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

[Phil Muldoon <pmuldoon@redhat.com>]

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