是否有一种简单的方法来覆盖列表对象的方法__getitem__？

Question

我正在尝试定义默认样式列表对象:

class ilist(list):
    def __init__(self,r=list(),dft=None):
        list.__init__(self,r)
        self.dft=dft
    def __getitem__(self,n):
        if len(self)<=n:
            for i in range(n-len(self)+1):
                self.append(self.dft)
        for i,v in enumerate(self):
            if i+1==len(self):
                return v

x=ilist()
print x[4]
print x

有用.

>>> 
None
[None, None, None, None, None]  

但我认为查询我的ilist是很糟糕的.我尝试过以下方法:

def __getitem__(self,n):
    from operator import getitem
    if len(self)<=n:
        for i in range(n-len(self)+1):
            self.append(self.dft)
    return getitem(self,n)

但事实表明它完全等于自我[n]和原因 RuntimeError: maximum recursion depth exceeded

我也尝试借用父类list方法.但是表单是x.__getitem__(y).我不知道如何使它适应ilist.

所以最后我的可怕解决方案出来了.原始和蛮力..有任何有效或简单的解决方案吗？提前致谢.

Answer 1

使用super()访问原始__getitem__:

def __getitem__(self,n):
    while len(self) <= n:
        self.append(self.dft)
    return super(ilist, self).__getitem__(n)

演示:

>>> class ilist(list):
...     def __init__(self,r=list(),dft=None):
...         list.__init__(self,r)
...         self.dft=dft
...     def __getitem__(self, n):
...         while len(self) <= n:
...             self.append(self.dft)
...         return super(ilist, self).__getitem__(n)
... 
>>> il = ilist()
>>> il[3]
>>> il
[None, None, None, None]
>>> il[2] = 5
>>> il
[None, None, 5, None]
>>> il[2]
5

您可能也想支持切片:

def __getitem__(self, n):
    maxindex = n
    if isinstance(maxindex, slice):
        maxindex = maxindex.indices(len(self))[1]
    while len(self) <= maxindex:
        self.append(self.dft)
    return super(ilist, self).__getitem__(n)

如果你想支持任意索引的赋值,添加一个__setitem__方法:

def __setitem__(self, n, val):
    maxindex = n
    if isinstance(maxindex, slice):
        maxindex = maxindex.indices(len(self))[1]
    while len(self) <= maxindex:
        self.append(self.dft)
    return super(ilist, self).__setitem__(n, val)

但是你可以将默认值创建移出到辅助方法:

class ilist(list):
    def __init__(self, r=None, dft=None):
        if r is None:
            r = []
        list.__init__(self, r)
        self.dft=dft

    def _ensure_length(n):
        maxindex = n
        if isinstance(maxindex, slice):
            maxindex = maxindex.indices(len(self))[1]
        while len(self) <= maxindex:
            self.append(self.dft)

    def __getitem__(self, n):
        self._ensure_length(n)
        return super(ilist, self).__getitem__(n)

    def __setitem__(self, n, val):
        self._ensure_length(n)
        return super(ilist, self).__getitem__(n)