How to merge two dicts in Python
Python 3.9+: use the | operator. {**a, **b} still works for older versions. Right-hand side wins on key conflicts.
a = {"x": 1, "y": 2}
b = {"y": 20, "z": 30}
merged = a | b
print(merged) # {'x': 1, 'y': 20, 'z': 30}The | operator (Python 3.9+) is the modern default: a | b returns a new dict combining both, with b's values winning on any key both share. It reads left-to-right like set union.
Before 3.9, {**a, **b} is the equivalent one-liner using dict unpacking. Both produce the SAME result and both create a NEW dict — neither mutates a or b.
To mutate a in place, use a.update(b) or a |= b (3.9+). Both add b's keys and overwrite conflicts, returning None.
Variations
Merge in place
a = {"x": 1}
a |= {"y": 2}
# or: a.update({"y": 2})
print(a) # {'x': 1, 'y': 2}Mutates a. |= is 3.9+; .update() works everywhere.
Merge with dict unpacking (pre-3.9)
merged = {**a, **b}Same result as a | b, works on Python 3.5+.
Merge without overwriting
merged = {**b, **a} # a wins nowReverse the order so the earlier one takes precedence.
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