Making functions non override-able

Since Python has monkey patching, not only can you not make anything "private". Even if you could, someone could still monkeypatch in a new version of the method function.

You can use this kind of name as a "don't go near" warning.

class Foo( object ):
    def _roo( self ):
       """Change this at your own risk."""

That's the usual approach. Everyone can read your source. They were warned. If they boldly go where they were warned not to go, they get what they deserve. It doesn't work and you can't help them.

You can try to make this intentionally obcure with inner classes and "hidden" implementation modules that are called by the "private" methods. But... everyone has your source. You can't prevent anything. You can only advise people of the consequences of their actions.

Python 3.8 (released Oct/2019) adds final qualifier to typing.

A final qualifier was added to the typing module---in the form of a final decorator and a Final type annotation---to serve three related purposes:

• Declaring that a method should not be overridden
• Declaring that a class should not be subclassed
• Declaring that a variable or attribute should not be reassigned

from typing import final

class Base:
    @final
    def foo(self) -> None:
        ...

class Derived(Base):
    def foo(self) -> None:  # Error: Cannot override final attribute "foo"
                            # (previously declared in base class "Base")
        ...

It is in line with what your were asking and is supported by core Python now.

Have a look at PEP-591 for more details.

You can use a metaclass:

class NonOverridable(type):
    def __new__(self, name, bases, dct):
        if bases and "roo" in dct:
            raise SyntaxError, "Overriding roo is not allowed"
        return type.__new__(self, name, bases, dct)

class foo:
    __metaclass__=NonOverridable
    ...

The metatype's new is called whenever a subclass is created; this will cause an error in the case you present. It will accept a definition of roo only if there are no base classes.

You can make the approach more fancy by using annotations to declare which methods are final; you then need to inspect all bases and compute all final methods, to see whether any of them is overridden.

This still doesn't prevent somebody monkey-patching a method into a class after it is defined; you can try to catch these by using a custom dictionary as the classes' dictionary (which might not work in all Python versions, as classes might require the class dictionary to be of the exact dict type).