ジェネレータ

通常の関数と異なり、前回返した値を覚えているようです。
呼び出すと次の値を返します。

generator.py

def counter(num=10):
    for _ in range(num):
        yield 'run'

def greeting():
        yield 'good morning'
        yield 'good afternoon'
        yield 'good night'

for g in greeting():
    print(g)

g=greeting()
print(next(g)) #good morning
print(next(g)) #good afternoon

c=counter()
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
print(next(c))
# print(next(c)) エラー出る

出力：

good morning
good afternoon
good night
good morning
good afternoon
run
run
run
run
run
run
run
run
run
run