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:

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

Examples

x = [i for i in range(10)]
print x

# This will give the output:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

For the next example, assume we want to create a list of squares.

# You can either use loops:
squares = []

for x in range(10):
    squares.append(x**2)
 
print squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# Or you can use list comprehensions to get the same result:
squares = [x**2 for x in range(10)]

# print squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]