Comprehensions and generator expressions
More Python9 min readLesson 1 of 7
1.1 One expression instead of a loop
A list comprehension builds a list from an iterable in a single expression: square brackets, an expression for each item, then a for clause, optionally followed by an if filter. It is not faster than a well-written loop, but it says what is happening without an index variable.
Swap the square brackets for parentheses and you get a generator expression. It produces values on demand instead of building the whole list, which matters when the data is large or the first few values are all you need.
python
squares = [n * n for n in range(1, 6)]
print(squares)
evens = [n for n in squares if n % 2 == 0]
print(evens)
total = sum(n * n for n in range(1, 6))
print(total)Output
[1, 4, 9, 16, 25] [4, 16] 55
Use comprehensions for a single transformation or filter. If the expression needs nested loops and two conditions, a normal loop with a name is easier to read a month later.