How do I merge two dictionaries in Python?

The short version

On Python 3.9 and later, combine two dictionaries with the | operator. On older versions, use {**a, **b}. In both cases, when the same key appears in both, the value from the second (right-hand) dictionary wins, which is the behaviour to remember.

The modern way

a = {"x": 1, "y": 2}
b = {"y": 3, "z": 4}

merged = a | b
print(merged)   # {'x': 1, 'y': 3, 'z': 4}

Note y is 3, not 2: the right-hand dictionary overrode the left. This is a new merged dictionary; a and b are unchanged.

The way that works everywhere

Before Python 3.9, use double-star unpacking, which does the same thing.

merged = {**a, **b}

Merging in place

If you want to update an existing dictionary rather than create a new one, use update or the |= operator.

a.update(b)   # a is now modified
a |= b        # same effect on 3.9+

The difference matters: a | b leaves a alone, while a.update(b) changes a. Choose based on whether you want to keep the original.

The collision rule is the whole point

The one thing to internalise is which value survives a clash. In every form above, later wins. So a | b keeps b's values on overlap, and b | a keeps a's. If you actually want the first dictionary to take priority, simply swap the order.

b | a   # {'y': 2, 'z': 4, 'x': 1}  -> a's y wins now

Merging more than two

The operators chain, so you can fold several together, again with later winning.

c = {"w": 9}
a | b | c