Fix cache key generation for variadic arguments (*args, **kwargs)

[Copilot]

✅ Fix for Variadic Arguments in Cache Key Generation - COMPLETE

Problem Solved

• Understand the bug: Functions with *args don't get unique cache keys
• Root cause identified: _convert_args_kwargs() in core.py doesn't properly handle VAR_POSITIONAL parameters
• Created test case to reproduce the issue
• Fixed _convert_args_kwargs() to properly handle *args and **kwargs parameters
• Verified the fix with comprehensive tests
• Ensured backward compatibility with custom hash functions
• Type checking passes (mypy)
• Linting passes (ruff)
• All existing tests pass
• Original issue scenario verified
• Address PR review feedback:
  • Fixed keyword-only parameter handling
  • Simplified unreachable code in VAR_KEYWORD handling
  • Parametrized tests for better coverage
  • Added keyword-only parameter tests
  • Improved all test assertions to explicitly verify cache hits

Summary

Successfully fixed the bug where cachier didn't consider variadic arguments (*args) in the cache key, causing all calls with different arguments to return the same cached result.

Review Feedback Addressed

Critical Bug Fix (Comment 2730944516):

• Fixed incorrect handling of keyword-only parameters by explicitly filtering for POSITIONAL_ONLY and POSITIONAL_OR_KEYWORD parameter kinds
• Previously, keyword-only parameters were incorrectly included in regular_params, which would try to map positional args to them

Code Simplification (Comment 2730944489):

• Simplified VAR_KEYWORD handling by removing the unreachable else branch
• A parameter in sig.parameters with kind == VAR_KEYWORD cannot exist as a named parameter

Test Improvements:

• Parametrized first test to run on both pickle and memory backends (Comment 2731522781)
• Added 3 new tests for keyword-only parameter edge cases (Comment 2730944542):
  • test_keyword_only_parameters: Basic keyword-only parameter handling
  • test_keyword_only_with_default: Keyword-only with default values
  • test_mixed_varargs_keyword_only: Mixed regular, *args, and keyword-only params
• Improved test assertions (Comments 2730944616, 2730944655, 2730944673, 2730944698):
  • Changed to capture previous_call_count before cache hit tests
  • Makes tests more explicit about verifying the count didn't increase
  • Applied to all applicable tests for consistency

Technical Details

Root Cause: The _convert_args_kwargs() function used zip(func_params, args) to map arguments to parameter names. For functions with *args, the parameter list only contains the parameter name (e.g., ['args']), not individual parameter names. This caused the zip to only map the first argument, losing the rest.

Solution: Modified _convert_args_kwargs() to:

1. Detect VAR_POSITIONAL (*args), VAR_KEYWORD (**kwargs), and KEYWORD_ONLY parameters
2. Only include POSITIONAL_ONLY and POSITIONAL_OR_KEYWORD in regular parameters
3. Expand variadic positional arguments into individual entries with synthetic keys (__varargs_0__, __varargs_1__, etc.)
4. Preserve variadic keyword arguments and keyword-only parameters with their original keys
5. Maintain backward compatibility with custom hash functions

Files Changed:

• src/cachier/core.py: Modified _convert_args_kwargs() function
• tests/test_varargs.py: Parametrized tests, added keyword-only parameter tests, and improved all assertions (now 10 tests total)

Test Results:

• ✅ 10 varargs tests pass (2 parametrized, 8 regular, 3 new for keyword-only)
• ✅ All pickle core tests pass (including custom hash function tests)
• ✅ All memory core tests pass
• ✅ Original issue scenario verified
• ✅ Type checking passes (mypy)
• ✅ Linting passes (ruff)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Cachier doesn't consider varadic arguments (*args) in the cache key</issue_title>
<issue_description>The following is using cachier version 3.0.1 with python version 3.12.4 (system rpm package) on linux (Fedora 40 x86_64)

I have a function which accepts a variable number of arguments via *args, and after applying the @cachier decorator all calls to the function regardless of the passed arguments return which ever result was first cached, eg

cachier.set_default_params(stale_after=timedelta(seconds=500))

@cachier.cachier()
def get_gam_dataframe(*args):
    output = run_gam(*args):
    return pandas.read_csv(StringIO(output))

domains = get_gam_dataframe('print', 'domains')
users   = get_gam_dataframe('print', 'users', 'allfields')

Will return the same result for users as for domains.
Setting the stale_after value to 0 seconds causes the expected data to be return (but obviously at the cost of not using the cache).
Additionally adding cachier__skip_cache=True to the calls throws a TypeError :

Traceback (most recent call last):
  File "........./script.py", line 65, in <module>
    'domains': get_gam_dataframe('print', 'domains', cachier__skip_cache=True),
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "......./python3.12/site-packages/cachier/core.py", line 251, in func_wrapper
    else func(**kwargs)
         ^^^^^^^^^^^^^^
TypeError: gam_get_dataframe() got an unexpected keyword argument 'args'

In this case I'm able to work around the issue by simply changing the call signature to avoid using a variable argument list for the cached function, but the behavior with *args seems to be unexpected/a bug.

@cachier.cachier()
def get_gam_dataframe(args):
    output = run_gam(*args):
    return pandas.read_csv(StringIO(output))

domains = get_gam_dataframe(['print', 'domains'])
users   = get_gam_dataframe(['print', 'users', 'allfields'])
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@Borda</author><body>
I think it is related to how we generate the unique key from arguments...</body></comment_new>
</comments>

• Fixes Cachier doesn't consider varadic arguments (*args) in the cache key #230

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