Comprehensions

In Python it's very common to build up new lists while looping over old lists. Partly this is because we don't mutate lists very often while looping over them.

There are three reasons we don't mutate lists often:

• Python's functions use a call-by-assignment style which means mutating lists passed to us might upset the caller of our function
• Adding things to lists or removing things from lists can cause issues while looping
• Python makes it difficult to mutate lists (we have to go out of our way to get the index of the current item while looping)

Because we build up new lists from old ones so often, Python has a special syntax to help us with this very common operation: list comprehensions.

See below for resources on list comprehensions set comprehensions, dictionary comprehensions, and [generator expressions.

Screencasts on comprehensions

• What are list comprehensions?
• Turning a for loop into a comprehension
• Why use list comprehensions?
• Set and dictionary comprehensions
• When should you not use list comprehensions?
• How to write a generator expression

Recommended Resources

The first resource, Comprehensible Comprehensions is the one I'd most recommend if you're up for 30 minutes of video. If you need something text-based or shorter, you might want to check out the second resource, my article on comprehensions.

• New to comprehensions and have 30 minutes? Watch my talk Comprehensible Comprehensions
• Don't have 30 minutes? Read my article Python List Comprehensions: Explained Visually
• Wondering when not to use comprehensions? See my article Overusing list comprehensions
• Have 3 hours to really dive into these? Work through my PyCon tutorial, List Comprehensions and Generator Expressions