Using keyword arguments in __getitem__ method in Python

Question

I want to define a class Foo whose objects can be used like, foo[1, a=2].

I tried to achieve this by decorating the __getitem__ method of Foo but with no success. Below is the example code.

def decorator(func):
    def func_(*args, **kewargs):
        if 'a' in kewargs:
            args = list(args) + [kewargs['a']]
            return func(*args)
        else:
            return func(*args)
    return func_

class Foo(object):
    @decorator
    def __getitem__(self, *items):
        return items
foo = Foo()

>>> foo.__getitem__(2, a=10)
(2, 10)
>>> foo[2, a=10]
SyntaxError: invalid syntax

So foo[...] is not equivalent to foo.__getitem__(...), something behind the scene is done for the former. My question is what exactly and how can I make foo[2, a=10] to work, if at all.

Answer 1

Python allows implicit tuple creation (without parentheses):

In [2]: tup = 1, 2, 3

In [3]: tup
Out[3]: (1, 2, 3)

And it works the same inside square brackets:

In [4]: d = {(1, 2, 3): 4}

In [5]: d[1, 2, 3]
Out[5]: 4

But (2, a=10) is not a valid tuple literal:

In [6]: (2, a=10)
  File "<ipython-input-1-7dc03602f595>", line 1
    (2, a=10)
         ^
SyntaxError: invalid syntax

Simply put, you can't make foo[2, a=10] to work, because it's a syntax error no matter how you tweak your __getitem__ implementation.

I'd probably define an ordinary method e.g. get and use it like Foo.get(2, a=10).