Python 类表示方法

发布于 2024-10-19 06:34:04 字数 343 浏览 2 评论 0原文

我知道方法 __repr__ 和 __str__ 的存在是为了给出类实例的正式和非正式表示。但是类对象是否也存在等价物，以便在打印类对象时可以显示它的良好表示？

>>> class Foo:
...     def __str__(self):
...         return "instance of class Foo"
...
>>> foo = Foo()
>>> print foo
instance of class Foo
>>> print Foo
__main__.Foo

原文

I know that methods __repr__ and __str__ exist to give a formal and informal representation of class instances. But does an equivalent exist for class objects too, so that when the class object is printed, a nice representation of it could be shown?

>>> class Foo:
...     def __str__(self):
...         return "instance of class Foo"
...
>>> foo = Foo()
>>> print foo
instance of class Foo
>>> print Foo
__main__.Foo

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三生一梦 2024-10-26 06:34:04

当您调用 print(foo) 时，会调用 foo 的 __str__ 方法。 __str__ 位于 foo 的类中，即 Foo。

同样，当您调用 print(Foo) 时，会调用 Foo 的 __str__ 方法。 __str__ 位于 Foo 类中，通常为 type。您可以使用元类更改它：

class FooType(type):
    def __str__(cls):
        return 'Me a Foo'
    def __repr__(cls):
        return '<Foo>'

class Foo(object):
    __metaclass__=FooType
    def __str__(self):
        return "instance of class Foo"

print(Foo)
# Me a Foo

print(repr(Foo))
# <Foo>

When you call print(foo), foo's __str__ method is called. __str__ is found in the class of foo, which is Foo.

Similarly, when you call print(Foo), Foo's __str__ method is called. __str__ is found in the class of Foo, which is normally type. You can change that using a metaclass:

class FooType(type):
    def __str__(cls):
        return 'Me a Foo'
    def __repr__(cls):
        return '<Foo>'

class Foo(object):
    __metaclass__=FooType
    def __str__(self):
        return "instance of class Foo"

print(Foo)
# Me a Foo

print(repr(Foo))
# <Foo>

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南冥有猫 2024-10-26 06:34:04

您也许可以使用元类来做到这一点，但是据我所知，普通类没有通用的解决方案。

如果只是您自己的类，您可以采用在元数据中包含特定类变量的编码标准，即：

class Whatever(object):
    classAuthor = "me"
    classCreated = "now"

或者如果您使用支持类装饰器的Python，您可以使用装饰器自动为您注释它或强制元数据存在。

但是...也许你只是想要AClass.__name__？

You might be able to do this with a metaclass, but AFAIK, there's no general solution for normal classes.

If it's just your own classes, you could adopt a coding standard of including a particular class variable with your metadata, ie:

class Whatever(object):
    classAuthor = "me"
    classCreated = "now"

Or if you're using a python that supports class decorators, you could use a decorator to annotate it for you automatically or enforce that the metadata is there.

But... maybe you just want AClass.__name__ ?

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哑剧 2024-10-26 06:34:04

在我看来，不能为类创建自定义 repr 字符串是件好事；类的要点是创建该类的实例。

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-柠檬树下少年和吉他 2024-10-26 06:34:04

您不能在类类型上使用 __repr__ 或 __str__，但可以使用 docstring 显示有关该类的信息

>>> class Foo:
...     """Foo's description"""
...     def __str__(self):
...         return "instance of class Foo"
... 
>>> foo = Foo()
>>> print foo
instance of class Foo
>>> print Foo.__doc__
Foo's description
>>> Foo.__doc__
"Foo's description"

You cannot use the __repr__ or __str__ on the class type but you can use the docstring to present information about the class

>>> class Foo:
...     """Foo's description"""
...     def __str__(self):
...         return "instance of class Foo"
... 
>>> foo = Foo()
>>> print foo
instance of class Foo
>>> print Foo.__doc__
Foo's description
>>> Foo.__doc__
"Foo's description"

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