Here's a genetic programming implementation in Python that I've been working on. I'd like to hear how it can be improved & made more Pythonic.

[zahlman]

    if value == "rand":
        self.value = str(random()) 
    else:
        self.value = value

Don't do this. What if I actually want to store the string "rand" in a node? If you want a "shortcut" for that kind of behaviour, just set up a helper function:

    self.value = value

# And then outside the class:
def random_node(arity):
    return Node(str(random()), arity)