Subclassing numpy.ndarray - 为什么__array_finalize__在这里没有被调用两次？

Question

根据this的子类ndarray的底层引擎，保证调用__array_finalize__方法，无论子类是直接实例化，作为视图转换还是从模板创建。

特别是，当明确调用构造函数时，调用方法的顺序是__new__→__array_finalize__→__init__。

我有以下简单的子类ndarray它允许额外的title属性。

class Frame(np.ndarray):  
    def __new__(cls, input_array, title='unnamed'): 
     print 'calling Frame.__new__ with title {}'.format(title) 
     self = input_array.view(Frame) # does not call __new__ or __init__ 
     print 'creation of self done, setting self.title...' 
     self.title = title 
     return self 

    def __array_finalize__(self, viewed): 
     # if viewed is None, the Frame instance is being created by an explicit 
     # call to the constructor, hence Frame.__new__ has been called and the 
     # title attribute is already set 
     # 
     # if viewed is not None, the frame is either being created by view 
     # casting or from template, in which case the title of the viewed object 
     # needs to be forwarded to the new instance 
     print '''calling Frame.__array_finalize__ with type(self) == {} and 
     type(viewed) == {}'''.format(type(self), type(viewed)) 

     if viewed is not None: 
      self.title = getattr(viewed, 'title', 'unnamed') 

     print self.title 

产生以下输出：

>>> f = Frame(np.arange(3), 'hallo') 
calling Frame.__new__ with title hallo 
calling Frame.__array_finalize__ with type(self) == <class '__main__.Frame'> and 
     type(viewed) == <type 'numpy.ndarray'> 
unnamed 
creation of self done, setting self.title... 
>>> f.title 
'hallo' 

正如你所看到的，__array_finalize__被称为行的结果

self = input_array.view(Frame) 

问：为什么__array_finalize__不会被调用再次作为__new__→__array_finalize__→__init__链的一部分？

Answer 1

在您链接到的文档中，它描述了ndarray.__new__将如何在其构建的阵列上调用__array_finalize__。当你创建实例为现有数组的view时，你的班级的__new__方法会导致这种情况发生。数组参数的view方法将为您调用ndarray.__new__，并在实例返回给您之前调用__array_finalize__方法。

您没有看到__array_finalize__调用了两次，因为您没有再次调用ndarray.__new__。如果您的__new__方法除了呼叫view之外还包含对super().__new__的呼叫，您可能会看到__array_finalized__呼叫两次。这样的行为可能会有问题（或者至少比必要的慢），所以毫无疑问你不会这么做！

当覆盖子类的方法被调用时，Python不会自动调用覆盖方法。取决于重写方法调用（或不调用）被覆盖的版本（直接与super或在这种情况下，通过另一个对象的view方法间接）。