Generator expressions are a high performance and memory efficient generalization of list comprehensions and generators. Imagine we want to sum up all even number ranging from 1 to 100.
Using list comprehension:
even_sum = sum([x for x in range(1, 100) if x % 2 == 0]) print(even_sum) #2450
This will prove inefficient in the case of a large range because it first creates a list, it iterates over it and then returns the sum. The same result can be achieved with a generator expression:
even_sum = sum(x for x in range(1, 100) if x % 2 == 0) print(even_sum) #2450
The generator expressions syntax says that it must be enclosed inside parenthesis (). A generator for squares of numbers:
squares = (x * x for x in range(1,10))
This generator can now be converted to a list with:
print(list(squares)) # [1, 4, 9, 16, 25, 36, 49, 64, 81]
Or, iterate over it with a for loop:
for item in squares: print(item)
This’ll print nothing, since a generator can only be iterated over once. To access values from a generator more than once, either save the values in a list, or define and then run the generator again.