List Comprehensions in Python

List comprehensions provide a concise way to create lists.

It consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The expressions can be anything, meaning you can put in all kinds of objects in lists.

The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it.

The list comprehension always returns a result list.

If you used to do it like this:

new_list = []
for i in old_list:
    if filter(i):
        new_list.append(expressions(i))
You can obtain the same thing using list comprehension. Notice the append method has vanished!

new_list = [expression(i) for i in old_list if filter(i)]

Syntax

The list comprehension starts with a ‘[‘ and ‘]’, square brackets, to help you remember that the result is going to be a list.

The basic syntax uses square brackets.

[ expression for item in list if conditional ]

This is equivalent to:

for item in list:
    if conditional:
        expression

new_list = [expression(i) for i in old_list if filter(i)]

new_list The new list (result).

expression(i) Expression is based on the variable used for each element in the old list.

for i in old_list The word for followed by the variable name to use, followed by the word in the old list.

if filter(i)

Apply a filter with an If-statement.

new_range = [i * i for i in range(5) if i % 2 == 0]

Which corresponds to:

*result* = [*transform* *iteration* *filter* ]

The * operator is used to repeat. The filter part answers the question if the item should be transformed.