Generators

Introduction to generators and yield

You already know how to build a list inside a function and return it. Generators are functions that hand back values one at a time, on demand, using yield. This lesson shows what changes when you swap return for yield.

So far, when a function had to return many values, you built a list inside the function and returned the whole thing.

Here is a function that returns the first three squares as a list:

def squares():
	result = []
	for i in range(3):
		result.append(i * i)
	return result
 
print(squares())

Python

Output

That works for three values. But imagine the function had to produce a million. The full list would sit in memory all at once, even if the caller only needs the values one at a time.

Generators solve this. They produce values on demand, one by one, without ever building the full collection. The keyword is yield.

Any function that contains yield is a generator function:

def my_gen():
	yield value_a
	yield value_b

Python

The squares example, rewritten as a generator:

def squares():
	for i in range(3):
		yield i * i
 
for x in squares():
	print(x)

Python

Output

Each yield pauses the function. The next request picks up where it left off, with all the local state intact:

def steps():
	print("start")
	yield 1
	print("middle")
	yield 2
	print("end")
 
for n in steps():
	print(n)

Python

Output

What will be the output?

def gen():
	yield 10
	yield 20
 
for x in gen():
	print(x)

Python

Calling a generator function does not run it. It returns a generator object:

def gen():
	yield 1
 
g = gen()
print(type(g).__name__)

Python

Output

Use next() to ask the generator for one value at a time:

def gen():
	yield 1
	yield 2
 
g = gen()
print(next(g))
print(next(g))

Python

Output

This is where generators pay off. The loop below would build a list of a billion numbers and crash on memory. The generator version pulls just the first three and stops, never computing the rest:

def counter():
	i = 0
	while True:
		yield i
		i += 1
 
g = counter()
print(next(g))
print(next(g))
print(next(g))

Python

Output

Pass the generator to list() to collect every value at once:

def evens(n):
	for i in range(n):
		if i % 2 == 0:
			yield i
 
print(list(evens(6)))

Python

Output

What will be the output?

def gen():
	for i in range(3):
		yield i + 10
 
print(list(gen()))

Python

What will be the output?

def gen():
	x = 0
	while x < 3:
		yield x
		x += 1
 
print(sum(gen()))

Python

What will be the output?

def gen():
	yield 1
	yield 2
 
g = gen()
print(list(g))
print(list(g))

Python

What will be the output?

def gen():
	print("a")
	yield 1
	print("b")
 
g = gen()
print("ready")

Python