how can I convert a dictionary to a string of keyword arguments?

The same syntax is used to accept arbitrary keyword arguments.

Python 2:

def somestring(**kwargs):
  return ', '.join('%s=%r' % x for x in kwargs.iteritems())

Python 3:

def somestring(**kwargs):
    return ", ".join(f"{key}={value}" for key, value in kwargs.items())

Note that dicts are arbitrarily ordered, so the resultant string may be in a different order than the arguments passed.