in bpo-22998.
My use case is JIT-compiling functions and function calls in Numba. We
reimplement the function calls ourselves, so need a complete mapping of
arguments to values.

[1st1]

> Do you guys have any good use case for such method?

A use case was given in bpo-22998.
My use case is JIT-compiling functions and function calls in Numba. We
reimplement the function calls ourselves, so need a complete mapping of
arguments to values.

This is a great use case ;-) Let's add it.

I propose the following method: BoundArguments.apply_defaults()

It will iterate through its parent Signature's parameters and assign default values to BoundArguments.arguments (when an arg is missing), including setting '()' for *args, and '{}' for **kwargs.

If you're OK with this, I can draft a patch.

[pitrou]

That sounds good to me, thank you!

[1st1]

FWIW it wasn't as easy as I thought it would be :) You were right, docs example is very basic.

Please take a look at the attached patch.

[python-dev]

New changeset ea61d8eb8a28 by Yury Selivanov in branch 'default':
bpo-24190: Add inspect.BoundArguments.apply_defaults() method.
https://hg.python.org/cpython/rev/ea61d8eb8a28

[1st1]

Thanks for the suggestion, Antoine!