Fix DrsSymbol and DrsIndexed pickle compatibility with SymPy v1.9+

[Copilot]

Problem

The test_pickle_within_drs test was failing with SymPy v1.9+ due to pickle incompatibility:

TypeError: DrsSymbol.__new__() missing 1 required positional argument: 'name'

This error occurred when trying to unpickle DrsSymbol and DrsIndexed objects within drudge scripts.

Root Cause

SymPy v1.9+ introduced the __getnewargs_ex__() method to its Symbol class, which takes precedence over the existing __getnewargs__() method during pickle deserialization. The issue manifested in two ways:

1. Argument Mismatch: SymPy's __getnewargs_ex__() returns (('symbol_name',), {}) providing only one argument, but DrsSymbol.__new__(cls, drudge, name) expects two arguments.

2. Missing Initialization: When __getnewargs_ex__() provides sufficient arguments for __new__, Python skips calling __setstate__(), leaving the _orig attribute uninitialized and causing AttributeError: 'DrsIndexed' object has no attribute '_orig'.

Solution

This PR implements a two-part fix:

1. Override __getnewargs_ex__() Methods

Added __getnewargs_ex__() methods to both DrsSymbol and DrsIndexed classes to provide the correct arguments for their respective __new__() methods:

def __getnewargs_ex__(self):
    """Get the arguments and keyword arguments for __new__.
    
    This method takes precedence over __getnewargs__ in Python 3.6+
    and is used by SymPy v1.9+.
    """
    return (None, self.name), {}  # For DrsSymbol
    # or 
    return ((None, self.base) + self.indices), {}  # For DrsIndexed

2. Ensure __setstate__() is Called

Modified __getstate__() methods to return {} instead of None, ensuring that __setstate__() is always called during unpickling to properly initialize the _orig attribute and other instance variables.

Testing

• ✅ The originally failing test_pickle_within_drs now passes
• ✅ All existing pickle-related tests continue to pass
• ✅ No regressions detected in the full test suite
• ✅ Maintains backward compatibility while ensuring forward compatibility with SymPy v1.9+

Files Changed

• drudge/drs.py: Added __getnewargs_ex__() methods and modified __getstate__() methods for both DrsSymbol and DrsIndexed classes

This fix follows Python's standard pickle protocol and should work reliably across different Python and SymPy versions.

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[coveralls]

Pull Request Test Coverage Report for Build 17519284021

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 87.996%

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Totals
Change from base Build 17518696745:	0.2%
Covered Lines:	3790
Relevant Lines:	4307

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💛 - Coveralls