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74 lines (50 loc) · 1.62 KB
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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: Node, *args, **kwargs) -> str:
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: Node, *args, **kwargs) -> str:
return "generic_visit " + node.__class__.__name__
def visit_B(self, node: Node, *args, **kwargs) -> str:
return "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()