visitor.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
http://peter-hoffmann.com/2010/extrinsic-visitor-pattern-python-inheritance.html

*TL;DR
Separates an algorithm from an object structure on which it operates.

An interesting recipe could be found in
Brian Jones, David Beazley "Python Cookbook" (2013):
- "8.21. Implementing the Visitor Pattern"
- "8.22. Implementing the Visitor Pattern Without Recursion"

*Examples in Python ecosystem:
- Python's ast.NodeVisitor: https://github.com/python/cpython/blob/master/Lib/ast.py#L250
which is then being used e.g. in tools like `pyflakes`.
- `Black` formatter tool implements it's own: https://github.com/ambv/black/blob/master/black.py#L718
"""


class Node(object):
    pass


class A(Node):
    pass


class B(Node):
    pass


class C(A, B):
    pass


class Visitor(object):
    def visit(self, node, *args, **kwargs):
        meth = None
        for cls in node.__class__.__mro__:
            meth_name = 'visit_' + cls.__name__
            meth = getattr(self, meth_name, None)
            if meth:
                break

        if not meth:
            meth = self.generic_visit
        return meth(node, *args, **kwargs)

    def generic_visit(self, node, *args, **kwargs):
        print('generic_visit ' + node.__class__.__name__)

    def visit_B(self, node, *args, **kwargs):
        print('visit_B ' + node.__class__.__name__)


def main():
    """
    >>> a, b, c = A(), B(), C()
    >>> visitor = Visitor()

    >>> visitor.visit(a)
    generic_visit A

    >>> visitor.visit(b)
    visit_B B

    >>> visitor.visit(c)
    visit_B C
    """


if __name__ == "__main__":
    import doctest
    doctest.testmod()