Using a self variable as a python method parameter

Question

Can I use my self variable as the method parameter such that I can achieve this:

myclass = My_Class(arg1=3)
output = myclass(1,2) # If the third argument is not given, use the self.arg1 variable
output = myclass(1,2,10) # If the third argument is given, use it instead of self.arg1

I tried the following code but it gives me an error

class My_Class():
 def __init__(self, arg1):
 self.arg1 = arg1
 
 def foo(self, x,y,z=self.arg1):
 return (x+y)**z

NameError: name 'self' is not defined

• John Gordon: You can't refer to self.arg1 in the parameter list like that.
• Klaus D.: The default value is evaluated only once at class initialization time. At this point no instance of the class and therefore no self exists.

Answer 1

The way that this might normally be done is to use None as the default parameter.

class My_Class():
 def __init__(self, arg1):
 self.arg1 = arg1
 
 def foo(self, x, y, z=None):
 if z is None:
 z = self.arg1
 return (x+y)**z

Answer 2

@2ps answer is excellent and is generally the accepted way to go. I just want to add that, if None is actually a valid argument to your function and you want to distinguish between None and "I didn't provide an argument", a little trick I've used in the past is to make my own private "none"-like object that no one else has access to.

_no_arg = object()
class My_Class():
 def __init__(self, arg1):
 self.arg1 = arg1
 
 def foo(self, x, y, z=_no_arg):
 if z is _no_arg:
 z = self.arg1
 return (x+y)**z