如何更改对象的实例的函数参数的默认值？

问题描述

如果我有对象

>>> class example_class():
>>>    def example_function(number, text = 'I print this: '):
>>>        print text, number

我可以更改example_function输入参数

I can change the example_function input parameter

>>> example_instance = example_class()
>>> print example_instace.example_function(3, text = 'I print that: ')

每次使用 example_instace 时，总是使用我打印：。是否可以更改 text 的默认值，以便我得到此行为：

Now I would like to always use I print that: every time I use example_instace. Is it possible to change the default value of text so that I get this behavior:

>>> example_instace = example_class()
>>> print example_instance.example_function(3)
I print this: 3
>>> default_value(example_instance.text, 'I print that: ')
>>> print example_instance.example_function(3)
I print that: 3

推荐答案

函数默认值与函数一起存储，函数对象用于创建方法包装器。您不能在每个实例的基础上更改默认值。

Function defaults are stored with the function, and the function object is used to create the method wrapper. You cannot alter that default on a per-instance basis.

相反，使用sentinel来检测默认值已被选中; 无是适用于无本身不是有效值的常见哨兵：

Instead, use a sentinel to detect that the default has been picked; None is a common sentinel suitable for when None itself isn't a valid value:

class example_class():
    _example_text_default = 'I print this: '
    def example_function(self, number, text=None):
        if text is None:
            text = self._example_text_default
        print text, number

，然后只需在每个实例的基础上设置 self._example_text_default 即可覆盖。

and then simply set self._example_text_default on a per-instance basis to override.

None 不是合适的标记，请为作业创建唯一的单例对象：

If None is not a suitable sentinel, create a unique singleton object for the job:

_sentinel = object()

class example_class():
    _example_text_default = 'I print this: '
    def example_function(self, number, text=_sentinel):
        if text is _sentinel:
            text = self._example_text_default
        print text, number

$ b b

，现在您可以使用 example_class（）。example_function（42，无）作为有效的非默认值。

and now you can use example_class().example_function(42, None) as a valid non-default value.

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