Merging dicts with the union operator

Merging two python dictionaries is a very common question on StackOverflow. Some of the better responses have been updated for later versions of Python, but I’ve seen this crop up a few times now to write it up here.

In Python 3.9+, merging two dictionaries is now staggeringly easy.

# New in Python 3.9

x = {"hello": "there", "general": "kenobi"}
y = {"general": "potato", "a": "b"}
z = x | y

# z = {'hello': 'there', 'general': 'potato', 'a': 'b'}

Notice that the order of preference is right to left. So values from y will override values from x. Some may think this is a bit unintuitive - the | operator is tradionally used to represent bitwise or, and generally when we evaluate or expressions, we stop evaluation as soon as we get a True. Which, in our example, would mean to stop if the value is found in x.

Do not think of the union operator as an or operator.

Start with x, and put y over the top.

Prior Solutions

For completeness, if you don’t have Python 3.9 yet (maybe because you’re waiting for tensorflow support and docker images… please come soon), here are the other various ways of going about merging dictionaries.

# Python 3.5+
z = {**x, **y}

# Python <3.5
z = x.copy()
z.update(y)

In terms of what NOT to do, never utilise the dict constructor when either merging or copying a dictionary.

# Do not do this
z = dict(x, **y)

Here I have used dict(x) to essentially copy the dictionary, and added in the extra keys from y.

This works great, until you have anything but a string for a dictionary key.

w = {"mixing": "types", 5: "danger"}
z = dict(x, **w)

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-7-7ebfe83d5cb9> in <module>
      1 w = {"mixing": "types", 5: "danger"}
----> 2 z = dict(x, **w)


TypeError: keywords must be strings

Remember, dictionaries aren’t tuples of keyword arguments, they are tuples of keys and values. The key’s can be anything so long as it can be hashed.

For your convenience, here’s the code in one block:

# New in Python 3.9

x = {"hello": "there", "general": "kenobi"}
y = {"general": "potato", "a": "b"}
z = x | y

# z = {'hello': 'there', 'general': 'potato', 'a': 'b'}
# Python 3.5+
z = {**x, **y}

# Python <3.5
z = x.copy()
z.update(y)
# Do not do this
z = dict(x, **y)
w = {"mixing": "types", 5: "danger"}
z = dict(x, **w)