How to check whether optional function parameter is set

Question

Is there an easy way in Python to check whether the value of an optional parameter comes from its default value, or because the user has set it explicitly at the function ca

Answer 1

I've seen this pattern a few times (e.g. library unittest, py-flags, jinja):

class Default:
    def __repr__( self ):
        return "DEFAULT"

DEFAULT = Default()

...or the equivalent one-liner...:

DEFAULT = type( 'Default', (), { '__repr__': lambda x: 'DEFAULT' } )()

Unlike DEFAULT = object(), this assists type-checking and provides information when errors occur -- frequently either the string representation ("DEFAULT") or the class name ("Default") are used in error messages.