ChainMap

Introduction

ChainMap groups multiple dictionaries into a single view, searching them in order.

Basic ChainMap

from collections import ChainMap

# Create chain from dicts
defaults = {"theme": "light", "language": "en"}
user_prefs = {"language": "fr", "notifications": True}

combined = ChainMap(user_prefs, defaults)

print(combined["theme"])       # light (from defaults)
print(combined["language"])    # fr (from user_prefs)
print(combined["notifications"]) # True

Modifications

combined = ChainMap(user_prefs, defaults)

# Changes affect first map only
combined["theme"] = "dark"
print(user_prefs)  # {'language': 'fr', 'theme': 'dark'}

# Add to first map
combined["new_key"] = "value"

Use Cases

from collections import ChainMap

# Command-line arguments with defaults
defaults = {"debug": False, "port": 8000}
args = {"port": 3000}
config = ChainMap(args, defaults)

# Variable scoping
local_vars = {"x": 1}
global_vars = {"y": 2}
all_vars = ChainMap(local_vars, global_vars)

Practice Problems

1. Create search path with ChainMap
2. Implement priority config loading
3. Merge multiple dictionaries with priority
4. Update ChainMap efficiently
5. Use ChainMap for namespace lookup