Dictionary Merging and Updating in Python 3.9

A quick peek into Python 3.9’s new merge | and update |= operators

Yong Cui, Ph.D.
Mar 6 · 5 min read
Photo by Pisit Heng on Unsplash

Python 3.9 is under active development and scheduled to be released in October this year. On Feb 26, the development team released its alpha 4 version. One of the new features, relevant to almost all Python programmers, is the introducing of new merge (|) and update (|=) operators.

Let’s take a quick peek what this is in this article.


Dictionaries

Dictionaries, commonly known as dict, are one of the most important built-in data types in Python . This data type is a flexibly-sized collection of key-value pairs, and it’s best known for having a constant time for data lookup because of its hashing implementation.

Here are some common usages:

# Declare a dict
student = {'name': 'John', 'age': 14}
# Get a value
age = student['age']
# age is 14
# Update a value
student['age'] = 15
# student becomes {'name': 'John', 'age': 15}
# Insert a key-value pair
student['score'] = 'A'
# student becomes {'name': 'John', 'age': 15, 'score': 'A'}

Merging Dictionaries — Old Ways

Sometimes, we need to merge two dictionaries for further processing. Prior to the official release of 3.9, there are several ways we can do this. Suppose we have two dicts: d1 and d2. We want to create a new dict called d3, a union of d1 and d2. To illustrate what we shall do if there are some overlapping keys between the merging dicts, we have another dict d2a, which has an overlapping key with d1.

# two dicts to start with
d1 = {'a': 1, 'b': 2}
d2 = {'c': 3, 'd': 4}
d2a = {'a': 10, 'c': 3, 'd': 4}
# target dict
d3 = {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Using the update() method

The first way is to use the dict’s method update(). The following code snippet shows you how to do it. Note that we’ll have to create a copy of d1 first, as the update() function will modify the original dict.

# create a copy of d1, as update() modifies the dict in-place
d3 = d1.copy()
# d3 is {'a': 1, 'b': 2}
# update the d3 with d2
d3.update(d2)
# d3 now is {'a': 1, 'b': 2, 'c': 3, 'd': 4}

When there are overlapping keys, we have to be more cautious regarding which values are to be kept. As you can see below, the dict that is passed as the argument in the update() method will “win” the game by having the value (i.e., 10) for the overlapping key (i.e., ‘a’).

d3 = d1.copy()
d3.update(d2a)
# d3 now is {'a': 10, 'b': 2, 'c': 3, 'd': 4}
# This is not the way that we want
d3 = d2a.copy()
d3.update(d1)
# d3 now is {'a': 1, 'c': 3, 'd': 4, 'b': 2}
# This is the way that we want

Unpacking dictionaries

The second way is to use the unpacking of the dictionaries. Similarly as the above method, “last seen wins” when there are overlapping keys.

# unpacking
d3 = {**d1, **d2}
# d3 is {'a': 10, 'b': 2, 'c': 3, 'd': 4}
# Not right
d3 = {**d2a, **d1}
# d3 is {'a': 1, 'c': 3, 'd': 4, 'b': 2}
# Good

Using dict(iterable, **kwarg)

One way to create a dict in Python is using the dict(iterable, **kwarg) method. Specifically relevant to the current topic is that when the iterable is a dict, a new dict will be created with the same key-value pairs. For the keyword arguments, we can pass another dict such that it will add the key-value pairs to the dict that is going to create. Note that this keyword argument dict will replace the value with the same key — something like “last seen wins.” Please see below for an example.

d3 = dict(d1, **d2)
# d3 is {'a': 1, 'b': 2, 'c': 3, 'd': 4}
# Good, it's what we want
d3 = dict(d1, **d2a)
# d3 is {'a': 10, 'b': 2, 'c': 3, 'd': 4}
# Not right, 'a' value got replaced

One thing to note is that this method only works when the keyword argument dict has strings as their keys. Like below, a dict using an int as its key won’t work.

>>> dict({'a': 1}, **{2: 3})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: keywords must be strings
>>> dict({'a': 1}, **{'2': 3})
{'a': 1, '2': 3}

Merging Dictionaries — New Feature

In the latest release of Python 3.9.0a4, we can use the merging operator | to merge two dicts very conveniently. An example is given below. You may have noticed that when there are overlapping keys between these two dicts, the last seen wins, and this behavior is consistent with what we’ve seen above, such as the update() method.

# use the merging operator |
d3 = d1 | d2
# d3 is now {'a': 1, 'b': 2, 'c': 3, 'd': 4}
# good
d3 = d1 | d2a
# d3 is now {'a': 10, 'b': 2, 'c': 3, 'd': 4}
# not good

Related to this merge operator is the augmented assignment version, which operates in-place (i.e., update the left side dict). Essentially, it functions the same way as the update() method. The following code snippet shows you its usage:

# Create a copy for d1
d3 = d1.copy()
# Use the augmented assignment of the merge operator
d3 |= d2
# d3 now is {'a': 1, 'b': 2, 'c': 3, 'd': 4}
# good
d3 |= d2a
# d3 now is {'a': 10, 'b': 2, 'c': 3, 'd': 4}
# not good

Closing Thoughts

This article reviewed the new feature of merging and updating dictionaries in Python 3.9. There are several other new updates/improvements in several modules, such as asyncio, math, and os modules. See the official website for more information. I can’t wait to check them out in its official release in October!

Are you ready for Python 3.9? Stay tuned for a review of its new features when it is officially released!

Better Programming

Advice for programmers.

Thanks to Zack Shapiro

Yong Cui, Ph.D.

Written by

Work at the nexus of biomedicine, data science & mobile dev. Love to write on these technological topics. Follow me @ycui01 on Twitter to get latest articles.

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