Question

I have a private method def __pickSide(self): in a parent class that I would like to override in the child class. However, the child class still calls the inherited def __pickSide(self):. How can I override the function? The child class’s function name is exactly the same as the parent’s function name.

Answer 1

Let’s look at the easiest example:

from dis import dis

class A(object):
  def __pick(self):
      print "1"

  def doitinA(self):
      self.__pick()

class B(A):
  def __pick(self):
      print "2"

  def doitinB(self):
      self.__pick()

b = B()
b.doitinA() # prints 1
b.doitinB() # prints 2

dis(A.doitinA)
print
dis(B.doitinB)

The disassembly is as follows:

  8           0 LOAD_FAST                0 (self)
              3 LOAD_ATTR                0 (_A__pick)
              6 CALL_FUNCTION            0
              9 POP_TOP
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE

 15           0 LOAD_FAST                0 (self)
              3 LOAD_ATTR                0 (_B__pick)
              6 CALL_FUNCTION            0
              9 POP_TOP
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE

As you can see, Python mangles function names that begin with two underscores (and accesses to such names!!) to a name that includes the class name – in this case _A__pick and _B__pick). That means that the class in which a function is defined determines which of the __pick methods is called.

The solution is simple, avoid pseudo-private methods by removing the double underscores. For example, use _pick instead of __pick.