On 2/09/20 5:14 am, Stephan Hoyer wrote:
On Tue, Sep 1, 2020 at 9:59 AM David Mertz <[email protected]
I think we need this for the same reason why we need **kwargs in normal
function calls: it makes it possible to forward dictionaries of
arguments to another method.
If we don't have dict unpacking, you'd have to write
obj.__getitem__(**kwargs) rather than obj[**kwargs], which is much more
verbose and goes against the principle that you shouldn't need to call
double-underscore methods in normal Python code.
It's also not even quite correct! As another message posted recently
pointed out, if 'other' happens to be a class object, __class_getitem__
should be called instead, if it exists:
def __getitem__(self, index, **kwds):
g = None
if isinstance(other, type):
g = getattr(other, '__class_getitem__', None)
if g is None:
g = other.__getitem__
return g(index, **kwds)
This is getting pretty tricky and convoluted. (I'm not sure it's
completely correct even now, since it doesn't check whether
__class_getitem__ is a class method.) It certainly won't remain
correct if the rules for indexing method lookup change in the
future.
If a[**kwds] is allowed, on the other hand, it's easy to write
index forwarding code that is straightforward, obvious and
future-proof.
--
Greg
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