Default value for option "python print-stack"

Default value for option "python print-stack"

[Maxim Akhmedov]

Hi,

Currently the default value for an option "python print-stack" is "message" 
that enables show only the messages of Python exceptions, but not their 
tracebacks nor line numbers.

Though, usually the message of a Python error itself is pretty useless as it 
doesn't point you where exactly the error happened. Like, suppose you have a 
pretty-printer written in Python, and then you get the following output:

(gdb) yp lastKey
Python Exception <type 'exceptions.UnboundLocalError'> local variable 
'value_data' referenced before assignment: 
Error occurred in Python command: local variable 'value_data' referenced before 
assignment

it doesn't really tell you anything, there may be lots of usages of a mentioned 
variable, and due to the dynamic nature of Python you may easily mess somewhere 
and it's particularly hard to find out the exact place by this message. Also, 
the exception message is duplicated.

I see two options here. 
First one is choosing "full" as a default value for this option. Is their any 
rationale for not doing that? Python tracebacks are usually not that large to 
intentionally suppress them.
Second one is appending the sourcefile:lineno to the message. Like: 
"/path/to/source.py:123". I believe it should be possible to extract this 
information from the traceback object you get after invoking the Python code.

From my point of view, it's a good idea to implement both of this options.


-- 
Best wishes,
Maxim Akhmedov
max42@yandex-team.com

Re: Default value for option "python print-stack"

[Simon Marchi]

On 2017-02-14 19:03, Maxim Akhmedov wrote:
> Hi,
> 
> Currently the default value for an option "python print-stack" is 
> "message"
> that enables show only the messages of Python exceptions, but not their
> tracebacks nor line numbers.
> 
> Though, usually the message of a Python error itself is pretty useless 
> as it
> doesn't point you where exactly the error happened. Like, suppose you 
> have a
> pretty-printer written in Python, and then you get the following 
> output:
> 
> (gdb) yp lastKey
> Python Exception <type 'exceptions.UnboundLocalError'> local variable
> 'value_data' referenced before assignment:
> Error occurred in Python command: local variable 'value_data' 
> referenced before
> assignment
> 
> it doesn't really tell you anything, there may be lots of usages of a 
> mentioned
> variable, and due to the dynamic nature of Python you may easily mess 
> somewhere
> and it's particularly hard to find out the exact place by this message. 
> Also,
> the exception message is duplicated.
> 
> I see two options here.
> First one is choosing "full" as a default value for this option. Is 
> their any
> rationale for not doing that? Python tracebacks are usually not that 
> large to
> intentionally suppress them.
> Second one is appending the sourcefile:lineno to the message. Like:
> "/path/to/source.py:123". I believe it should be possible to extract 
> this
> information from the traceback object you get after invoking the Python 
> code.
> 
> From my point of view, it's a good idea to implement both of this 
> options.

I agree with the first one, I spent so much time tracking bugs in my 
Python code before knowing about this setting.

Simon

Re: Default value for option "python print-stack"

[Phil Muldoon]

On 18/02/17 15:00, Simon Marchi wrote:
> On 2017-02-14 19:03, Maxim Akhmedov wrote:
>> Hi,
>>
>> Currently the default value for an option "python print-stack" is "message"
>> that enables show only the messages of Python exceptions, but not their
>> tracebacks nor line numbers.
>>
>> Though, usually the message of a Python error itself is pretty useless as it
>> doesn't point you where exactly the error happened. Like, suppose you have a
>> pretty-printer written in Python, and then you get the following output:
>>
>> (gdb) yp lastKey
>> Python Exception <type 'exceptions.UnboundLocalError'> local variable
>> 'value_data' referenced before assignment:
>> Error occurred in Python command: local variable 'value_data' referenced before
>> assignment
>>
>> it doesn't really tell you anything, there may be lots of usages of a mentioned
>> variable, and due to the dynamic nature of Python you may easily mess somewhere
>> and it's particularly hard to find out the exact place by this message. Also,
>> the exception message is duplicated.
>>
>> I see two options here.
>> First one is choosing "full" as a default value for this option. Is their any
>> rationale for not doing that? Python tracebacks are usually not that large to
>> intentionally suppress them.
>> Second one is appending the sourcefile:lineno to the message. Like:
>> "/path/to/source.py:123". I believe it should be possible to extract this
>> information from the traceback object you get after invoking the Python code.
>>
>> From my point of view, it's a good idea to implement both of this options.
> 
> I agree with the first one, I spent so much time tracking bugs in my Python code before knowing about this setting.
> 
> Simon

Originally, the message only approach was chosen because Python
pretty-printed values that caused exceptions in backtraces would often
generate large amounts of Python exceptions that totally messed up the
backtrace. This is always the case when the pretty-printed values are
not initialised. I have no strong position on message or stack-trace;
I prefer the former because there was thought put into the reasoning.

Just as a note, you can set the exception printing parameter, as well
as any other GDB parameter, in the user .gdbinit.  I've often mulled,
and I think I brought up once, a review of GDB default values (like
set print pretty on, for example). This will generate some
disagreements to this list, though! ;)

Cheers

Phil