[ python-Bugs-1754456 ] "yield" causes strange behaviour

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Bugs item #1754456, was opened at 2007-07-16 00:17
Message generated for change (Comment added) made by tebeka
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Category: Python Interpreter Core
Group: Python 2.5
>Status: Closed
Resolution: None
Priority: 5
Private: No
Submitted By: Miki Tebeka (tebeka)
Assigned to: Nobody/Anonymous (nobody)
Summary: "yield" causes strange behaviour

Initial Comment:
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 at 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 at lakshmi:python-bug - 14:15] $

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

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>Comment By: Miki Tebeka (tebeka)
Date: 2007-07-16 16:05

Message:
Logged In: YES 
user_id=358087
Originator: YES

OUCH, forgot about that sorry.

Closing.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2007-07-16 10:51

Message:
Logged In: YES 
user_id=80475
Originator: NO

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.

----------------------------------------------------------------------

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