James Hartley wrote:
> I can successfully override __getitem__() for rvalues, but the following
> example shows that more is required when used as an lvalue:
>
> ===8<-----
> #!/usr/bin/env python
>
> class Foo():
> def __init__(self, n):
> self.d = dict.fromkeys([i for i in range(0, n)])
>
> def __getitem__(self, i):
> return self.d[i]
>
> def main():
> foo = Foo(4)
> print(foo[0])
> foo[0] = 2 # not as an lvalue?
> print(foo[0])
>
> if __name__ == '__main__':
> main()
> ===8<-----
>
> Python 3.4 generates the following output when this example is executed:
>
> None
> Traceback (most recent call last):
> File "./test.py", line 17, in <module>
> main()
> File "./test.py", line 13, in main
> foo[0] = 2
> TypeError: 'Foo' object does not support item assignment
>
> I am surprised that the error states that the object itself cannot accept
> assignment. From the C++ perspective, the underlying dictionary should be
> exposed. Does Python overloading allow use of bracket overriding in
> lvalues?
Since Python doesn't allow multiple method definitions with different
signatures under the same name there's a dedicated __setitem__() method:
>>> class Foo:
... def __setitem__(self, index, value): print("Setting Foo()[{!r}] to
{!r}".format(index, value))
...
>>> foo = Foo()
>>> foo[42] = "bar"
Setting Foo()[42] to 'bar'
Of course with the above definition
>>> foo[42]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'Foo' object does not support indexing
but you already know how to fix that. To get a class that works like
Python's list or dict while only writing a few methods yourself you can
inherit from collections.MutableSequence or MutableMapping.
PS: Even without function overloading the developers might have chosen a
common method for both, e. g.
def __item__(self, index, *args):
if args:
[value] = args
# set item
else:
result = ... # calculate current value
return result
The actual choice is cleaner and a tad more efficient.
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