[Python-Dev] Make stacklevel=2 by default in warnings.warn()

[Walter Dörwald]

On 21 Sep 2015, at 9:18, Victor Stinner wrote:

> 2015-09-20 8:44 GMT+02:00 Serhiy Storchaka <storchaka at gmail.com>:
>> I propose to make the default value of stacklevel to be 2.
>> I think that unlikely this will break existing code.
>
> Consider this simple script:
> ---
> import warnings
> warnings.warn("here")
> ---
>
> Currrent output:
> ---
> x.py:3: UserWarning: here
> warnings.warn("here")
> ---
>
> => it shows the script name (x.py), the line number and the line, as 
> expected.
>
> Now try stacklevel=2:
> ---
> import warnings
> warnings.warn("here", stacklevel=2)
> ---
>
> New output:
> ---
> sys:1: UserWarning: here
> ---
>
> "sys:1" is not really useful :-/
>
> I would describe this as a regression, not an enhancement.
>
> It's hard to find a "good" default value. It's better to always
> specify stacklevel :-)

A "dynamic" stacklevel might help. Normally when you implement a call to 
warning.warn() inside a module, you want to report the innermost 
stacklevel that is outside of your module, because that's the spot where 
the error likely is. This could be done automatically (report the first 
module that is different from the one where the call to warning.warn() 
is), or by specifying a base package name or regular expression, i.e. 
report the innermost stackframe that is not from 
"mypackage.mysubpackage").

See http://bugs.python.org/issue850482

Bye,
    Walter Dörwald