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.

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)

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.

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.

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.

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
>>>

Implemented in #32242. Thanks for the PR!

It appears this change is also causing issues on the entire buildbot fleet