5. Assignment Expressions
By Bernd Klein. Last modified: 30 Mar 2023.
Introduction
Python version 3.8 came with a new feauture, which some programmers of Python longed for for quite a while. A new way to assign objects to variables has been introduced, i.e. the := operator. It gives programmers a convenient way to assign variables in the middle of expressions. If you look at the characters with a little imagination you can see a similarity with the eyes and tusks of a walrus, so this is why affectionately known as the walrus operator.
The assignment expressions have been discussed in PEP 572 and this is what was written about the naming:
During discussion of this PEP, the operator became informally known as "the walrus operator". The construct's formal name is "Assignment Expressions" (as per the PEP title), but they may also be referred to as "Named Expressions" (e.g. the CPython reference implementation uses that name internally).
We will introduce this way of assignment in this part of our Python tutorial.
A simple assignment can also be replaced by an assignment expression, even though it looks clumsy and is definitely not the intended use case of it:
x = 5
# can be written as:
(x := 5) # valid, but not recomended!
# the brackets are crucial
OUTPUT:
5
Let's look at a little Code example which only uses traditional assignments:
txt = 'Python needs training!'
ideal_length = 22
n = len(txt)
if n == ideal_length:
print(f'The length {n} is ideal!')
else:
print(f'{n} is not ideal!')
OUTPUT:
The length 22 is ideal!
We will use the new walrus operator in the following Python code snippet:
txt = 'Python needs training!'
ideal_length = 22
if (n := len(txt)) == ideal_length:
print(f'The length {n} is ideal!')
else:
print(f'The length {n} is not ideal!')
OUTPUT:
The length 22 is ideal!
Okay, you may say this is not very impressive and the first version might be considered as even more readable. So, let us have a look at a more useful usecase.
Live Python training
Enjoying this page? We offer live Python training courses covering the content of this site.
Beneficial applications of the Assignment Expressions
List Comprehension
In the following you will see a list comprehension with a walrus operator:
def f(x):
return x + 4
numbers = [3, 7, 2, 9, 12]
odd_numbers = [result for x in numbers if (result := f(x)) % 2]
odd_numbers
OUTPUT:
[7, 11, 13]
The above implementation is more efficient than a list comprehension without the assignment expression, because we will have to call the function twice:
def f(x):
return x + 4
numbers = [3, 7, 2, 9, 12]
odd_numbers = [f(x) for x in numbers if f(x) % 2]
odd_numbers
OUTPUT:
[7, 11, 13]
Regular Expressions
There is also a big advantage when we use regular expressions:
import re
txt = """The Python training course started at 2022-02-4
the other one at 2022-01-24
only one date per line, if at all
the dates may also be in this format 2020/10/15
or 20-10-04"""
for line in txt.split('\n'):
if (date := re.search(r'(\d{2,4})[-/](\d{2})[-/](\d{2})', line)):
year, month, day = date.groups()
print(year, month, day)
OUTPUT:
2022 01 24 2020 10 15 20 10 04
Reading Fixed Length Files
with open('training_course_info.txt') as fh:
while ((data := fh.read(52)) != ''):
print(data.rstrip())
OUTPUT:
Python Training Course for Beginners Berlin 08 Python Intermediate Course Hamburg 06 Python Advanced Trining Course Frankfurt 08
Usage in while Loops
In the chapter on while loops of our Python Tutorial, we had a little number guessing game:
import random
lower_bound, upper_bound = 1, 20
to_be_guessed = random.randint(lower_bound, upper_bound)
guess = 0
while guess != to_be_guessed:
guess = int(input("New number: "))
if guess > to_be_guessed:
print("Number too large")
elif guess < to_be_guessed:
print("Number too small")
else:
print("Congratulations. You made it!")
As you can see, we had to initialize guess to zero to be able to enter the loop. We can do the initialization directly in the loop condition with an assignment expression and simplify the whole code by this:
import random
lower_bound, upper_bound = 1, 20
to_be_guessed = random.randint(lower_bound, upper_bound)
while (guess := int(input("New number: "))) != to_be_guessed:
if guess > to_be_guessed:
print("Number too large")
elif guess < to_be_guessed:
print("Number too small")
else:
print("Congratulations. You made it!")
OUTPUT:
Number too small Number too large Number too small Congratulations. You made it!
Taken to the extreme:
We said in the beginning of this page that some Python programmers longed for this consstruct for quite a while. One reason why it was not introduced earlier was the fact that it can also be used to write code which is less readable if used to extensivel. The following code snippet is showing such an extreme example which is not recommended to use:
a, b, c = 1, 2, 3
x = 4
y = (c := (a := x*2.3) + (b := x*4.5 -3))
y
OUTPUT:
24.2
c
OUTPUT:
24.2
a
OUTPUT:
9.2
Live Python training
Enjoying this page? We offer live Python training courses covering the content of this site.
Upcoming online Courses