Comprehensions condense for loops into a one-line expression.
A list comprehension creates a list:
:::python
squares = [x ** 2 for x in range(10)]
Produces the same result as:
:::python
squares = []
for x in range(10):
squares.append(x ** 2)
The comprehension may contain an if clause to filter the list:
:::python
squares = [x ** 2 for x in range(10) if x % 3 == 0]
Note that you can also place an if on the other side of the for as a ternary expression.
This leads to a different result:
:::python
squares = [x ** 2 if x % 3 == 0 else -1 for x in range(10)]
It is perfectly fine to place one comprehension inside another. The following code creates a 5x5 matrix:
:::python
[[x * y for x in range(5)] for y in range(5)]
Note that you can concatenate the for statements without the extra brackets.
In that case the result is a flat list:
:::python
[x * y for x in range(5) for y in range(5)]
Comprehensions that produce a list of strings are often found together with join(), e.g. to format text output:
:::python
';'.join([char.upper() for char in 'abcde'])
This variant produces dictionaries:
:::python
ascii_table = {x: chr(x) for x in range(65, 91)}
The same with a set:
:::python
unique = {x.upper() for x in 'Hello World'}