Question

根据关于子类ndarray的{{3}}引子，无论子类是直接实例化，作为视图转换还是从模板创建，都可以保证调用__array_finalize__方法

特别是，当显式调用构造函数时，调用的方法的顺序是__new__ - ＆gt; __array_finalize__ - ＆gt; __init__。

我有ndarray的以下简单子类，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'

如您所见，__array_finalize__作为行

的结果被调用

 self = input_array.view(Frame)

问题：为什么__array_finalize__不会被__new__ - ＆gt;再次调用__array_finalize__ - ＆gt; __init__链？

Answer 1

在您链接到的文档中，它描述了ndarray.__new__如何在其构造的阵列上调用__array_finalize__。当您将实例创建为现有数组的__new__时，您的类的view方法会导致这种情况发生。数组参数的view方法为您调用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__被叫两次，因为您没有再次呼叫ndarray.__new__。如果__new__方法除super().__new__来电之外还包含对view的调用，您可能会看到__array_finalized__被调用两次。这样的行为可能是错误的（或者至少比必要的慢），所以你不会这样做也就不足为奇了！

当调用重写子类的方法时，Python不会自动调用重写方法。调用（或不调用）被覆盖的版本（直接使用super或者在本例中间接通过另一个对象的view方法）取决于重写方法。

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

1 个答案: