Hello,
As I understood, now assert_warn just print message to stdout for warning.

https://github.com/ActivisionGameScience/assertpy/blob/master/assertpy/assertpy.py#L1104

        if self.kind == 'warn':
            print(out)

assert_warn - is a good idea but it's not useful in production.

I suppose to expand signature by following :

def assert_warn(val, description='', logger=None):

then, in error function:

        if self.kind == 'warn':
            if self.logger:
                logger.warning(out)
            else:
                print(out)  # actually it's better to print to stderr or use warning module

in this case we can pass particular logger (logging. getLogger('name') instance) or logging module to invoke warning() function. Or pass nothing to use default behavior.