Source: 📖 Effective Python item 29
In order to needless repetition, assignment expressions can be used within comprehensions. They can be used in any portion of the comprehension, but it's best to only use them within the conditional portion as using them anywhere else will result in the created variable to leak into the containing scope which can lead to obscure bugs.
original = [1, 2, 3, 4, 5, 6, 7, 8, 9]
a = [square for x in original if (square := x**2) > 16]
print(a)
>>>
[25, 36, 49, 64, 81]
The above example uses an assignment expression to create the square variable, eliminating the need to run the **2 operation twice:
original = [1, 2, 3, 4, 5, 6, 7, 8, 9]
a = [x**2 for x in original if x**2 > 16]
print(a)
>>>
[25, 36, 49, 64, 81]
This is not the greatest example, but in more complex comprehensions and expressions this can reduce the length of your code and improve readability.