"yield" causes strange behaviour

[tebeka]

BPO	1754456
Nosy	@rhettinger, @tebeka
Files	yield.py: yield.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2007-07-16.13:05:22.000>
created_at = <Date 2007-07-15.21:17:25.000>
labels = ['interpreter-core']
title = '"yield" causes strange behaviour'
updated_at = <Date 2007-07-16.13:05:22.000>
user = 'https://github.com/tebeka'

bugs.python.org fields:

activity = <Date 2007-07-16.13:05:22.000>
actor = 'tebeka'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2007-07-15.21:17:25.000>
creator = 'tebeka'
dependencies = []
files = ['2422']
hgrepos = []
issue_num = 1754456
keywords = []
message_count = 3.0
messages = ['32512', '32513', '32514']
nosy_count = 2.0
nosy_names = ['rhettinger', 'tebeka']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1754456'
versions = ['Python 2.5']

[tebeka]

Running the attached files produces:
ONE PASS

However if you uncomment line 46 and run the file again, you don't get any output.

A pdb session of the file produces:
[mtebeka@lakshmi:python-bug - 14:14] $pdb yield.py

/home/mtebeka/work/python-bug/yield.py(3)<module>()
-> from imaplib import IMAP4
(Pdb) b 72
Breakpoint 1 at /home/mtebeka/work/python-bug/yield.py:72
(Pdb) c
/home/mtebeka/work/python-bug/yield.py(72)main()
-> one_pass()
(Pdb) s
--Call--
/usr/local/lib/python2.5/bdb.py:318: RuntimeWarning: tp_compare didn't return -1 or -2 for exception
i = max(0, len(stack) - 1)
The program finished and will be restarted
/home/mtebeka/work/python-bug/yield.py(3)<module>()
-> from imaplib import IMAP4
(Pdb) q
[mtebeka@lakshmi:python-bug - 14:15] $

(Sorry for the length of the example, this is the smaller version that still reproduces the bug)

[rhettinger]

Miki, I don't see a bug here. The presence of "yield" is significant during compilation not just when the line is executed. When the yield appears anywhere in a function body, it is not a function anymore, it is a generator. So, when the "yield" is uncommented, the call to one_pass returns a generator which doesn't start running until its next() method is called. In contrast, when the "yield" is commented, you have a function that starts running immediately when one_pass() is called.

Here is a simplified comparison of the generator versus function:

>>> def one_pass():
...     print 'running'
...     yield 1
...
>>> g = one_pass()
>>> g.next()
running
1
>>> def one_pass():
...     print 'running'
...     # yield 1
...
>>> one_pass()
running

If you still see a bug, please elaborate. If not, please close this report.

[tebeka]

OUCH, forgot about that sorry.

Closing.