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Generators

yield from and chaining generators

When one generator wants to hand off to another iterable, writing a for loop with yield works but feels noisy. yield from does the same thing in one line.

A common pattern is one generator that wants to forward every value of another iterable.

You can do that with a plain loop and yield, but the loop is just plumbing:

def letters():
	for c in "abc":
		yield c
 
print(list(letters()))

Python

Output

yield from does the exact same thing in one line:

def letters():
	yield from "abc"
 
print(list(letters()))

Python

Output

The keyword takes any iterable: a list, a string, a generator, anything you can loop over.

yield from some_iterable

Python

Forwarding a list works just as well:

def gen():
	yield from [10, 20, 30]
 
print(list(gen()))

Python

Output

And forwarding another generator:

def numbers():
	yield 1
	yield 2
 
def wrapper():
	yield from numbers()
 
print(list(wrapper()))

Python

Output

What will be the output?

def gen():
	yield from range(3)
 
print(list(gen()))

Python

 [1, 2, 3]
 Output

 range(0, 3)
 Output

 [0, 1, 2]
 Output

You can chain several generators by stacking yield from statements:

def first():
	yield 1
	yield 2
 
def second():
	yield 3
 
def chained():
	yield from first()
	yield from second()
 
print(list(chained()))

Python

Output

What will be the output?

def a():
	yield "x"
 
def b():
	yield "y"
	yield from a()
 
print(list(b()))

Python

 ['y', 'x']
 Output

 ['x', 'y']
 Output

 ['y']
 Output

A small wrapper that delegates to range() becomes a one-liner:

def count_up_to(n):
	yield from range(1, n + 1)
 
print(list(count_up_to(4)))

Python

Output

What will be the output?

def gen():
	yield from [1, 2]
	yield 3
 
print(list(gen()))

Python

 [1, 2]
 Output

 [3, 1, 2]
 Output

 [1, 2, 3]
 Output

What will be the output?

def gen():
	yield from "ab"
	yield from "cd"
 
print("".join(gen()))

Python

 ab
 Output

 abcd
 Output

 cdab
 Output

What will be the output?

def gen():
	yield from []
 
print(list(gen()))

Python

 []
 Output

 Error
 Output

 None
 Output

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