Python custom class indexing

Question

Is it possible to make a class in python that can be indexed with square brackets but not derived from a different indexed type?

I'm interested in making a class with optional indexes, that would behave like this:

class indexed_array():
    def __init__(self, values):
        self.values = values

    def __sqb__(self, indices):   #This is a made up thing that would convert square brackets to a function
        if len(indices) == 2:
            return self.values[indices[0]][indices[1]]
        elif len(indices) == 1:
            return self.values[indices[0]][0]

myarray = indexed_array([[1,2,3], [4,5,6], [7,8,9]])
print myarray[1, 1]     # returns 5
print myarray[1]        # returns 4

Is there a real method like my __sqb__? Alternatively, can you index a custom class another way?

Yes. You are looking for `__getitem__`. See here: https://docs.python.org/3.5/reference/datamodel.html#special-method-names. However, it looks like you just want to reinvent `numpy` array. — juanpa.arrivillaga, Jan 16 '17 at 22:32
You would be needing `__getitem__` and `__index__` magic functions. Also check: https://www.python.org/dev/peps/pep-0357/ — Moinuddin Quadri, Jan 16 '17 at 22:36
`__getitem__` looks like just what I wanted. @juanpa.arrivillaga does numpy allow for having a default index like my example? — ericksonla, Jan 16 '17 at 22:40
@juanpa.arrivillaga by "default index" I believe @ericksonia means the way that `myarray[1]` returns `values[1][0]` (not `values[1]`) — donkopotamus, Jan 16 '17 at 22:55
In that case, no. Indices are ordered and are accessed in order. — juanpa.arrivillaga, Jan 16 '17 at 22:56

score 19 · Accepted Answer · answered Jan 16 '17 at 22:35

You are need to implement __getitem__. Be aware that a single index will be passed as itself, while multiple indices will be passed as a tuple.

Typically you might choose to deal with this in the following way:

class indexed_array:
    def __getitem__(self, indices):
        # convert a simple index x[y] to a tuple for consistency
        if not isinstance(indices, tuple):
            indices = tuple(indices)

        # now handle the different dimensional cases
        ...

Python custom class indexing

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