[issue36537] except statement block incorrectly assumes end of scope(?).

[Saim Raza]

Saim Raza  added the comment:

> Knowing that pdb stops just before the interpreter executes the line after 
> pdb.set_trace(), what prevents you from writing the pdb.set_trace() statement 
> before the last line of the except clause ?

Of course, that can be done. The issue here is to get the correct and expected 
behavior from pdb/ipdb.

On a similar note, users might want to put just the set_trace() call and no 
other code in the block while writing the code or during debugging the 
exception caught.

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Xavier de Gaye]

Xavier de Gaye  added the comment:

> Now, I need to *inconveniently* put some dummy code after the ser_trace call 
> every time I want to access the exception inside the except block

Knowing that pdb stops just before the interpreter executes the line after 
pdb.set_trace(), what prevents you from writing the pdb.set_trace() statement 
before the last line of the except clause ?

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[issue36537] except statement block incorrectly assumes end of scope(?).

[SilentGhost]

SilentGhost  added the comment:

I cannot imagine that the fix would be straightforward or that there is much 
use of this particular pattern. Perhaps, a note in the docs suggesting 
post_mortem() for except clauses over set_trace() would be more appropriate.

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Saim Raza]

Saim Raza  added the comment:

Thanks, SilentGhost! However, should we try to fix set_trace as well to avoid 
hassles to other users?

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[issue36537] except statement block incorrectly assumes end of scope(?).

[SilentGhost]

SilentGhost  added the comment:

Saim, a .post_mortem could be used instead. As I noted, it works just fine.

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Saim Raza]

Saim Raza  added the comment:

This is pretty unintuitive from a user's stand point. Now, I need to 
*inconveniently* put some dummy code after the ser_trace call every time I want 
to access the exception inside the except block.

Also, this is a change in behavior from Python 2.7. Is this documented 
somewhere?

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Xavier de Gaye]

Xavier de Gaye  added the comment:

The *** Tracing *** section [1] of Objects/lnotab_notes.txt in the cpython 
repository provides detailed explanations for when the line trace function is 
called.

[1] 
https://github.com/python/cpython/blob/9d7b2c0909b78800d1376fd696f73824ea680463/Objects/lnotab_notes.txt#L63

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Xavier de Gaye]

Xavier de Gaye  added the comment:

@Saim wrote
> https://docs.python.org/3/reference/compound_stmts.html#the-try-statement says

This is correct, the disassembly of foo.py below demonstrates that point: 
BEGIN_FINALLY at bytecode index 36 starts the implementation of the 'finally' 
clause that deletes 'err'. But pdb is a line debugger, not a debugger at the 
bytecode level, so when 'foo = 1' is replaced by 'import pdb; pdb.set_trace()', 
the first line where pdb may stop is at line 6 and since foo() last line is 
line 5, pdb stops at the 'return' event for this function.

===   foo.py   
def foo():
try:
1/0
except Exception as err:
foo = 1

import dis
dis.dis(foo)
===
  2   0 SETUP_FINALLY   12 (to 14)

  3   2 LOAD_CONST   1 (1)
  4 LOAD_CONST   2 (0)
  6 BINARY_TRUE_DIVIDE
  8 POP_TOP
 10 POP_BLOCK
 12 JUMP_FORWARD38 (to 52)

  4 >>   14 DUP_TOP
 16 LOAD_GLOBAL  0 (Exception)
 18 COMPARE_OP  10 (exception match)
 20 POP_JUMP_IF_FALSE   50
 22 POP_TOP
 24 STORE_FAST   0 (err)
 26 POP_TOP
 28 SETUP_FINALLY8 (to 38)

  5  30 LOAD_CONST   1 (1)
 32 STORE_FAST   1 (foo)
 34 POP_BLOCK
 36 BEGIN_FINALLY
>>   38 LOAD_CONST   0 (None)
 40 STORE_FAST   0 (err)
 42 DELETE_FAST  0 (err)
 44 END_FINALLY
 46 POP_EXCEPT
 48 JUMP_FORWARD 2 (to 52)
>>   50 END_FINALLY
>>   52 LOAD_CONST   0 (None)
 54 RETURN_VALUE
===

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Saim Raza]

Saim Raza  added the comment:

https://docs.python.org/3/reference/compound_stmts.html#the-try-statement says

=
except E as N:
foo
=

is converted to the following statement:

=
except E as N:
try:
foo
finally:
del N
=

In the examples in this thread, foo is 'import pdb; pdb.set_trace()'. So, I was 
expecting 'del N' to be executed only after executing foo. This implies that 
err should have been available in the scope.

Further, isn't the code execution supposed to be stopped at the set_trace() 
line and not on the next line. Should it at least respect the scope 
(indentation) and not execute the '--Return--' line event until the user 
actually executes the next line (x=1 in your example)?

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Xavier de Gaye]

Xavier de Gaye  added the comment:

This is not a bug.
On the first pdb session from the OP:

True
--Return--
> (5)()->None
-> import pdb; pdb.set_trace()
(Pdb) print("err" in locals())
False  <-- BUG??

Pdb has stopped on a '--Return--' line event, so we are already outside the 
scope of the except clause and the 'err' local variable cannot be accessed 
outside the except block. The following code snippet shows that pdb stops at 
the line following the set_trace() call:

==
def foo():
try:
1/0
except Exception as err:
import pdb; pdb.set_trace()
x= 1

foo()
==

And the corresponding pdb session:

$ python foo.py
> /path/to/foo.py(6)foo()
-> x= 1
(Pdb) err
*** NameError: name 'err' is not defined
(Pdb)

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +xdegaye

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[issue36537] except statement block incorrectly assumes end of scope(?).

[SilentGhost]

SilentGhost  added the comment:

Can confirm also on 3.6, but this seems to be something particular to 
set_trace, when pdb is entered via post_mortem, the variable is available in 
locals().

--
nosy: +SilentGhost, barry

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[issue36537] except statement block incorrectly assumes end of scope(?).

[Saim Raza]

New submission from Saim Raza :

If pdb.set_trace() is the last statement in the first code snippet, variable 
'err' is (wrongly?) excluded from locals(). Adding any code after the 
pdb.set_trace() statement makes 'err' available in locals.

In [2]: try:
   ...: raise ValueError("I am ValueError")
   ...: except ValueError as err:
   ...: print("err" in locals())
   ...: import pdb; pdb.set_trace()
   ...:
True
--Return--
> (5)()->None
-> import pdb; pdb.set_trace()
(Pdb) print("err" in locals())
False  <-- BUG??
(Pdb) c

In [3]: try:
   ...: raise ValueError("I am ValueError")
   ...: except ValueError as err:
   ...: print("err" in locals())
   ...: import pdb; pdb.set_trace()
   ...: import os # Dummy code - makes variable 'err' available inside the 
debugger.
   ...:
True
> (6)()->None
-> import os # Dummy code - makes variable err available inside debugger
(Pdb) print("err" in locals())
True

In [4]: sys.version_info
Out[4]: sys.version_info(major=3, minor=7, micro=3, releaselevel='final', 
serial=0)

FTR, the variable 'err' is available in both cases in case of Python 2.7. Also, 
this happens with ipdb as well.

Please note that I am aware that I need to assign the variable 'err' to some 
variable inside the except block to access it outside the except block. 
However, the pdb statement is still inside the except block in both cases.

--
components: Library (Lib)
messages: 339510
nosy: Saim Raza
priority: normal
severity: normal
status: open
title: except statement block incorrectly assumes end of scope(?).
type: behavior
versions: Python 3.7

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