[issue25474] Weird behavior when setting f_trace in a context manager

[Terry J. Reedy]

Terry J. Reedy added the comment:

Thank you Xavier.  I guess the lesson is that physical line tracing works best 
(and most as one would hope or expect) when physical lines are logical lines 
(or compound statement header lines).

--
resolution:  -> not a bug
stage: test needed -> resolved

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[issue25474] Weird behavior when setting f_trace in a context manager

[Fred Gansevles]

Fred Gansevles added the comment:

Xavier, thanks! you found it.
If I look the code again, I see that with zero, one, four and five
the context-manager (i.e. Context()) and the target (one .. five) are on
the same code-line
In the case of two and three they are on a different line.

Now, with the dependency of the trace function on the *physical line* it all 
make sense.

Fred.

--
status: open -> closed

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[issue25474] Weird behavior when setting f_trace in a context manager

[Xavier de Gaye]

Xavier de Gaye added the comment:

The difference lies in the line numbers. When tracing lines, the interpreter 
operates on the basis of physical lines, not logical lines 
(https://docs.python.org/3/reference/lexical_analysis.html#line-structure).  
The local trace function is called on the first instruction of each physical 
line (see the comment in maybe_call_line_trace() in Python/ceval.c).

In the first case of as_context.py, the 'zero' case, we have:

 25  74 SETUP_EXCEPT49 (to 126)

 27  77 LOAD_NAME6 (Context)
 80 CALL_FUNCTION0 (0 positional, 0 keyword pair)
 83 SETUP_WITH  20 (to 106)
 86 STORE_NAME   7 (zero)

 28  89 LOAD_NAME8 (print)
 92 LOAD_CONST   5 ('zero')
 95 LOAD_NAME7 (zero)
 ...

After 'CALL_FUNCTION' has been run and the trace function set up, the __raise() 
local trace function is invoked for the first time upon running the 'LOAD_NAME' 
instruction, since 'SETUP_WITH' and 'STORE_NAME' are not the first instruction 
of line 27.

In the 'two' case, we have:

 41 >>  214 SETUP_EXCEPT49 (to 266)

 42 217 LOAD_NAME6 (Context)
220 CALL_FUNCTION0 (0 positional, 0 keyword pair)
223 SETUP_WITH  20 (to 246)

 43 226 STORE_NAME  11 (two)

 44 229 LOAD_NAME8 (print)
232 LOAD_CONST   9 ('two')
235 LOAD_NAME   11 (two)


Here the __raise() local trace function is invoked for the first time upon 
running the 'STORE_NAME' instruction.

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[issue25474] Weird behavior when setting f_trace in a context manager

[Fred Gansevles]

Fred Gansevles added the comment:

Xavier, thanks for looking at my post.
But, since all six invocations of the context manager are the same
- I did an 'ast.parse' and 'ast.dump' and the the six calls were *exactly* the 
same (save lineno and col_offset) - why does 'zero', 'one', 'four' and 'five' 
get assigned but 'two' and 'three' not ?

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[issue25474] Weird behavior when setting f_trace in a context manager

[Xavier de Gaye]

Xavier de Gaye added the comment:

This is an artefact that occurs as the result of raising an exception in the 
local trace function.

Disassembling the first case of failure of as_context.py with the attached 
script gives the following:

  9   0 LOAD_GLOBAL  0 (context)
  3 CALL_FUNCTION0 (0 positional, 0 keyword pair)
  6 SETUP_WITH  20 (to 29)

 10   9 STORE_FAST   0 (two)

 11  12 LOAD_GLOBAL  1 (print)
 15 LOAD_CONST   1 ('two')
 18 LOAD_FAST0 (two)
 21 CALL_FUNCTION2 (2 positional, 0 keyword pair)
 24 POP_TOP
 25 POP_BLOCK
 26 LOAD_CONST   0 (None)
>>   29 WITH_CLEANUP
 30 END_FINALLY

 12  31 LOAD_GLOBAL  1 (print)
 34 LOAD_CONST   2 ('TWO')
 37 LOAD_FAST0 (two)
 40 CALL_FUNCTION2 (2 positional, 0 keyword pair)
 43 POP_TOP
 44 LOAD_CONST   0 (None)
 47 RETURN_VALUE

The __raise() local trace function is invoked on line 10 upon executing the 
STORE_FAST bytecode (this can be confirmed by running foo() under the control 
of pdb).  The exception raised by __raise() causes the assignement to 'two' to 
fail, hence the tracebacks in as_context.py.  This does not seem to be a bug in 
python.

--
nosy: +xdegaye
Added file: http://bugs.python.org/file40922/as_context_dis.py

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[issue25474] Weird behavior when setting f_trace in a context manager

[Terry J. Reedy]

Terry J. Reedy added the comment:

ZERO <__main__.Context object at 0x00874AF3E0B8>
ONE <__main__.Context object at 0x00874AF326D8>
Traceback (most recent call last):
  File "F:\Python\mypy\tem.py", line 45, in 
print ('TWO', two)
NameError: name 'two' is not defined
Traceback (most recent call last):
  File "F:\Python\mypy\tem.py", line 53, in 
print ('THREE', three)
NameError: name 'three' is not defined
FOUR <__main__.Context object at 0x00874AF32748>
FIVE <__main__.Context object at 0x00874B157EB8>

--
nosy: +ncoghlan, terry.reedy
stage:  -> test needed
versions: +Python 2.7, Python 3.4, Python 3.5, Python 3.6

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[issue25474] Weird behavior when setting f_trace in a context manager

[Fred Gansevles]

New submission from Fred Gansevles:

I'm playing with the idea of making a DSL based on anonynous code blocks

I discovered that the behaviour of the context manager is different in some 
cases if there are line-continuations in the 'with' command

I've attached a script that reproduces this behaviour.

With both Python 2.7.6 and Python 3.4.3 I get the same results.


Fred.

--
files: as_context.py
messages: 253426
nosy: Fred Gansevles
priority: normal
severity: normal
status: open
title: Weird behavior when setting f_trace in a context manager
type: behavior
Added file: http://bugs.python.org/file40858/as_context.py

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