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我有这样一个类结构：

list --(parent)--> ClipList --(parent)--> VolumeList

ClipList 只能存储 Clip 对象，而 VolumeList - 只能存储 Volume 对象。这有效（即从 ClipList 继承的方法适用于 VolumeList），因为 Volume 类继承自 Clip 类。因此，我尝试使用 Volume 和 VolumeList 类实现的目的是让 Clip 和 ClipList 的风格与原来的不同，从而拥有自己的类。

当我尝试从 VolumeList 调用 Volume 对象时，就会出现问题。为了找到一个被调用的剪辑的位置，我运行了一个二分搜索（因为列表是有序的）。搜索是一个简单的二进制搜索，并且脱离了上下文可以工作，所以我认为不值得在这里发布代码。有问题的一点是这个条件：

if ( self == [] ):
        return None

现在，二分查找是在ClipList类中实现的，并且仅由 VolumeList 继承。上述条件在使用 ClipList 时没有引起任何问题，但是当使用（已填充的）VolumeList 运行时，条件

self == []

是，由于某种原因，实际上True。因此，二分查找总是返回 None，并且我无法通过名称（它们的排序）访问列表中的任何 Volume 对象，它们只能通过索引访问。

现在，填充的 VolumeList 看起来像这样（虽然大得多）：

[<__main__.Volume instance at 0xb6f3b1ac>, <__main__.Volume instance at 0xb6f3b24c>]

另外值得一提的是 ClipList 与 VolumeList 位于不同的模块中 - 这可能有作用，但我不确定。

如果需要，我可以发布更多代码，但我觉得问题（和解决方案）在于上面的位；如果与上述位分开测试，其他代码似乎可以工作。

问题可能出在哪里，为什么会出现这种情况True？

这是代码：

  1. ClipList.findIndex：

    def findIndex(self, clip, start = 0, end = None, pos = 0):

    # Finds the index of the place where the given clip should be added.
    # It does that by looking at the *tapes* of the clips, and finds
    # a place where the *tape* of the clip could be inserted without
    # distorting the order. It then looks at clip's timecode to put it
    # in the right place according to the timecode. The 'pos' parameter
    # carries the information about the index which we will return.

    # Define the right end place if none is specified
    if ( end == None ):
        end = len(self) - 1

    # But first of all, check if there is a need to find the place of the
    # clip - i. e. if the list isn't empty.
    if ( not self ):
        return None

    # The second check that we have to make - whether it isn't the case
    # that we have already found what we need, i. e. the closest element
    # in the list to the clip we search.
    if ( start > end ):
        return pos

    # Now, we can implement the actual search. To do that,
    # we use the binary search algorithm.
    middle = (start + end) / 2
    currTape = Clip.physicalTape( self[middle].VOLUME )
    goalTape = Clip.physicalTape( clip.VOLUME )

    if ( currTape > goalTape ):
        return self.findIndex(clip, start, middle -1, middle )

    if ( currTape < goalTape ):
        return self.findIndex(clip, middle + 1, end, middle + 1)

    if ( currTape == goalTape ):

        currTimecode = self[middle].IN
        goalTimecode = clip.IN

        if ( currTimecode > goalTimecode ):
            return self.findIndex(clip, start, middle - 1, middle )

        if ( currTimecode < goalTimecode ):
            return self.findIndex(clip, middle + 1, end, middle + 1)

  2. VolumeList.__getitem__：

    def __getitem__(self, key):
    # Is called whenever someone tries to get an item of the list
    # by calling 'volumes[ some_text_or_number ]'. VolumeList
    # will act as a normal list in all occassions except where the
    # key is a string. There, it will search for a volume label or
    # a tape number.

    if ( isinstance( key, basestring ) ):

        # Before checking whether the argument is a volume 
        # number, try to find if the user hasn't made an inquiry
        # about a tape. Tapes can only be called by calls such as
        # 'volumes[ "HD00345" ]', and *not* any other calls.
        if ( len(key) == 7 and key[:2].upper() == 'HD' 
                           and key[2:].isdigit() ):

            # Try to find the start and end of the sequence of 
            # volumes from the tape. To do this, search for an
            # 'infinitely small' and 'infinitely big' volumes from
            # the tape.
            start = self.findIndex( Volume( key.upper() + 'A', inFrame = -1 ) )
            end   = self.findIndex( Volume( key.upper() + 'Z', inFrame = 9999999 ) )

            if ( start != None and end != None ):
                return self[start:end]
            else:
                raise KeyError( 'The tape is not represented in this VolumeList!' )

        # Otherwise, assume that the user has called a particular
        # volume.
        volume = Volume.volumeLabel( key )

        if ( volume != None ):
            index = self.findIndex( Volume( volume, inFrame = -1 ) )

            if ( index < len( self ) and index != None ):

                if ( self[ index ].LABEL == volume ):
                    return list.__getitem__(self, index)
                else:
                    print 'The volume is here but not exactly'
                    raise KeyError( 'The volume is not in the list!' )
            else:
                raise KeyError( 'The volume is not in the list!' )
        else:
            raise KeyError( 'Tried to call a volume by an invalid volume label!' )

    else:
        return list.__getitem__(self, key)
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