Python类的奇怪行为 [英] Odd behaviour of python's class

问题描述

这里是一个python类：

Here is a python class:

class TxdTest(object):
    def __init__(self, name = '', atrributes = []):
        self.name = name
        self.attributes = atrributes

b $ b

然后我使用它像这样：

and then I use it like this:

def main():
    for i in range(3):
        test = TxdTest()
        print test.name
        print test.attributes
        test.name = 'a'
        test.attributes.append(1)

那么，结果是什么？结果是：

so, what's the result? The result is:

[]

[1]

[1，1]

为什么类中的self.attributes仍然获得值？

Why the 'self.attributes' in class still obtain the value ?

推荐答案

将可变对象（如列表）传递给python中的函数或方法时，调用函数或方法。这意味着无论何时调用该函数或方法（在这种情况下，您的 __ init __ 方法），每次都使用完全相同的列表，继续前面的修改。

When passing a mutable object (like a list) to a function or method in python, a single object is used across all invocations of the function or method. This means that ever time that function or method (in this case, your __init__ method) is called, the exact same list is used every time, holding on to modifications made previously.

如果您希望有一个空列表作为默认列表，您应该执行以下操作：

If you want to have an empty list as a default, you should do the following:

class TxdTest(object):
    def __init__(self, name = '', atrributes = None):
        self.name = name
        if attributes is None
             self.attributes = []
        else:
             self.attributes = attributes

有关此工作原理的详细说明，请参阅：最少惊讶在Python中：可变默认参数，如bgporter所述。

For a detailed explanation of how this works see: "Least Astonishment" in Python: The Mutable Default Argument, as mentioned by bgporter.

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