Python: Use private static method as default

Question

I have a class member which accepts a function:

class A:

    def func(self, method):
       ...

I want to set a default method since that behavior is desired 99% of the time. This default behavior is static since it does not depend on any members of the class. However, I would like this default method to be private and invisible to the user. Is there any way of accomplishing that?

This is what I have tried:

1. class A:
   
       @staticmethod
       def __meth(x):
          pass
   
       def func(self, method = meth):
          pass
   

   Error: 'staticmethod' object is not callable

2. class A:
   
       @staticmethod
       def __meth(x):
          pass
   
       def func(self, method = A.__meth):
          pass
   

   Error: NameError: name 'A' is not defined

3. class A:
   
       @staticmethod
       def __meth(x):
          pass
   
       def func(self, method = self.__meth):
          pass
   

   Error: NameError: name 'self' is not defined

I am using Python 3.5 and do not want to rely on newer features.

Answer 1

It's fairly idiomatic to use None as the default and assign it as needed:

class A:

    @staticmethod
    def __meth(x):
        print(x)

    def func(self, method=None):
        if method is None:
            method = self.__meth

        method("x")

Answer 2

The problems start with your default parameter. These parameters are evaluated whilst the class definition is being read, and so class A is not yet defined.

You should handle it like a normal default parameter:

class A:

    @staticmethod
    def __meth(x):
       print('meth')

    def func(self, method = None):
        if method is None:
            self.__meth(1)
        else:
             method()


def foo():
    print('foo')

a = A()

a.func()
a.func(foo)

Output:

meth
foo

Answer 3

You can delay name resolution by putting it into a lambda:

class A:

   @staticmethod
   def __meth(x):
       pass

   def func(self, method = lambda s: A.__meth(s)):
       pass