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Default None not empty

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.

raw .md Null Handling Python

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