Python 类 : Why can't I use the method len() inside __eq__(self, 其他)？

Question

https://gist.github.com/rjurney/1e8454af8e44312d02d7

class FrozenSortedTuple:
  """A frozenset that cares about order of tuples. And is not actually frozen."""
  def __init__(self, vals):
    if type(vals) not in [list, set, tuple]:
      raise Exception('Value not a list or set')
    self.vals = list(vals)

  def __eq__(self, other):
    if len(self.vals) != len(other):
      return False
    if type(self.vals) != type(other):
      return False
    if type(other) not in [list, set, tuple]:
      return False
    other_list = list(other)
    for i,item in enumerate(self.vals):
      if item != other_list[i]:
        return False
    return True

在 iPython 中调用:

In [2]: a = ['a','b']

In [3]: b = ['b','a']

In [4]: c = ['a','b']

In [5]: a == b
Out[5]: False

In [6]: FrozenSortedTuple(a)
Out[6]: <__main__.FrozenSortedTuple instance at 0x103c56200>

In [7]: fa = FrozenSortedTuple(a)

In [8]: fb = FrozenSortedTuple(b)

In [9]: fa == fb

错误:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-317181571e4d> in <module>()
----> 1 fa == fb

<ipython-input-1-ef99f0af5061> in __eq__(self, other)
     15 
     16   def __eq__(self, other):
---> 17     if len(self.vals) != len(other):
     18       return False
     19     if type(self.vals) != type(other):

AttributeError: FrozenSortedTuple instance has no attribute '__len__'

我很困惑。

Answer 1

如果您尝试直接比较两个 FrozenSortedTuples 值的结构和内容，请将所有 other 实例更改为 other.vals。

def __eq__(self, other):
    if len(self.vals) != len(other.vals):
      return False
    if type(self.vals) != type(other.vals):
      return False
    if type(other.vals) not in [list, set, tuple]:
      return False
    other_list = list(other.vals)
    for i,item in enumerate(self.vals):
      if item != other_list[i]:
        return False
    return True

当然，如果 other 不是 FrozenSortedTuple，这将无法工作。例如，fa == 23 将不起作用，因为数字 23 没有“vals”属性。