用不同的格式选项实现 __str__ 方法的 Pythonic 方法是什么? [英] what is the Pythonic way to implement a __str__ method with different format options?

问题描述

我想创建一个 __str__ 方法，根据用户选择创建各种格式的字符串.

我想出的最好的方法是制作一个 __str__(**kwargs) 方法，这似乎可以正常工作，但它与 str(obj) 不兼容 或 print(obj).换句话说，我必须使用 print(obj.__str__(style='pretty')) 而不是 print(obj, style='pretty').

解决方案

实施 object.__format__() 方法，然后用户可以使用 format() 函数 和 str.format() 方法:

print(format(obj, 'pretty'))

或

print('这个对象很漂亮:{:pretty}'.format(obj))

您可能希望将大部分格式处理委托给 str.__format__:

def __format__(self, spec):if spec.endswith('漂亮'):美化 = self.pretty_version()返回 prettified.__format__(spec[:-6])返回 str(self).__format__(spec)

这样您仍然可以支持默认 str.__format__ 方法支持的所有字段宽度和填充对齐选项.

演示:

<预><代码>>>>类 Foo():... def __str__(self):... 返回 'plain foo'...定义漂亮版本(自我):...返回'漂亮的foo'... def __format__(self, spec):...如果 spec.endswith('pretty'):... 美化 = self.pretty_version()...返回 prettified.__format__(spec[:-6])...返回 str(self).__format__(spec)...>>>f = Foo()>>>打印(f)纯 foo>>>打印(格式(f))纯 foo>>>打印(格式(f，'漂亮'))漂亮的 foo>>>打印(格式(f，'> 20pretty'))漂亮的 foo>>>print('这个对象很漂亮:{:^20pretty}!'.format(f))这个对象很漂亮:漂亮的 foo ！

I'd like to create a __str__ method that creates the string in various formats according to user choice.

The best I have come up with is to make a __str__(**kwargs) method, and this seems to work ok, but it isn't compatible with str(obj) or print(obj). In other words I have to use print(obj.__str__(style='pretty')) rather than print(obj, style='pretty').

解决方案

Implement the object.__format__() method instead, and a user can then specify the formatting required with the format() function and str.format() method:

print(format(obj, 'pretty'))

or

print('This object is pretty: {:pretty}'.format(obj))

You probably want to delegate most of the handling of the format on to str.__format__:

def __format__(self, spec):
    if spec.endswith('pretty'):
        prettified = self.pretty_version()
        return prettified.__format__(spec[:-6])
    return str(self).__format__(spec)

That way you can still support all the field width and padding alignment options that the default str.__format__ method supports.

Demo:

>>> class Foo():
...     def __str__(self):
...         return 'plain foo'
...     def pretty_version(self):
...         return 'pretty foo'
...     def __format__(self, spec):
...         if spec.endswith('pretty'):
...             prettified = self.pretty_version()
...             return prettified.__format__(spec[:-6])
...         return str(self).__format__(spec)
... 
>>> f = Foo()
>>> print(f)
plain foo
>>> print(format(f))
plain foo
>>> print(format(f, 'pretty'))
pretty foo
>>> print(format(f, '>20pretty'))
          pretty foo
>>> print('This object is pretty: {:^20pretty}!'.format(f))
This object is pretty:      pretty foo     !

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