[Python-ideas] Decorators for variables

Matthias welp boekewurm at gmail.com
Fri Apr 1 13:14:41 EDT 2016

> An example of the transformation would help here

An example, that detects cycles in a graph, and doesn't do an update if
the graph has cycles.

class prevent_cycles(property):
        This uses nodes that point to only one other node: if there is a
        in the current subgraph, it will detect that within O(n)
    def __init__(self, value):
        self._value = None

    def getter(self):
        return self._value

    def setter(self, value):
        if not turtle_and_hare(value):
            self._value = value
            raise Exception("cycle detected, shutting down")

    def deleter(self):
        del self._value

    def turtle_and_hare(self, other):
            Generic turtle and hare implementation. true if cycle, false if
            Returns True if cyclic from this point, false if it is not.
        turtle = other
        hare = other
        fieldname = self.__name__
        # this assuming that properties have access to their name, but that
        # would also be the same as a function.

        while True:
            if hare is None:
                return False

            hare = getattr(hare, fieldname)

            if hare is None:
                return False
            if hare is turtle:
                return True

            hare = getattr(hare, fieldname)
            turtle = getattr(turtle, fieldname)

            if hare is turtle:
                return True

class A(object):
    def __init__(self, parent):
        self.parent = parent

This would prevent cycles from being created in this object A, and would
some highly reusable code. The same can be done for @not_none, etc, to
some states which may be unwanted.
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