python 40 lines · 8 steps

Five ways to merge dicts in Python

A tour of comprehension, in-place union, reduce, recursion, and ChainMap approaches to combining mappings.

Explained by highlit
Intermediate dictionaries merging recursion chainmap reduce
1from collections import ChainMap
2from functools import reduce
3from typing import Any, Iterable, Mapping
4 
5 
6def merge(*dicts: Mapping[str, Any]) -> dict[str, Any]:
7 return {k: v for d in dicts for k, v in d.items()}
8 
9 
10def merge_spread(*dicts: Mapping[str, Any]) -> dict[str, Any]:
11 result: dict[str, Any] = {}
12 for d in dicts:
13 result |= d
14 return result
15 
16 
17def merge_reduce(dicts: Iterable[Mapping[str, Any]]) -> dict[str, Any]:
18 return reduce(lambda acc, d: {**acc, **d}, dicts, {})
19 
20 
21def deep_merge(base: Mapping[str, Any], override: Mapping[str, Any]) -> dict[str, Any]:
22 merged = dict(base)
23 for key, value in override.items():
24 if (
25 key in merged
26 and isinstance(merged[key], Mapping)
27 and isinstance(value, Mapping)
28 ):
29 merged[key] = deep_merge(merged[key], value)
30 else:
31 merged[key] = value
32 return merged
33 
34 
35def first_wins_view(*dicts: Mapping[str, Any]) -> ChainMap[str, Any]:
36 return ChainMap(*dicts)
37 
38 
39def last_wins_view(*dicts: Mapping[str, Any]) -> ChainMap[str, Any]:
40 return ChainMap(*reversed(dicts))
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Later dicts win by default in most merges, since their keys overwrite earlier ones.
  2. 2Deep merging requires recursion to combine nested mappings instead of replacing them wholesale.
  3. 3ChainMap gives a lazy layered view without copying, and reversing the layers flips precedence.

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