[New-bugs-announce] [issue10544] yield expression inside generator expression does nothing

Inyeol Lee report at bugs.python.org
Fri Nov 26 20:21:19 CET 2010

New submission from Inyeol Lee <inyeol.lee at gmail.com>:

Simple coroutine with for loop works:

>>> def pack_a():
        while True:
            L = []
            for i in range(2):

>>> pa = pack_a()
>>> next(pa)
>>> pa.send(1)
>>> pa.send(2)
[1, 2]

If using list comprehension (generator expression), it fails:

>>> def pack_b():
        while True:
            L = [(yield) for i in range(2)]

>>> pb = pack_b()
<endless loop here>

I understand what's going on here - generator expression is converted to nested function and there's no way to either stop the execution inside nested function (since it's not started yet!) or send() a value to its yield expression. Still I think this behavior is a bug and needs fixed.

- best fix would make it behave the same as for loop.
- if it's not fixable, yield expression inside genexp should not be allowed.

components: Interpreter Core
messages: 122475
nosy: Inyeol.Lee
priority: normal
severity: normal
status: open
title: yield expression inside generator expression does nothing
type: behavior
versions: Python 3.1

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