• Fixed get_default_valid_fields() and get_valid_fields() methods in filters.py
• Changed context={} default parameter to context=None to prevent mutable default anti-pattern
• Added proper None checking with context = {} assignment inside methods

Why this fix is important:

• Mutable default arguments (context={}) create shared state across function calls
• Same dict object gets reused, potentially causing unexpected side effects
• This is a well-known Python anti-pattern that can lead to bugs

What was changed:

• Line 249: get_default_valid_fields(self, queryset, view, context=None)
• Line 285: get_valid_fields(self, queryset, view, context=None)
• Added 'if context is None: context = {}' in both methods

Testing results:

• All existing filter tests pass (pytest tests/test_filters.py)
• Custom verification script confirms fix works correctly
• Maintains backward compatibility
• No breaking changes to API

Addresses GitHub issue#9741

#9741

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Description

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