[issue37647] Wrong lineno in traceback when formatting strings with % and multilines

[Bruno P. Kinoshita]

Bruno P. Kinoshita  added the comment:

Hi,

Thanks for the explanation. Could you elaborate a bit on this one:

>In Python 3.8 the traceback points to the start of the subexpression that 
>raises an exception.

Just wanted to understand why I get the desired line number when I call 
Z().__str__(), but when I call simply Z() is returns - as you clarified - the 
start of the subexpression.

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[issue37647] Wrong lineno in traceback when formatting strings with % and multilines

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Line 9 is not where the error actually happened. The exception is raised not 
when you call Z(), but when you implicitly call Z.__str__() when evaluate the % 
operator.

In Python 3.8 the traceback points to the start of the subexpression that 
raises an exception.

So this issue has been fixed in 3.8. The fix was not backported to 3.7 because 
it is a large change which affects other code that traces execution or analyses 
tracebacks (e.g. coverage.py).

--
resolution:  -> out of date
stage:  -> resolved
status: open -> closed

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[issue37647] Wrong lineno in traceback when formatting strings with % and multilines

[Bruno P. Kinoshita]

Bruno P. Kinoshita  added the comment:

Hi Ammar, thanks for the quick reply and for the suggestion.

Tried on the latest version on master, and looks like it's indeed different, 
though still looks like it is displaying the wrong line number.

```
kinow@localhost:~/Development/python/workspace/cpython$ python
Python 3.9.0a0 (heads/master:b530a4460b, Jul 22 2019, 18:38:44) 
[GCC 7.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
kinow@localhost:~/Development/python/workspace/cpython$ python z.py
Traceback (most recent call last):
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 14, in 

print(list(raise_generator()))
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 7, in 
raise_generator
yield 'three values are: %s %s %s' % (
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 3, in 
__str__
raise ValueError('Z error')
ValueError: Z error
```

It is now displaying the line number 7, where the yield statement begins, 
instead of where it actually happened (line 9).

And once again, if I replace `Z()` by `Z().__str__()`, then it displays the 
right line number.

```
kinow@localhost:~/Development/python/workspace/cpython$ python z.py
Traceback (most recent call last):
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 14, in 

print(list(raise_generator()))
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 9, in 
raise_generator
Z().__str__(),  # traceback must point to this lineno 9
  File "/home/kinow/Development/python/workspace/cpython/z.py", line 3, in 
__str__
raise ValueError('Z error')
ValueError: Z error
```

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[issue37647] Wrong lineno in traceback when formatting strings with % and multilines

[Ammar Askar]

Ammar Askar  added the comment:

Please try this on the latest version of Python, there was a behavior change 
implemented in issue12458 that might make this a non-issue.

--
nosy: +ammar2, serhiy.storchaka

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[issue37647] Wrong lineno in traceback when formatting strings with % and multilines

[Bruno P. Kinoshita]

New submission from Bruno P. Kinoshita :

Hi,

Tested on Python 3.7 on Ubuntu LTS. But I believe this affects previous 
versions too, as found it from an issue posted in Jinja2 in 2013: 
https://github.com/pallets/jinja/issues/276

This code simulates the issue where the traceback has the wrong line number 
(without using any Jinja2 code):

```
class Z(object):
def __str__(self):
raise ValueError('Z error')


def raise_generator():
yield 'three values are: %s %s %s' % (
'primeiro',
Z(),  # traceback must point to this lineno 9
'terceiro'  # but points to this lineno 10 (__str__ only, __eq__ is OK)
)


print(list(raise_generator()))
```

The output:

```
Traceback (most recent call last):
  File 
"/home/kinow/Development/python/workspace/cylc-flow/cylc/flow/tests/Z.py", line 
14, in 
print(list(raise_generator()))
  File 
"/home/kinow/Development/python/workspace/cylc-flow/cylc/flow/tests/Z.py", line 
10, in raise_generator
'terceiro'  # but points to this lineno 10 (__str__ only, __eq__ is OK)
  File 
"/home/kinow/Development/python/workspace/cylc-flow/cylc/flow/tests/Z.py", line 
3, in __str__
raise ValueError('Z error')
ValueError: Z error
```

Jinja uses something similar to the class Z to raise errors when the template 
has undefined variables. The curious part is that if instead of simply 
formatting the string with "%s" % (Z()) you use "%s" % (str(Z())) or if you 
callZ().__str__(), then the traceback reports the correct line number.

Not sure if intentional, but would be nice if the traceback reported the 
correct line number, and I think other applications could end up having the 
same issue.

This is my first issue, so I apologize if I did not include enough information, 
or if there is anything missing in this ticket. Let me know if I need to update 
it with more information or formatting the code.

Would be happy to work on this if this is considered an easy-to-fix issue.

Thanks all for your work on Python,

Bruno P. Kinoshita

--
components: Interpreter Core
messages: 348277
nosy: kinow
priority: normal
severity: normal
status: open
title: Wrong lineno in traceback when formatting strings with % and multilines
type: behavior
versions: Python 3.7

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