In order to move the warnings.showwarning() API forward to support the new 
'line' argument, a DeprecationWarning is needed for implementations that 
lack support for it.

From the Python side a simple check for the 'line' argument using 
'inspect' will work. For the C side, a check that there is more than 1 
default argument should suffice, although a quick check of what other ways 
to introspect Python function objects from C code should be done first 
before settling on this solution.

Brett, can you list the places showwarning is used?

On Sat, May 3, 2008 at 2:11 PM, Benjamin Peterson
<report@bugs.python.org> wrote:
>
>  Benjamin Peterson <musiccomposition@gmail.com> added the comment:
>
>  Brett, can you list the places showwarning is used?
>

Lib/warnings.py and Python/_warnings.c. In both cases just search for
"showwarning"; there is only a single call site in both files.

r62720 and r62722 have the fix (and a fix for the fix). In the Python code 
I check for an argument named 'line' and in the C code I make sure there 
are at least two default arguments. Both assume a Python implementation 
for showwarning(), which should be a reasonable assumption.