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"""

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:

pass

class A(Node):

pass

class B(Node):

pass

class C(A, B):

pass

class Visitor:

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()