__getitem__ With Slices On A List Of Lists

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

Solution 1:

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
   ifisinstance(row, int) andisinstance(col, (int, slice)):
      return self._Cells[row][col]
   elifisinstance(row, slice) andisinstance(col, (int, slice)):
      return [r[col] for r in self._Cells[row]]
   else:
      raise TypeError, "Invalid argument type"

Solution 2:

The return from __getitem__ when index is a slice is a list of lists:

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

The second slice indexes into this list, rather than applying the slice to each item in the list.

To get the behaviour you're looking for, you could make NestedLists.__getitem__ return a new class, NestedListsSlice when index is a slice. This could then define its own __getitem__ to apply the second index to:

classNestedLists:
    ...
    def__getitem__(self, index):
        ...
        elifisinstance(index, slice):
            return NestedListsSlice(self._Cells[index])
        ...

classNestedListsSlice():
    def__init__(self, _Cells):
        self._Cells = _Cells
    def__getitem__(self, index):
        ifisinstance(index, int) orisinstance(index, slice):
            returnlist(x[index] for x in self._Cells)
        else:
            raise TypeError