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

Guido van Rossum guido at python.org
Mon Sep 21 17:08:25 CEST 2015

I think it's been conclusively shown that we should not change the default. Instead I recommend updating the docs with an example showing when stacklevel=2 is appropriate.

On Mon, Sep 21, 2015 at 2:05 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On 21 September 2015 at 17:21, Nathaniel Smith <njs at pobox.com> wrote: > On Sep 21, 2015 12:15 AM, "Victor Stinner" <victor.stinner at gmail.com> wrote: >> >> Would it be too verbose to display two frames or more by default? >> Maybe depending on the action (ex: only if the warning is emitted only >> once). > > It's not just about how it gets displayed -- the frame that it gets > "attributed" to is also used when comparing against the warnings filters to > determine what action to take. What if stacklevel=1 makes it match a filter > with action "ignore", and stacklevel=2 makes it match a filter with action > "raise"? > > (This is a common example I've encountered in the context of wanting to set > up CI tests so that if my code uses deprecated functionality then I want > to fail the test so I notice, but if some library I'm using uses deprecated > functionality internally then that's not my problem. This breaks down when > the library makes the common error of issuing DeprecationWarnings with > stacklevel=1, because that makes python think that they are deprecating > themselves, not warning that I did something deprecated.)

As Victor notes, though, that's not the right default for scripts that are issuing deprecation warnings to end users - there, they really are deprecating themselves. It's also not the right default for pretty much any warning other than DeprecationWarning - for those, you're generally wanting to say "we hit this problematic code path", not report anything about your caller. Passing "stacklevel=2" for API deprecations is by no means obvious though, so perhaps it makes sense to add a "warnings.warndeprecated(message)" function that's implemented as: def warndeprecated(message, stacklevel=1): return warnings.warn(message, DeprecationWarning, stacklevel=(2+stacklevel)). Cheers, Nick. -- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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