[ python-Bugs-1248119 ] pdb 'next' does not skip list comprehension

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Bugs item #1248119, was opened at 2005-07-30 14:12
Message generated for change (Comment added) made by isandler
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Joseph Heled (pepster)
Assigned to: Nobody/Anonymous (nobody)
Summary: pdb 'next' does not skip list comprehension

Initial Comment:
pdb next command forces you to step over each list element.

This seem to be a reincarnation of this old bug.
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=523995&group_id=5470

Version:
Python 2.4.1 (#2, Mar 30 2005, 21:51:10) 
[GCC 3.3.5 (Debian 1:3.3.5-8ubuntu2)] on linux2



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Comment By: Ilya Sandler (isandler)
Date: 2005-07-30 20:20

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While this behaviour indeed feels like a bug I'm starting to
think that it is not..

Observation1: 'step' should stop at every iteration of a
list comprehension

Observation2: the only difference between 'step' and 'next'
 is that 'step' steps into function calls (which is not the
case here)

So, it seems like 'next' should also stop at every iteration
of a list comprehenstion. Ie current behaviour is not a bug...

Would supporting a numeric argument for the 'next' command
make sense? 

So that 'next 1' would mean "stop when actual line number
increases by at least 1"...




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Comment By: Ilya Sandler (isandler)
Date: 2005-07-30 17:59

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A bit more information. I looked into what happens in
ceval.c and apparently the current behaviour is intentional..

Looks like this piece of code:

	else if (frame->f_lasti <= *instr_prev) {
		/* jumping back in the same line forces a trace event */
		result = call_trace(func, obj, frame,
				    PyTrace_LINE, Py_None);
	}

in maybe_call_line_trace() is responsible for the extra
"line" events..
Seems like this piece of code was added to fix bug #765624.
in ceval.c:2.386

So, should this (1248119) bug be dealt with by skipping
extra line events in bdb?

Any thoughts? Or am I totally lost?


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Comment By: Ilya Sandler (isandler)
Date: 2005-07-30 17:33

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I changed bdb.py to print the kind of event it receives as
well as as line numbers and here is a session which
illustrates the bug...

 bagira:~/python/dist/src/bug-next1248119> cat t
 #../python
 y=[1,2,3,4]
 x=[ i+1 for i in y]
 print x
 bagira:~/python/dist/src/bug-next1248119> ../python -m pdb t
 event: call; line 1, file <string>
 event: line; line 1, file <string>
 event: call; line 2, file t
 event: line; line 2, file t
 > /home/ilya/python/dist/src/bug-next1248119/t(2)?()
 -> y=[1,2,3,4]
 (Pdb) n
 event: line; line 3, file t
 > /home/ilya/python/dist/src/bug-next1248119/t(3)?()
 -> x=[ i+1 for i in y]
 (Pdb) n
 event: line; line 3, file t
 > /home/ilya/python/dist/src/bug-next1248119/t(3)?()
 -> x=[ i+1 for i in y]
 (Pdb) n
 event: line; line 3, file t
 > /home/ilya/python/dist/src/bug-next1248119/t(3)?()
 -> x=[ i+1 for i in y]
 (Pdb) 
 event: line; line 3, file t
 > /home/ilya/python/dist/src/bug-next1248119/t(3)?()
 -> x=[ i+1 for i in y]

So it appears that the interpreter generates "line" events
for every iteration of the loop..

Would this be a bug in the interpreter (and not in pdb/bdb)?




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