Generators

Generators

A generator produces a sequence of values one at a time, pausing between each one. It never stores the whole list in memory.

A simple generator

Use yield instead of return:

def countdown(n):
    while n > 0:
        yield n
        n -= 1

for num in countdown(3):
    print(num)
# 3, 2, 1

yield vs return

• return gives one value and the function is done.
• yield gives a value, then PAUSES. The next call continues right after the yield.

Generator objects

Calling a generator function gives you a generator object, not the values:

gen = countdown(3)
print(next(gen))  # 3
print(next(gen))  # 2

next() steps through

next(gen) runs the generator up to the next yield. When it's empty, it raises StopIteration.

Generator expressions

Like list comprehensions but with parentheses:

squares = (x * x for x in range(1000000))  # tiny memory

A list of a million items would use lots of memory; the generator uses almost none.

Why generators?

• Huge or infinite sequences without using memory.
• Lazily produce values only when needed.
• Great for streaming data, reading big files.

TL;DR

• yield makes a function a generator.
• Generators pause and resume between values.
• next() pulls each value; StopIteration ends them.
• Use them for big or infinite sequences.