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Comprehensions are Common Python

What has your experience of comprehensions been? Do you agree with this post? Do you disagree? Please comment below.


Comprehensions are well covered elsewhere so we'll only briefly introduce them below before making the specific points relevant to the When of Python. In summary, it will be argued that comprehensions are such an important part of Python we should treat them as Common Python but that we should only use this compressed syntax when it is relatively easy to understand.


List Comprehensions

List comprehensions are a good starting point to explain the syntax and value of Python comprehensions. A common need in programming is to take a collection of items and return a modified or filtered collection. For example, maybe we want the square of every number in a list: [1, 2, 3] => [1, 4, 9].

We could create this new list using a for loop:

numbers = [1, 2, 3]
squares = []
for num in numbers:
    squares.append(num ** 2)

Or using map we could write:

def square(x):
    return x ** 2

numbers = [1, 2, 3]
squares = map(square, numbers)

In pseudocode what we want is collect into a list the square of every item in the numbers list. In Python we can write code where the syntax is very similar to the pseudocode using a comprehension:

## squares is a list of the square of every item in the numbers list
squares = [num ** 2 for num in numbers]

And if we want to only include numbers < 3 the syntax is still very similar to a simple pseudo code version of what is desired.

## squares is a list of the square of every item in the numbers list
## if the number < 3
squares = [num ** 2 for num in numbers if num < 3]

See also How to write a Python list comprehension in 3 simple steps for a useful animation.

Other Comprehensions

Python 3 introduced other comprehensions including dictionary and set comprehensions. For example:

names = [('Charles', 'George'), ('Camilla', 'Shand'), ('Charles', 'Dickens')]
unique_fnames = {fname for fname, _lname in names}

Comprehensions are Common Python

Comprehensions are Common Python (see Welcome to the When of Python). This point was actually controversial in a Python mailing list debate in 2018. The objection to treating comprehensions as Common Python was that developers working with Python often come from other languages which don't have comprehension constructs. It was suggested that little would be lost avoiding their usage but the benefit would be that Rust developers, for example, could work on Python code without having to learn an unfamiliar concept.

Against this position I think comprehensions are an elegant, readable way of expressing simple tasks into relatively simple code (once the approach has been learned). For a defence of comprehensions I refer to Raymond Hettinger's classic talk on Transforming Code into Beautiful, Idiomatic Python. Hettinger argues that we shouldn't break atoms of thought into subatomic particles. He advocates instead for concise, expressive one-liners where one logical line of code equals one sentence in English.

Another advocate of comprehensions1 is Rodrigo Girão Serrão What learning APL taught me about Python. He suggests the reason comprehensions are so good is their readability and that the reason they're so readable is that they highlight the most important part of the code, the data transformation. So instead of being hidden away at the bottom-right of the code, like it is in a loop, the transformation is prominently displayed top-left.

Comprehensions are a beautiful part of Python and we shouldn't cripple the language to keep it compatible with as many other languages as possible.

Certainly, comprehensions are very widespread in Python code, so we should include them in Common Python - that is, those features we should all know so we can basically read each other's work.

Note - just because we consider comprehensions Common Python and not Situational Python doesn't mean we shouldn't be thoughtful about when to use them and when not to.


Being one-liners (even if stretched over several lines) comprehensions can attract the same "cleverness" which plagues a lot of code. The desire to fit complexity into a single line can overwhelm concerns with readability, maintainability, and avoiding bugs. As a general rule of thumb, if a comprehension stops being easy to read, use an alternative approach instead e.g. a for loop or map. Note - Walrus operators in comprehensions should generally be viewed as "tricky" and not worth the cost.

  1. By his own admission, Rodrigo is partially obsessed with comprehensions and considers them "insanely useful" - he has even written a book with over 200 exercises on the topic!