类方法生成“TypeError:...获得多个关键字参数值..."；

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问题描述

如果我这样定义一个带有关键字参数的类方法:

If I define a class method with a keyword argument thus:

class foo(object):
  def foodo(thing=None, thong='not underwear'):
    print thing if thing else "nothing" 
    print 'a thong is',thong

调用该方法会生成一个TypeError:

calling the method generates a TypeError:

myfoo = foo()
myfoo.foodo(thing="something")

...
TypeError: foodo() got multiple values for keyword argument 'thing'

怎么回事?

推荐答案

问题在于python中传递给类方法的第一个参数始终是调用该方法的类实例的副本，通常标记为自我.如果类是这样声明的:

The problem is that the first argument passed to class methods in python is always a copy of the class instance on which the method is called, typically labelled self. If the class is declared thus:

class foo(object):
  def foodo(self, thing=None, thong='not underwear'):
    print thing if thing else "nothing" 
    print 'a thong is',thong

它的行为符合预期.

说明:

没有self作为第一个参数，当myfoo.foodo(thing="something")被执行时，foodo方法被调用带参数 (myfoo, thing="something").然后将实例 myfoo 分配给 thing(因为 thing 是第一个声明的参数)，但是 python 也尝试分配 "something" 到 thing，因此异常.

Without self as the first parameter, when myfoo.foodo(thing="something") is executed, the foodo method is called with arguments (myfoo, thing="something"). The instance myfoo is then assigned to thing (since thing is the first declared parameter), but python also attempts to assign "something" to thing, hence the Exception.

为了演示，请尝试使用原始代码运行:

To demonstrate, try running this with the original code:

myfoo.foodo("something")
print
print myfoo

你会输出如下:

<__main__.foo object at 0x321c290>
a thong is something

<__main__.foo object at 0x321c290>

您可以看到thing"已分配给foo"类的实例myfoo"的引用.文档的这一部分解释了函数参数的工作原理.

You can see that 'thing' has been assigned a reference to the instance 'myfoo' of the class 'foo'. This section of the docs explains how function arguments work a bit more.

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Thing