Python etc / yield and comprehensions

Published: 22 October 2020, 18:00

Accidentally, yield can be used in generator expressions and comprehensions:

[(yield i) for i in 'ab']
# <generator object <listcomp> at 0x7f2ba1431f48>

list([(yield i) for i in 'ab'])
# ['a', 'b']

list((yield i) for i in 'ab')
# ['a', None, 'b', None]

This is because yield can be used in any function (turning it into a generator) and comprehensions are compiled into functions:

>>> dis.dis("[(yield i) for i in range(3)]")
0 LOAD_CONST     0 (<code object <listcomp> ...>)
2 LOAD_CONST     1 ('<listcomp>')

This produces a warning in Python 3.7 and will raise SyntaxError in python 3.8+. However, yield inside lambda still can be used:

a = lambda x: (yield x)
# [1]