Python Tip: List Comprehensions

Build a complicated list in a single line

Learn how to build lists with comprehensions, how to filter them, and when a regular for loop is the better choice.

Python Tip: List Comprehensions

Want to build lists in a cleaner way?

List comprehension allows us to build a complicated list simply, often in a single line.

It is one of the clearest markers of “Pythonic” code.

Building a List the Long Way

Imagine we have a list of item prices and we want a second list with 8% sales tax added to each price.

The usual approach is to create an empty list, loop over the prices, and append each new value.

prices = [12.99, 4.50, 29.00, 7.25]

with_tax = []
for price in prices:
    with_tax.append(round(price * 1.08, 2))

print(with_tax)

[14.03, 4.86, 31.32, 7.83]

This works, but there is a cleaner way to do it…

The Same List in One Line

Python gives us a shorter way to make lists, called a “list comprehension”. A list comprehension is a single expression, written inside square brackets, that builds an entire list at once.

Here is the same calculation on the same prices.

with_tax = [round(price * 1.08, 2) for price in prices]

print(with_tax)

[14.03, 4.86, 31.32, 7.83]

Writing a list comprehension is like writing a for loop backwards. We start with the logic we want to run, round(price * 1.08, 2), and then we say what we are looping over, for price in prices. The square brackets around both parts collect every result into a new list.

Keeping Only Some Items

A comprehension can also skip items. If we add if at the end, Python keeps only the prices for which the condition is True.

Let’s say we want the prices over $10.

expensive = [price for price in prices if price > 10]

print(expensive)

[12.99, 29.0]

The logic on the left is just price, so each kept item goes into the new list unchanged. A comprehension can filter a list without changing any values.

Looping Over Two Levels

Data often arrives nested. Suppose our prices are grouped by order, so orders is a list of lists, and we want one flat list containing every price.

A comprehension can use two for clauses to do this.

orders = [[12.99, 4.50], [29.00], [7.25, 3.75]]

all_prices = [price for order in orders for price in order]

print(all_prices)

[12.99, 4.5, 29.0, 7.25, 3.75]

The two for clauses go in the same order we would write two nested loops: for order in orders on the outside, then for price in order on the inside. The second clause is allowed to use order because the first clause has already defined it.

When Not to Use a Comprehension

The convention is to only write a comprehension when we actually want the list it builds.

If we simply want to run something for each item, such as printing it or writing it to a file, a regular for loop is easier to read.

# We want the list, so a comprehension fits
with_tax = [price * 1.08 for price in prices]

# We only want to print, so a loop is clearer
for price in prices:
    print(price)

Wrap-Up

If you are building a new list from an existing collection, use a comprehension.

If you are doing something for each item, use a for loop.

Write one backwards: the logic first, then the loop that feeds it.

Follow me for more tips.
- Shep4