__getitem__，列表中有切片 [英] __getitem__ with slices on a list of lists

问题描述

我正在创建一个代表列表列表的类. __getitem__使我头疼.一切都进行得很顺利，直到我引入slice作为参数为止.

I'm creating a class representing a list of lists. __getitem__ is giving me headaches. Everything goes swimmingly until I introduce slices as parameters.

演示代码

# Python version 2.7.5

class NestedLists:
   _Cells = [['.', '.', '.', '.', '.'],
             ['.', '.', 'N', '.', '.'],
             ['.', 'C', 'A', 'T', '.'],
             ['.', '.', 'P', '.', '.'],
             ['.', '.', '.', '.', '.']]

   def __getitem__(self, index):
      if isinstance(index, int):
         return self._Cells[index]
      elif isinstance(index, slice):
         return self._Cells[index]
      else:
         raise TypeError, "Invalid argument type"

nested = NestedLists()

print "Expecting A"
print nested[2][2]

print "Expecting CAT"
print nested[2][1:4]

print "Expecting ..N.."
print "          .CAT."
print "          ..P.."
print nested[1:4]

print "Expecting .N."
print "          CAT"
print "          .P."
print nested[1:4][1:4]

下面的输出

Expecting A
A
Expecting CAT
['C', 'A', 'T']
Expecting ..N..
          .CAT.
          ..P..
[['.', '.', 'N', '.', '.'], ['.', 'C', 'A', 'T', '.'], ['.', '.', 'P', '.', '.']]
Expecting .N.
          CAT
          .P.
[['.', 'C', 'A', 'T', '.'], ['.', '.', 'P', '.', '.']]

显然，正在发生的是第二个[]运算符调用被应用到第一个[...]操作符的输出，但仍位于最外层列表的上下文中.但是，解决方案使我难以理解.

Apparently, what is happening is that the second [] operator call is being applied to the output of the first... but staying in the context of the outermost list. The solution eludes me however.

推荐答案

您可能希望将多维访问的语法从obj[row][col]更改为使用单个元组索引:obj[row, col].这是numpy的ndarray类型使用的格式，它非常有用，因为它使您可以立即查看索引的所有维度.您可以编写__getitem__，以便进行任意尺寸的切片:

You probably want to change your syntax for mutidimensional access from obj[row][col] to using a single tuple index: obj[row, col]. This is the format numpy's ndarray type uses, and its very helpful, as it lets you see all the dimentions of the index at once. You can write a __getitem__ that will allow slicing in any dimension:

def __getitem__(self, index):
   row, col = index
   if isinstance(row, int) and isinstance(col, (int, slice)):
      return self._Cells[row][col]
   elif isinstance(row, slice) and isinstance(col, (int, slice)):
      return [r[col] for r in self._Cells[row]]
   else:
      raise TypeError, "Invalid argument type"

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