Python：替换全局变量

我试图生成一个Ω的西格玛代数。 我正在寻找如何替换全局变量。 这里Ω=[1,2,3]

global sigma
sigma=[[]]
def buildDTree(sofar, todo):
    global sigma
    sal=[]
    if len(todo)==0:
        return binaryTree(sofar)
    else:
        withelt=buildDTree(sofar + [todo[0]],todo[1:])
        withoutelt=buildDTree(sofar, todo[1:])
        here=binaryTree(sofar)
        here.setLeftBranch(withelt)
        here.setRightBranch(withoutelt)
        sal+=(here.getLeftBranch().getValue())
        sigma+=[sal]
        return here
buildDTree([], [1,2,3])
print sigma

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最简单的方法是将关键字参数添加到递归函数中，并返回结果sigma：

def buildDTree(sofar, todo, sigma=None):
    if sigma is None:
        sigma = [[]]
    sal = []
    if not todo:
        return sigma, binaryTree(sofar)
    else:
        _, withelt = buildDTree(sofar + [todo[0]], todo[1:], sigma)
        _, withoutelt = buildDTree(sofar, todo[1:], sigma)
        here = binaryTree(sofar)
        here.setLeftBranch(withelt)
        here.setRightBranch(withoutelt)
        sal += here.getLeftBranch().getValue()
        sigma += [sal]
        return sigma, here

sigma, _ = buildDTree([], [1,2,3])

这至少使

sigma

成为递归调用的“局部”

更好的方法是创建一个类：

class DTreeBuilder(object):
    def __init__(self):
        self.sigma = [[]]

    def buildDTree(self, sofar, todo):
        sal = []
        if len(todo) == 0:
            return binaryTree(sofar)
        else:
            withelt = buildDTree(sofar + [todo[0]], todo[1:])
            withoutelt = buildDTree(sofar, todo[1:])
            here = binaryTree(sofar)
            here.setLeftBranch(withelt)
            here.setRightBranch(withoutelt)
            sal += here.getLeftBranch().getValue()
            self.sigma += [sal]
            return here

然后像这样使用它：

builder = DTreeBuilder()
builder.build([], [1,2,3])
print builder.sigma

---

编写一个类并使用一个属性。