Python course / Working Python

Comprehensions

Write the list-building loop you already know as one expression, and recognise when not to. A comprehension stops being clever the moment a reader has to go through it twice.

list, dict, set
filtering
nested fors
lambda
key functions
generator expressions
any and all

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filter, transform, summarise

rows = [("tea", 350), ("jam", 225), ("oats", 180), ("honey", 400)]

cheap = [name for name, pence in rows if pence < 300]
prices = {name: pence / 100 for name, pence in rows}

print("cheap:", cheap)
print("tea in pounds:", prices["tea"])
print("total pence:", sum(pence for _, pence in rows))
print("any at four pounds or more:", any(pence >= 400 for _, pence in rows))

cheap: ['jam', 'oats']
tea in pounds: 3.5
total pence: 1155
any at four pounds or more: True

What you will learn

Write the list-building loop you already know as one expression.
Keep only the items you want, and know what runs first.
Recognise a comprehension that has stopped being clear, and turn it back into a loop.
Build a dictionary from another collection in one expression.
Hand values to a function one at a time, without building a list first.
Ask whether some or every item passes a test, and stop as soon as the answer is known.

The lessons in this module

From loop to comprehension
Filtering
Changing and choosing at once
When not to use one
Dict comprehensions
Set comprehensions
Two fors in one comprehension
Lists of lists
Choosing a value, or choosing an item
lambda
key functions
map and filter
Generator expressions
any and all
Where the loop variable goes
The walrus in a comprehension
The supplier's list
Module review: the market stall

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