blocks|key|855372|text|请参阅abc+module。基本上，您在类上定义__metaclass__+=+abc.ABCMeta，然后用@abc.abstractmethod修饰每个抽象方法。除非重写了所有抽象方法，否则无法实例化从该类派生的类。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|855373|如果您的类已经在使用元类，那么可以从ABCMeta而不是type派生它，这样您就可以继续使用自己的元类。|855374|一种廉价的替代方法(也是引入abc模块之前的最佳实践)是让您的所有抽象方法只引发一个异常(NotImplementedError是一个很好的异常)，这样从它派生的类就必须覆盖该方法才能有用。|855375|然而，abc解决方案更好，因为它根本不会实例化这样的类(即，它“失败得更快”)，还因为您可以在派生类中使用super()函数提供每个方法的默认或基本实现。|855376|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/library/abc.html^0|O|R|1J|J|3|A|0|0|I|7|S|4|0|E|3|19|J|0|3|3|1H|7|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@$9|Y|A|Z|1|10]]|E|$]]|$1|F|3|G|5|6|7|11|8|@$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|M|$N|$5|O|P|Q|E|$R|S]]]]

See the <a href="http://docs.python.org/library/abc.html" rel="noreferrer">abc module</a>. Basically, you define <code>__metaclass__ = abc.ABCMeta</code> on the class, then decorate each abstract method with <code>@abc.abstractmethod</code>. Classes derived from this class cannot then be instantiated unless all abstract methods have been overridden.

If your class is already using a metaclass, derive it from <code>ABCMeta</code> rather than <code>type</code> and you can continue to use your own metaclass.

A cheap alternative (and the best practice before the <code>abc</code> module was introduced) would be to have all your abstract methods just raise an exception (<code>NotImplementedError</code> is a good one) so that classes derived from it would have to override that method to be useful. 

However, the <code>abc</code> solution is better because it keeps such classes from being instantiated at all (i.e., it "fails faster"), and also because you can provide a default or base implementation of each method that can be reached using the <code>super()</code> function in derived classes.

blocks|key|855412|text|在介绍abc之前，您会经常看到这种情况。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|855413|class+Base(object):
++++def+go(self):
++++++++raise+NotImplementedError("Please+Implement+this+method")


class+Specialized(Base):
++++def+go(self):
++++++++print+"Consider+me+implemented"|code-block|syntax|javascript|855414|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Before abc was introduced you would see this frequently.

<pre><code>class Base(object):
 def go(self):
 raise NotImplementedError("Please Implement this method")


class Specialized(Base):
 def go(self):
 print "Consider me implemented"
</code></pre>

blocks|key|855459|text|抽象基类是很有魔力的。我定期使用它们实现一些东西，并对自己的聪明感到惊讶，不久之后，我发现自己完全被自己的聪明搞糊涂了(尽管这可能只是个人的限制)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|855460|实现这一点的另一种方法(如果你问我，应该在python+std库中)是制作一个装饰器。|855461|def+abstractmethod(method):
++++"""
++++An+@abstractmethod+member+fn+decorator.
++++(put+this+in+some+library+somewhere+for+reuse).

++++"""
++++def+default_abstract_method(*args,+**kwargs):
++++++++raise+NotImplementedError('call+to+abstract+method+'+
++++++++++++++++++++++++++++++++++%2B+repr(method))
++++default_abstract_method.__name__+=+method.__name__++++
++++return+default_abstract_method


class+Shape(object):

++++def+__init__(self,+shape_name):
+++++++self.shape+=+shape_name

++++@abstractmethod
++++def+foo(self):
++++++++print+"bar"
++++++++return

class+Rectangle(Shape):
++++#+note+you+don't+need+to+do+the+constructor+twice+either
++++pass++

r+=+Rectangle("x")
r.foo()|code-block|syntax|javascript|855462|装饰器不是我写的。我突然想到有人会这么做的。你可以在这里找到：http://code.activestate.com/recipes/577666-abstract-method-decorator/+Good+one+jimmy2times。请注意该页r.e底部的讨论。装饰器的类型安全性。(如果有人愿意的话，这可以用inspect模块来修复)。|offset|length|855463|entityMap|0|LINK|mutability|MUTABLE|url|http://code.activestate.com/recipes/577666-abstract-method-decorator/^0|0|0|0|V|1X|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

Abstract base classes are deep magic. Periodically I implement something using them and am amazed at my own cleverness, very shortly afterwards I find myself completely confused by my own cleverness (this may well just be a personal limitation though).

Another way of doing this (should be in the python std libs if you ask me) is to make a decorator.

<pre><code>def abstractmethod(method):
 """
 An @abstractmethod member fn decorator.
 (put this in some library somewhere for reuse).

 """
 def default_abstract_method(*args, **kwargs):
 raise NotImplementedError('call to abstract method ' 
 + repr(method))
 default_abstract_method.__name__ = method.__name__ 
 return default_abstract_method


class Shape(object):

 def __init__(self, shape_name):
 self.shape = shape_name

 @abstractmethod
 def foo(self):
 print "bar"
 return

class Rectangle(Shape):
 # note you don't need to do the constructor twice either
 pass 

r = Rectangle("x")
r.foo()
</code></pre>

I didn't write the decorator. It just occurred to me someone would have. You can find it here: <a href="http://code.activestate.com/recipes/577666-abstract-method-decorator/" rel="noreferrer">http://code.activestate.com/recipes/577666-abstract-method-decorator/</a> Good one jimmy2times. Note the discussion at the bottom of that page r.e. type safety of the decorator. (That could be fixed with the inspect module if anyone was so inclined).

blocks|key|1069887|text|您可以使用six和abc高效地为python2和python3构造一个类，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1069888|import+six
import+abc

@six.add_metaclass(abc.ABCMeta)
class+MyClass(object):
++++"""
++++documentation
++++"""

++++@abc.abstractmethod
++++def+initialize(self,+para=None):
++++++++"""
++++++++documentation
++++++++"""
++++++++raise+NotImplementedError|code-block|syntax|javascript|1069889|This+is+an+awesome+document+of+it.|offset|length|1069890|entityMap|0|LINK|mutability|MUTABLE|url|https://pymotw.com/3/abc/^0|0|0|0|Y|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

You can use six and abc to construct a class for both python2 and python3 efficiently as follows: 

<pre><code>import six
import abc

@six.add_metaclass(abc.ABCMeta)
class MyClass(object):
 """
 documentation
 """

 @abc.abstractmethod
 def initialize(self, para=None):
 """
 documentation
 """
 raise NotImplementedError
</code></pre>

<a href="https://pymotw.com/3/abc/" rel="nofollow noreferrer">This is an awesome document of it.</a>

I am having trouble in using inheritance with Python. While the concept seems too easy for me in Java yet up till now I have been unable to understand in Python which is surprising to me at least.

I have a prototype which follow:

<pre><code>class Shape():
 def __init__(self, shape_name):
 self.shape = shape_name

class Rectangle(Shape):
 def __init__(self, name):
 self.shape = name
</code></pre>

In the above code how can I make an abstract method that would need to be implemented for all the subclasses?

Abstract methods in Python

Python

Java

我在Python中使用继承时遇到了问题。虽然这个概念在Java中对我来说似乎太容易了，但到目前为止，我还不能理解Python，这至少让我感到惊讶。我有一个原型，如下：class Shape():   def __init__(self, shape_name):       self.shape = shape_nam...

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