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Currently, the warnings.showformat() function take between 4 and 6 parameters and it's not possible to add new parameters.
warnings.showformat() calls warnings.formatwarnings() with 5 parameters. Again, it's not easy to pass new parameters.
I would like to add a new "source" parameter for ResourceWarning: see issue #26567 "ResourceWarning: Use tracemalloc to display the traceback where an object was allocated when a ResourceWarning is emitted".
To make warnings extensible, I propose to:
* Add new showmsg() and formatmsg() functions to the warnings module, these functions take a warnings.WarningMessage instance
* The _warnings module calls warnings.showmsg() rather than warnings.showwarning()
* For backward compatibility, warnings.showmsg() calls warnings.showwarning() if warnings.showwarning() was replaced. Same for warnings.formatmsg(): call warnings.formatwarning() if replaced.
Attached patch implements these changes.
With these changes, the warnings looks a little bit more like the logging module and its logging.LogRecord class.
The patch is incomplete, it doesn't add much tests yet and it doesn't touch the documentation. I would prefer to get the new API (and the whole idea) approved before going too far.
Note: If warnings.showwarning is deleted ("del warnings.showwarning"), showmsg() uses its own implementation. Same for formatmsg().

I'm not sure about the method names showmsg() and formatmsg().
Maybe: showwarnmsg() and formatwarnmsg()?
First I used warnings.show() and warnings.format(), but "format()" is already the name of a builtin function.
By the way, I would prefer to use underscore between names, like show_msg() or show_warn_msg(). But I don't know if it's ok to use a different naming scheme in an existing module :-/

New changeset 79888b970cc4 by Victor Stinner in branch 'default':
Add _showwarnmsg() and _formatwarnmsg() to warnings
https://hg.python.org/cpython/rev/79888b970cc4 

I pushed a change which adds a private _showwarnmsg(), but I'm still interested to make this new function public!
I pushed the change to unblock the issue #26567.

New changeset 9c92352324e8 by Victor Stinner in branch 'default':
Fix test_logging
https://hg.python.org/cpython/rev/9c92352324e8 

To make warning testing saner, in numpy we added basically my own version of catch_warnings on steroids, which needed/will need changing because of this.
Unless I missed it somewhere, this change should maybe put into the release notes to warn make it a bit easier to track down what is going on.

New changeset 7eebeb8fb84e2a9cb73903a08c59cf1d3b32cee0 by Victor Stinner in branch 'master':
bpo-30812: Fix test_warnings, restore _showwarnmsg (#2504)
https://github.com/python/cpython/commit/7eebeb8fb84e2a9cb73903a08c59cf1d3b32cee0

New changeset d0aac5da59b1bbd113e6081d7c807ad5bced8a05 by Victor Stinner in branch '3.6':
bpo-30812: Fix test_warnings, restore _showwarnmsg (#2504) (#2507)
https://github.com/python/cpython/commit/d0aac5da59b1bbd113e6081d7c807ad5bced8a05