Скачать или смотреть Can I Use final value in If Part of List Comprehension?

Explore how Python's list comprehensions work and learn how to maximize efficiency using assignment expressions for cleaner code.
---
This video is based on the question https://stackoverflow.com/q/64898556/ asked by the user 'Robo Robok' ( https://stackoverflow.com/u/4403732/ ) and on the answer https://stackoverflow.com/a/64898622/ provided by the user 'chepner' ( https://stackoverflow.com/u/1126841/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Can I use final value in if part of list comprehension?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding List Comprehensions in Python

In Python programming, list comprehensions offer a concise way to create lists. However, they can sometimes lead to redundancy, especially when dealing with complex calculations within filters. A common question among Python developers is whether it’s possible to compute a value and use that computed value in the if part of a list comprehension.

The Problem Statement

Consider a scenario where you have a list of numbers from 1 to 10 and you want to find the squares of these numbers that are less than 1000. You want to do this efficiently, without repeating the square calculation. Here’s an example of how this could be approached:

[[See Video to Reveal this Text or Code Snippet]]

While this works, it requires calculating n ** 2 twice: once for the list and again for the filter condition. This redundancy can be inefficient, especially if the computation is complex or if the list is significantly large.

Exploring Alternative Solutions

One common approach to avoid redundancy is using the filter function. For example:

[[See Video to Reveal this Text or Code Snippet]]

This method definitely avoids the redundancy of calculations within a list comprehension. However, if you want to stick to a single list comprehension, there’s an even better solution introduced in Python 3.8.

Leveraging the Assignment Expression

Python 3.8 introduced the assignment expression operator :=, commonly known as the "walrus operator". This allows you to assign a value to a variable and return that value simultaneously. Here’s how you can use it in a list comprehension to avoid double calculations:

[[See Video to Reveal this Text or Code Snippet]]

How It Works

Single Calculation: The expression (n2 := n ** 2) computes the square of n once and assigns it to the variable n2.

Conditional Check: The value of n2 is then used directly in the condition of the if statement to ensure it’s less than 1000.

Clear and Efficient: This not only makes the code cleaner but also optimizes performance by eliminating unnecessary calculations.

Conclusion

Python's list comprehensions are powerful tools for building lists efficiently. When faced with the need to perform the same computation multiple times, consider using the assignment expression introduced in Python 3.8. It allows you to both calculate and utilize the value seamlessly within a single comprehension, significantly improving your code's readability and performance.

By leveraging these techniques, you can write more efficient and elegant Python code. Happy coding!