Question

I am translating some code from lisp to Python.

In lisp, you can have a let construct with the variables introduced declared as special and thus having dynamic scope. (See http://en.wikipedia.org/wiki/Dynamic_scope#Dynamic_scoping)

How can I do likewise in Python? It seems the language does not support this directly, if true, what would be a good way to emulate it?

Answer 1

I feel Justice is plain right in his reasoning here.

On the other hand — I can’t resist implementing proof of concept for still another programing paradigm “unnatural” to Python — I simply love doing this. 🙂

So, I created a class whose objects’attributes are scopped just like you require (and can be created dynamically). As I said, it is just in a proof of concept state – but I think most usual errors, (like trying to access a variable ina scope it is not defined at all) should have errors raised, even if not the proper ones (IndexError due to a stack underflow instead of AttributeError, for example)

import inspect


class DynamicVars(object):
    def __init__(self):
        object.__setattr__(self, "variables", {})

    def normalize(self, stackframe):
        return [hash(tpl[0]) for tpl in stackframe[1:]]

    def __setattr__(self, attr, value):
        stack = self.normalize(inspect.stack())
        d = {"value": value, "stack": stack}
        if not attr in self.variables:
            self.variables[attr] = []
            self.variables[attr].append(d)
        else:
            our_value = self.variables[attr]
            if our_value[-1]["stack"] == stack:
                our_value[-1]["value"] = value
            elif len(stack) <= len(our_value):
                while our_value and stack !=  our_value["stack"]:
                    our_value.pop()
                our_value.append(d)
            else: #len(stack) > len(our_value):
                our_value.append(d)
    def __getattr__(self, attr):
        if not attr in self.variables:
            raise AttributeError
        stack = self.normalize(inspect.stack())
        while self.variables[attr]:
            our_stack = self.variables[attr][-1]["stack"]
            if our_stack == stack[-len(our_stack):]:
                break
            self.variables[attr].pop()
        else:
            raise AttributeError
        return self.variables[attr][-1]["value"]


# for testing:
def c():
    D = DynamicVars()
    D.c = "old"
    print D.c
    def a():
        print D.c
    a()
    def b():
        D.c = "new"
        a()
    b()
    a()
    def c():
        D.c = "newest"
        a()
        b()
        a()
    c()
    a()

c()

2020 update – Another similar question showed up, and I crafted a hack that needs no special namespace objects (but which resorts to using inner things from cPython, like updating the locals() to actual variables: https://stackoverflow.com/a/61015579/108205 (works with Python 3.8)