feat(python): Enable creation of independently reusable Config instances

[alexander-beedie]

At the moment we set the options on Config init (or via the associated @staticmethod); when used as a context manger or function decorator this means we need to explicitly supply all the options on each usage - we cannot currently reuse an instance because scope exit resets the options to the state they were in at scope entry, and having the same object enter a new scope will not set the options again (as the options are set at init).

• This PR enables reusable Config instances. A new init parameter "apply_on_context_enter" is introduced which defers applying the indicated options until context/function scope is entered.

• Because the options are now stored and set on scope entry, bundles of Config options can now be created/managed centrally and then cleanly reused throughout a codebase without having to set all the options explicitly each time.

• Minor drive-by; added proper __eq__ and __ne__ comparison methods for Config instances.

• Backwards-compatible update; no change to the current default behaviour.

Example

Declare some Config instances that take effect only when entering context/function scope:

import polars as pl

cfg_verbose = pl.Config(
  verbose=True,
  apply_on_context_enter=True,
)

cfg_markdown = pl.Config(
  tbl_formatting="MARKDOWN",
  float_precision=3,
  apply_on_context_enter=True,
)

Can now use (and reuse) these objects as decorators or context managers without having to explicitly re-declare all the parameters:

@cfg_markdown
def write_frame_to_stdout(df: pl.DataFrame) -> None:
    sys.stdout.write(str(df))

with cfg_markdown:
    print(df)

@cfg_verbose
def do_various_things():
    ...

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.52%. Comparing base (0b218d8) to head (3dfb310).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/config.py	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20053      +/-   ##
==========================================
+ Coverage   79.50%   79.52%   +0.01%     
==========================================
  Files        1562     1563       +1     
  Lines      216939   217160     +221     
  Branches     2459     2462       +3     
==========================================
+ Hits       172479   172693     +214     
- Misses      43901    43907       +6     
- Partials      559      560       +1     

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[ritchie46]

Nice one!