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Merge Dictionaries

Zero-dependency Python snippets for merging multiple dictionaries using the standard library.

7 snippets available in this sub-category.

Simple

Merge with update()

dict merge update inplace data-structures

Merge two dicts in-place (right overwrites left)

d1 = {"a": 1, "b": 2}
d2 = {"b": 3, "c": 4}
d1.update(d2)
print(d1)  # {'a': 1, 'b': 3, 'c': 4}

Notes

  • update() modifies the first dict
  • Overwrites keys present in both

Merge with dict unpacking (Python 3.5+)

dict merge unpacking new data-structures

Merge two or more dicts into a new dict

d1 = {"a": 1, "b": 2}
d2 = {"b": 3, "c": 4}
merged = {**d1, **d2}
print(merged)  # {'a': 1, 'b': 3, 'c': 4}

Notes

  • Later dicts overwrite earlier ones
  • Works for any number of dicts: {**d1, **d2, **d3, ...}

Merge with | operator (Python 3.9+)

dict merge or operator data-structures

Merge dicts with | (returns new dict)

d1 = {"a": 1, "b": 2}
d2 = {"b": 3, "c": 4}
merged = d1 | d2
print(merged)  # {'a': 1, 'b': 3, 'c': 4}

Notes

  • d1 | d2 returns a new dict
  • d1 |= d2 updates in-place

Complex

Merge multiple dicts (reduce)

dict merge multiple reduce data-structures

Merge a list of dicts (rightmost wins)

from functools import reduce

dicts = [{"a": 1}, {"b": 2}, {"a": 3}]
merged = reduce(lambda a, b: {**a, **b}, dicts)
print(merged)  # {'a': 3, 'b': 2}

Notes

  • Useful for merging arbitrary number of dicts

Deep merge (recursive)

dict merge deep recursive data-structures

Recursively merge nested dictionaries

def deep_merge(d1, d2):
    result = d1.copy()
    for k, v in d2.items():
        if k in result and isinstance(result[k], dict) and isinstance(v, dict):
            result[k] = deep_merge(result[k], v)
        else:
            result[k] = v
    return result


d1 = {"a": 1, "b": {"x": 10}}
d2 = {"b": {"y": 20}, "c": 3}
print(deep_merge(d1, d2))  # {'a': 1, 'b': {'x': 10, 'y': 20}, 'c': 3}

Notes

  • Only merges nested dicts, not lists/sets

Conflict resolution (custom merge)

dict merge conflict custom data-structures

Merge dicts with custom conflict resolution

def merge_with_conflict(d1, d2, conflict):
    result = d1.copy()
    for k, v in d2.items():
        if k in result:
            result[k] = conflict(result[k], v)
        else:
            result[k] = v
    return result


d1 = {"a": 1, "b": 2}
d2 = {"b": 3, "c": 4}
merged = merge_with_conflict(d1, d2, lambda x, y: x + y)
print(merged)  # {'a': 1, 'b': 5, 'c': 4}

Notes

  • conflict is a function: (old, new) -> result

Edge cases: empty dicts, overlapping keys

dict merge edge-case data-structures

Handle edge cases for merging

print({**{}, **{}})  # {}
print({**{"a": 1}, **{}})  # {'a': 1}
print({**{}, **{"a": 1}})  # {'a': 1}

Notes

  • Merging with empty dict returns the other dict

🔗 Cross Reference

🏷️ Tags

dict, merge, update, unpacking, or, deep, conflict, edge-case, data-structures

📝 Notes

  • Use update(), unpacking, or | for shallow merges
  • Use recursion for deep merges