Allow decimal.localcontext to accept keyword arguments to set context attributes

[stevendaprano]

BPO	47135
Nosy	@ncoghlan, @stevendaprano, @corona10, @dignissimus
PRs	gh-91291: Accept attributes as keyword arguments in decimal.localcontext #32242
Files	sam_ezeh.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 = None
closed_at = None
created_at = <Date 2022-03-27.10:16:04.212>
labels = ['library', '3.11']
title = 'Allow decimal.localcontext to accept keyword arguments to set context attributes'
updated_at = <Date 2022-04-01.20:49:19.136>
user = 'https://github.com/stevendaprano'

bugs.python.org fields:

activity = <Date 2022-04-01.20:49:19.136>
actor = 'sam_ezeh'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-03-27.10:16:04.212>
creator = 'steven.daprano'
dependencies = []
files = ['50713']
hgrepos = []
issue_num = 47135
keywords = ['patch']
message_count = 6.0
messages = ['416114', '416364', '416507', '416511', '416513', '416514']
nosy_count = 4.0
nosy_names = ['ncoghlan', 'steven.daprano', 'corona10', 'sam_ezeh']
pr_nums = ['32242']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue47135'
versions = ['Python 3.11']

[stevendaprano]

Whenever I use decimal and need to change the context temporarily, without fail I try to write

with decimal.localcontext(prec=10):
    ...

or similar. Then I get surprised that it fails, and re-write it as

with decimal.localcontext() as ctx:
    ctx.prec = 10
    ...

Let's make the first version work. localcontext should accept keyword arguments corresponding to the same arguments accepted by Context, and set them on the context given.

A proof-of-concept wrapper function:

def localcontext(ctx=None, **kwargs):
    if ctx is None:
        ctx = decimal.getcontext().copy()
    for key, value in kwargs.items():
        setattr(ctx, key, value)
    return decimal.localcontext(ctx)

I think this would be a valuable, and useful, improvement to the decimal API.

[ncoghlan]

Seems reasonable to me, but I think a full implementation would want to throw an error for keyword args that don't already exist as context attributes (otherwise typos would fail silently)

[dignissimus]

I'm looking into adding this

Seems reasonable to me, but I think a full implementation would want to throw an error for keyword args that don't already exist as context attributes (otherwise typos would fail silently)

For _pydecimal, I think this would automatically happen automatically as Context.__setattr__ raises AttributeError when it's passed a name that isn't a context attribute.

For _decimal this can be done with keyword arguments and PyArg_ParseTupleAndKeywords.

[dignissimus]

I've uploaded a patch and it seems to work, which I'm very proud of.

I'll create some tests, amend documentation and create a news entry. After that, I'll create a pull request on GitHub.

[stevendaprano]

I'm not sure what the implementation uses to enforce this, but decimal
contexts already seem to reject arbitrary attributes. So a naive
implementation that just setattr()'s the keyword arguments will
automatically fail:

>>> from decimal import getcontext
>>> ctx = getcontext()
>>> setattr(ctx, 'precision', 10)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'decimal.Context' object has no attribute 'precision'

But you are absolutely correct that however we enforce it, we should
avoid allowing typos to silently fail.

[dignissimus]

This is what functionality looks like when supplying incorrect attribute names with the patch.

Python 3.11.0a6+ (heads/bpo-47135-dirty:d4bb38f82b, Apr  1 2022, 20:01:56) [GCC 11.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import _pydecimal
>>> ctx = _pydecimal.getcontext()
>>> ctx.precision = 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/run/media/sam/OS/Git/cpython/Lib/_pydecimal.py", line 3974, in __setattr__
    raise AttributeError(
    ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'decimal.Context' object has no attribute 'precision'
>>> with _pydecimal.localcontext(precision=10) as ctx:
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/run/media/sam/OS/Git/cpython/Lib/_pydecimal.py", line 506, in localcontext
    setattr(ctx, key, value)
    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/run/media/sam/OS/Git/cpython/Lib/_pydecimal.py", line 3974, in __setattr__
    raise AttributeError(
    ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'decimal.Context' object has no attribute 'precision'
>>> import decimal
>>> ctx = decimal.getcontext()
>>> ctx.precision = 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'decimal.Context' object has no attribute 'precision'
>>> with decimal.localcontext(precision=10) as ctx:
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'precision' is an invalid keyword argument for this function
>>>

[JelleZijlstra]

Implemented in #32242. Thanks for the PR!