在Python中使用元類實現工廠設計模式

Question

我在元類中找到了很多鏈接，其中大多數提到它們對於實現工廠方法很有用。你能給我看一個使用元類來實現設計模式的例子嗎？

Answer 1

我很想聽聽人們對這個意見，但我認爲這是你想要做

class FactoryMetaclassObject(type): 
    def __init__(cls, name, bases, attrs): 
     """__init__ will happen when the metaclass is constructed: 
     the class object itself (not the instance of the class)""" 
     pass 

    def __call__(*args, **kw): 
     """ 
     __call__ will happen when an instance of the class (NOT metaclass) 
     is instantiated. For example, We can add instance methods here and they will 
     be added to the instance of our class and NOT as a class method 
     (aka: a method applied to our instance of object). 

     Or, if this metaclass is used as a factory, we can return a whole different 
     classes' instance 

     """ 
     return "hello world!" 

class FactorWorker(object): 
    __metaclass__ = FactoryMetaclassObject 

f = FactorWorker() 
print f.__class__ 

內容示例

你會看到的結果是：type'str'

Answer 2

沒有必要。你可以override a class's __new__() method爲了返回一個完全不同的對象類型。

Answer 3

你可以找到wikibooks/python，here和here一些有用的例子