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4d3ed1f75b
Interactive notebook demonstrating Claude's memory tool and context editing capabilities with code review examples. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
115 lines
3.5 KiB
Python
115 lines
3.5 KiB
Python
"""
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Cache manager with mutable default argument bug.
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This is one of Python's most common gotchas.
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"""
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from datetime import datetime
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from typing import Dict, List, Optional
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class CacheManager:
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"""Manage cached data with TTL support."""
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def __init__(self):
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self.cache = {}
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def add_items(
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self, key: str, items: List[str] = [] # BUG: Mutable default argument!
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) -> None:
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"""
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Add items to cache.
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BUG: Using [] as default creates a SHARED list across all calls!
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This is one of Python's classic gotchas.
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"""
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# The items list is shared across ALL calls that don't provide items
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items.append(f"Added at {datetime.now()}")
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self.cache[key] = items
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def add_items_fixed(self, key: str, items: Optional[List[str]] = None) -> None:
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"""Add items with proper default handling."""
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if items is None:
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items = []
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items = items.copy() # Also make a copy to avoid mutation
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items.append(f"Added at {datetime.now()}")
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self.cache[key] = items
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def merge_configs(
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self, name: str, overrides: Dict[str, any] = {} # BUG: Mutable default!
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) -> Dict[str, any]:
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"""
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Merge configuration with overrides.
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BUG: The default dict is shared across all calls!
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"""
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defaults = {"timeout": 30, "retries": 3, "cache_enabled": True}
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# This modifies the SHARED overrides dict
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overrides.update(defaults)
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return overrides
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def merge_configs_fixed(
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self, name: str, overrides: Optional[Dict[str, any]] = None
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) -> Dict[str, any]:
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"""Merge configs properly."""
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if overrides is None:
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overrides = {}
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defaults = {"timeout": 30, "retries": 3, "cache_enabled": True}
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# Create new dict to avoid mutation
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result = {**defaults, **overrides}
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return result
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class DataProcessor:
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"""Another example of the mutable default bug."""
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def process_batch(
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self, data: List[int], filters: List[str] = [] # BUG: Mutable default!
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) -> List[int]:
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"""
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Process data with optional filters.
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BUG: filters list is shared across calls!
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"""
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filters.append("default_filter") # Modifies shared list!
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result = []
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for item in data:
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if "positive" in filters and item < 0:
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continue
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result.append(item * 2)
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return result
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if __name__ == "__main__":
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cache = CacheManager()
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# Demonstrate the bug
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print("=== Demonstrating Mutable Default Argument Bug ===\n")
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# First call with no items
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cache.add_items("key1")
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print(f"key1: {cache.cache['key1']}")
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# Second call with no items - SURPRISE! Gets the same list
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cache.add_items("key2")
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print(f"key2: {cache.cache['key2']}") # Will have TWO timestamps!
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# Third call - even worse!
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cache.add_items("key3")
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print(f"key3: {cache.cache['key3']}") # Will have THREE timestamps!
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print("\nAll keys share the same list object!")
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print(f"key1 is key2: {cache.cache['key1'] is cache.cache['key2']}")
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print("\n=== Using Fixed Version ===\n")
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cache2 = CacheManager()
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cache2.add_items_fixed("key1")
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cache2.add_items_fixed("key2")
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cache2.add_items_fixed("key3")
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print(f"key1: {cache2.cache['key1']}")
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print(f"key2: {cache2.cache['key2']}")
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print(f"key3: {cache2.cache['key3']}")
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print(f"\nkey1 is key2: {cache2.cache['key1'] is cache2.cache['key2']}") |