Python Comprehensions

List Comprehensions are a special kind of syntax that let us create lists out of other lists, and are incredibly useful when dealing with numbers and with one or two levels of nested for loops.

From the Python 3 tutorial List comprehensions provide a concise way to create lists. [...] or to create a subsequence of those elements that satisfy a certain condition.

Read Python Comprehensions: A step by step Introduction for a more in-deep or introduction.

List comprehension

This is how we create a new list from an existing collection with a For Loop:

>>> names = ['Charles', 'Susan', 'Patrick', 'George']

>>> new_list = []
>>> for n in names:
...     new_list.append(n)
...
>>> new_list
# ['Charles', 'Susan', 'Patrick', 'George']

And this is how we do the same with a List Comprehension:

>>> names = ['Charles', 'Susan', 'Patrick', 'George']

>>> new_list = [n for n in names]
>>> new_list
# ['Charles', 'Susan', 'Patrick', 'George']

We can do the same with numbers:

>>> n = [(a, b) for a in range(1, 3) for b in range(1, 3)]
>>> n
# [(1, 1), (1, 2), (2, 1), (2, 2)]

Adding conditionals

If we want new_list to have only the names that start with C, with a for loop, we would do it like this:

>>> names = ['Charles', 'Susan', 'Patrick', 'George', 'Carol']

>>> new_list = []
>>> for n in names:
...     if n.startswith('C'):
...         new_list.append(n)
...
>>> print(new_list)
# ['Charles', 'Carol']

In a List Comprehension, we add the if statement at the end:

>>> new_list = [n for n in names if n.startswith('C')]
>>> print(new_list)
# ['Charles', 'Carol']

To use an if-else statement in a List Comprehension:

>>> nums = [1, 2, 3, 4, 5, 6]
>>> new_list = [num*2 if num % 2 == 0 else num for num in nums]
>>> print(new_list)
# [1, 4, 3, 8, 5, 12]

Set and Dict comprehensions The basics of list comprehensions also apply to sets and dictionaries.

Set comprehension

>>> b = {"abc", "def"}
>>> {s.upper() for s in b}
{"ABC", "DEF"}

Dict comprehension

>>> c = {'name': 'Pooka', 'age': 5}
>>> {v: k for k, v in c.items()}
{'Pooka': 'name', 5: 'age'}

A List comprehension can be generated from a dictionary:

>>> c = {'name': 'Pooka', 'first_name': 'Oooka'}
>>> ["{}:{}".format(k.upper(), v.upper()) for k, v in c.items()]
['NAME:POOKA', 'FIRST_NAME:OOOKA']