[Python-bugs-list] [ python-Bugs-462937 ] continue inside try confuses while loop

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Bugs item #462937, was opened at 2001-09-19 10:46
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Category: Python Interpreter Core
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Amit Patel (amitp)
Assigned to: Thomas Wouters (twouters)
Summary: continue inside try confuses while loop

Initial Comment:
If there's a continue inside a try inside a while with
a break inside a function, well, things go wacky.  I'll
post code and give a description:

#!/usr/bin/env python2

def bug(n):
  print 'Top'
  x = 0
  while 1:
    print 'x = %d' % x
    x = x + 1
    if x > n: break
    try:
      y = float(42)
      if y > 10:
        continue
    except ValueError:
      print 'Error'
  print 'Bottom'

bug(4)  # no loop
bug(3)  # loops



When you run bug(4), it does what you expect -- prints
Top, prints x= for various values of x, then prints
Bottom, and returns.

When you run bug(3), it prints Top, various values of
x, and then instead of 'break' getting out of the loop,
it jumps to the top of the function and prints Top again !!

This seems really weird to me.  Odd values of n behave
differently than even values of n !



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>Comment By: Thomas Wouters (twouters)
Date: 2001-09-24 12:35

Message:
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Gah. found it. How embarrassing (as I suspected.) My only solace 
is that Jeremy accepted the patch <wink>. The ugly hack of 
re-creating the just-popped frame-block in the case of a continue 
swapped the 'handler' and 'stacklevel' arguments to 
PyBlock_BlockSetup! And any subsequent break or exception would 
use the wrong handler and the wrong stack level to pop to.

Don't ask me *how* I found it; I'm afraid something will go wrong 
with the fabric of space if I admit I found such an obscure bug 
with such little effort :)

Fixed in revision 2.277 of Python/ceval.c.



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Comment By: Guido van Rossum (gvanrossum)
Date: 2001-09-19 21:50

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I can reproduce this in 2.2 too.  Thomas, can you look at
this? I believe you wrote the continue-in-try code...

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Comment By: Gerhard Häring (ghaering)
Date: 2001-09-19 19:42

Message:
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FYI: Python 1.5.2 and 2.0.1 say:

gerhard@lilith:/tmp > /opt/python20/bin/python 
python-continue-bug-2.py
SyntaxError: 'continue' not supported inside 'try' clause 
(python-continue-bug-2.py, line 10)

This problem seems to have started with the 2.1 branch.


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Comment By: Amit Patel (amitp)
Date: 2001-09-19 13:18

Message:
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I'm attaching a variant of the program which runs and then
causes "SystemError: unknown opcode".


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Comment By: Amit Patel (amitp)
Date: 2001-09-19 10:50

Message:
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Bah, it destroyed my formatting.  I'll upload a separate file.

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