Question

I’m trying to figure out how to pass optional arguments from optparse. The problem I’m having is if an optparse option is not specified, it defaults to a None type, but if I pass the None type into a function, it yells at me instead of using the default (Which is understandable and valid).

conn = psycopg2.connect(database=options.db, hostname=options.hostname, port=options.port)

The question is, how do I use the function’s defaults for optional arguments but still pass in user inputs if there is an input without having a huge number of if statements.

Answer 1

Define a function remove_none_values that filters a dictionary for none-valued arguments.

def remove_none_values(d):
    return dict((k,v) for (k,v) in d.iteritems() if not v is None)

kwargs = {
  'database': options.db,
  'hostname': options.hostname,
  ...
}
conn = psycopg2.connect(**remove_none_values(kwargs))

Or, define a function wrapper that removes none values before passing the data on to the original function.

def ignore_none_valued_kwargs(f):
    @functools.wraps(f)
    def wrapper(*args, **kwargs):
        newkwargs = dict((k,v) for (k,v) in d.iteritems() if not v is None)
        return f(*args, **kwargs)
    return wrapper

my_connect = ignore_none_valued_kwargs(psycopg2)
conn = my_connect(database=options.db, hostname=options.hostname, port=options.port)