Possible to use more than one argument on __getitem__?

Question

I am trying to use

__getitem__(self, x, y): 

on my Matrix class, but it seems to me it doesn't work (I still don't know very well to use python). I'm calling it like this:

print matrix[0,0] 

Is it possible at all to use more than one argument? Thanks. Maybe I can use only one argument but pass it as a tuple?

Answer 1

__getitem__ only accepts one argument (other than self), so you get passed a tuple.

You can do this:

class matrix:     def __getitem__(self, pos):         x,y = pos         return "fetching %s, %s" % (x, y)  m = matrix() print m[1,2] 

outputs

fetching 1, 2 

See the documentation for object.__getitem__ for more information.

Answer 2

Indeed, when you execute bla[x,y], you're calling type(bla).__getitem__(bla, (x, y)) -- Python automatically forms the tuple for you and passes it on to __getitem__ as the second argument (the first one being its self). There's no good way^[1] to express that __getitem__ wants more arguments, but also no need to.

---

^[1] In Python 2.* you can actually give __getitem__ an auto-unpacking signature which will raise ValueError or TypeError when you're indexing with too many or too few indices...:

>>> class X(object): ...   def __getitem__(self, (x, y)): return x, y ...  >>> x = X() >>> x[23, 45] (23, 45) 

Whether that's "a good way" is moot... it's been deprecated in Python 3 so you can infer that Guido didn't consider it good upon long reflection;-). Doing your own unpacking (of a single argument in the signature) is no big deal and lets you provide clearer errors (and uniform ones, rather than ones of different types for the very similar error of indexing such an instance with 1 vs, say, 3 indices;-).