NameError:名称"self"未定义 [英] NameError: name 'self' is not defined

问题描述

为什么这样的结构

class A:
    def __init__(self, a):
        self.a = a

    def p(self, b=self.a):
        print b

给出错误NameError: name 'self' is not defined?

推荐答案

默认参数值在函数定义时求值，但是self是仅在函数调用时可用的参数.因此，参数列表中的参数不能互相引用.

Default argument values are evaluated at function define-time, but self is an argument only available at function call time. Thus arguments in the argument list cannot refer each other.

将参数默认为None并在代码中为此添加测试是一种常见的模式:

It's a common pattern to default an argument to None and add a test for that in code:

def p(self, b=None):
    if b is None:
        b = self.a
    print b

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