Editing Generator (computer programming) (section)

===Python===

Generators were added to [[Python (programming language)|Python]] in version 2.2 in 2001.<ref name="python"/> An example generator:

<syntaxhighlight lang="python">
from typing import Iterator

def countfrom(n: int) -> Iterator[int]:
    while True:
        yield n
        n += 1

# Example use: printing out the integers from 10 to 20.
# Note that this iteration terminates normally, despite
# countfrom() being written as an infinite loop.

for i in countfrom(10):
    if i <= 20:
        print(i)
    else:
        break

# Another generator, which produces prime numbers indefinitely as needed.
import itertools

def primes() -> Iterator[int]:
    """Generate prime numbers indefinitely as needed."""
    yield 2
    n = 3
    p = [2]
    while True:
        # If dividing n by all the numbers in p, up to and including sqrt(n),
        # produces a non-zero remainder then n is prime.
        if all(n % f > 0 for f in itertools.takewhile(lambda f: f * f <= n, p)):
            yield n
            p.append(n)
        n += 2
</syntaxhighlight>

In Python, a generator can be thought of as an [[iterator]] that contains a frozen [[stack frame]]. Whenever <code>next()</code> is called on the iterator, Python resumes the frozen frame, which executes normally until the next <code>yield</code> statement is reached. The generator's frame is then frozen again, and the yielded value is returned to the caller.

PEP 380 (implemented in Python 3.3) adds the <code>yield from</code> expression, allowing a generator to delegate part of its operations to another generator or iterable.<ref name="pep380">[https://www.python.org/dev/peps/pep-0380/ PEP 380 -- Syntax for Delegating to a Subgenerator]</ref>

====Generator expressions====

Python has a syntax modeled on that of [[list comprehension]]s, called a generator expression that aids in the creation of generators.
The following extends the first example above by using a generator expression to compute squares from the <code>countfrom</code> generator function:
<syntaxhighlight lang="python">
squares = (n * n for n in countfrom(2))

for j in squares:
    if j <= 20:
        print(j)
    else:
        break
</syntaxhighlight>