如何在Python中创建动态范围变量? [英] How to create dynamical scoped variables in Python?

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问题描述

我正在将一些代码从Lisp转换为Python.

I am translating some code from lisp to Python.

在lisp中,您可以使用let构造,其中引入的变量声明为special,因此具有动态作用域. (请参见 http://en.wikipedia.org/wiki/Dynamic_scope#Dynamic_scoping )

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)

我该如何在Python中做同样的事情?似乎该语言不直接支持此功能,如果为true,将有什么更好的方法来模仿它?

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?

推荐答案

在他的推理中,我认为正义是正确的.

I feel Justice is plain right in his reasoning here.

另一方面-我无法抗拒为Python的另一种非自然的"编程范例实现概念证明-我只是喜欢这样做. :-)

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. :-)

因此,我创建了一个类,其对象属性的定义与您的要求相同(并且可以动态创建).就像我说的那样,它只是处于概念验证状态-但是我认为大多数常见的错误(例如尝试访问根本没有定义的作用域中的变量)都应该引发错误,即使不是正确的错误(IndexError)例如由于堆栈下溢而不是AttributeError)

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年更新-出现了另一个类似的问题,我设计了一种不需要特殊名称空间对象的hack(但是该方法使用了cPython的内部功能,例如将locals()更新为实际变量) : https://stackoverflow.com/a/61015579/108205 (适用于Python 3.8)

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)

这篇关于如何在Python中创建动态范围变量?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!

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