subscripting with 2 arguments in python

Question

Assume I have a class X which has 2 attributes : i and j.

I want to have :

x = X((1,2,3),(2,3,4)) #this would set i to (1,2,3) and j to (2,3,4)

I now want subscripting to work in the following way :

a, b = x[1,2] #a should now be 2 and b should now be 3

At the moment I'm trying this :

    def __getitem__(self, i, j):
        return self.x[i] , self.y[j]

However this keeps giving me the error that getitem takes in exactly 3 arguments but 2 is given (when I try to print out x[1,2] for instance)

Cat Plus Plus · Accepted Answer · 2011-10-25 17:50:47Z

7

Comma is the tuple packing operator. x[1, 2] calls x.__getitem__((1, 2)).

def __getitem__(self, ij):
   i, j = ij
   return self.x[i], self.y[j]

answered Oct 25, 2011 at 17:50

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6 Comments

Arnab Datta Over a year ago

wow, that was quick..Could you give me a pointer to where this is explained in more detail?

Cat Plus Plus Over a year ago

@ArnabDatta: T *ptr = &internet;. Seriously, though, I'd imagine tuples are explained in any Python tutorial.

Arnab Datta Over a year ago

call me a noob, but I tried searching the internet for getitem with multiple params, subscripting with multiple params, and looked through explanations of the getitem method without any luck. The fact that comma is the tuple packing operator is not obvious to someone new to python, so the sarcasm isnt exactly well recieved in this case.

Russell Borogove Over a year ago

@ArnabDatta - tuple packing and unpacking isn't specific to __getitem__ and is described somewhat here: docs.python.org/tutorial/…

Cat Plus Plus Over a year ago

@ArnabDatta: This is not related to __getitem__. __getitem__ takes an object (as in x[obj] is equivalent to x.__getitem__(obj)), you pass a tuple here (I'm pretty sure the official tutorial remarks that it's comma that creates a tuple, not parenthesis). There's nothing more to it. Also, joke and sarcasm is not the same thing.

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