Generator expression

Question

Accepted Answer

sum(x*x for x in range(1000000)) uses a generator expression - it produces one value at a time and never allocates a million-element list. sum([x*x for x in range(1000000)]) builds the entire list first, then sums it - same result, much higher peak memory. The syntax is nearly identical: swap the brackets for parentheses (or just omit them when the expression is the only argument to a function call).