sum(x * x for x in range(10**8)) versus sum([x * x for x in range(10**8)]). What is the key difference?

Options

1. The generator version is always faster because it skips the loop
2. The generator version holds one item at a time, so memory stays flat (Correct option)
3. Both versions are identical after Python compiles them to bytecode
4. The list version is lazy and only builds values when sum asks

Answer

The generator version uses roughly constant memory; the list version builds 100M items first

Why

With square brackets [...] Python first builds a complete list of 100 million numbers in memory, and only then sum adds them.

With parentheses (...) it is a generator expression: each number is created only when sum asks for the next one, so only one number exists at a time.

Same result, far less memory.

Remember

Use generators for big data you only loop over once.

Key terms

• Generator
• Lazy evaluation
• List comprehension