python - 如何在 Python 中区分未分配的变量和零？

Question

一些外部代码运行以下代码的我的功能：

def __init__(self,weights=None,threshold=None):

    print "weights: ", weights
    print "threshold: ", threshold

    if weights:
        print "weights assigned"
        self.weights = weights
    if threshold:
        print "threshold assigned"
        self.threshold = threshold

这段代码输出：

weights:  [1, 2]
threshold:  0
weights assigned

即打印运算符的行为就像threshold是零，而if运算符的行为就像它没有定义一样。

正确的解释是什么？怎么了？参数的状态是什么threshold，如何识别？

Answer 1

使用if weights is not None而不是if weights.

更多细节：当您说if weights您要求 Pythonweights在布尔上下文中进行评估时，许多事情可能是“假等价”（或“假”）0，包括空字符串、空容器等。如果您只想检查对于一个None值，你必须明确地这样做。

Answer 2

您可以显式测试一个None值。

def __init__(self,weights=None,threshold=None):
    print "weights: ", weights
    print "threshold: ", threshold

    if weights is not None:
        print "weights assigned"
        self.weights = weights
    if threshold is not None:
        print "threshold assigned"
        self.threshold = threshold