Comprehensions

Python offers you different types of comprehensions: list, dict, and set.

We'll concentrate on the first one for now, and then it will be easy to explain the other two.

A list comprehension is a quick way of making a list. Usually the list is the result of some operation that may involve applying a function, filtering, or building a different data structure.

Let's start with a very simple example I want to calculate a list with the squares of the first 10 natural numbers. How would you do it? There are a couple of equivalent ways:

squares.map.py

# If you code like this you are not a Python guy! ;)
>>> squares = []
>>> for n in range(10):
...     squares.append(n ** 2)
...
>>> list(squares)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# This is better, one line, nice and readable
>>> squares = map(lambda n: n**2, range(10))
>>> list(squares)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

The preceding example should be nothing new for you...