Always use None as the default value for optional parameters and class attributes instead of empty collections ([], {}, "") or other mutable defaults. This prevents sharing mutable state between instances and makes null checks more explicit.

Example:

# ❌ Bad - Using mutable defaults
class ToolAdapter:
    original_tools: List[BaseTool] = []  # Shared between instances
    def __init__(self, items: List[str] = []):  # Mutable default
        self.items = items

# ✅ Good - Using None as default
class ToolAdapter:
    def __init__(self):
        self.original_tools: List[BaseTool] = []  # Instance-specific
    
    def process(self, items: Optional[List[str]] = None):
        items = items or []  # Convert None to empty list
        # Process items...

This approach:

Prevents shared state bugs from mutable defaults
Makes optional parameters explicit
Forces conscious initialization of collections
Enables clear null checking patterns

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