---
title: "chain map"
url: https://perrotta.dev/2026/01/chain-map/
last_updated: 2026-01-10
---


```python
from collections import ChainMap

a = {1: 2}
b = {1: 3}
c = {2: 4}

# a has more priority than b, which has more priority than c
d = ChainMap(a, b, c)

assert len(d) == 2
assert d[1] == 2
assert d[2] == 4
d[1] = 3
assert d[1] == 3
assert a[1] == 3
d[2] = 10
assert d[2] == 10
assert a[2] == 10
assert c[2] == 4

del d[2]
assert 2 not in a

e = d.new_child({1: 10})
assert e[1] == 10
```