Thank you, Guido, for your interest in this discussion.
You wrote:
> So __keyfn__ has literally only two allowed values, None and True.
>
That's my proposal, for now. I'm happy for further allowed values to be
added via another PEP. In fact, that is my preference.
You also wrote:
> Could you just start over? And explain the name? And why it can't be
> False/True?
Consider
>>> d[1, 2] = 'val'
>>> d.__setitem__((1,2), 'val')
Here I call (1, 2) the key. As in
def __setitem__(key, val):
# set the value
The passage
>>> d[1, 2] = 'val'
>>> d.__setitem__((1,2), 'val')
goes via tuple. If tuple didn't already exist, we'd have to invent it. And
so here tuple is what I call "the key function".
The key function is the function that is the intermediary between
>>> d[EXPRESSION]
>>> d.__getitem__(key)
When there is a key function, the signatures are
__getitem__(key)
__delitem__(key)
__setitem__(key, val)
So we're allowed to have
class A:
__keyfn__ = None
def __setitem__(self, val, a, b, c d):
# set the value
Recall that dict has, implicitly, a way of getting a key from permitted
arguments. The meaning of
class B:
__keyfn__ = True
def __getitem__(key):
# Get the value
is that the key is produced by exactly the same method as in dict.
The meaning of
__keyfunc__ = True
is "produce a key from the arguments, in exactly the same way as in dict".
Or in other words, "Yes, it's True. We do have a keyfn. Use the default,
dict, keyfn."
I think (None, True) works better than (False, True). This is because a
further PEP might allow the user to supply a custom keyfn. So while it is
at this time a binary choice, there might be further choices in future.
> Can you explain (in words or with examples) what happens in each case? You
> were so excited to show off one case that you never showed how the other
> case would work
>
How about:
class A:
__keyfn__ = None
def __setitem__(self, val, x=0, y=0, z=0):
print((val, x, y, z))
>>> a = A()
>>> a[1, z=2] = 'hello'
('hello', 1, 0, 2)
[Above copied from
https://mail.python.org/archives/list/python-ideas@python.org/message/P3AW6GNIYDDOTVQ2FKWYD7XYNZ5P5QBS/
]
And also
class C:
__keyfn__ = True
def __setitem__(self, *argv, **kwargs):
print(f'argv={argv} | kwargs={kwargs}')
>>> c = C()
>>> c[1] = 'val'
argv=(1, 'val') | kwargs={}
>>> c[1, 2] = 'val'
argv=((1, 2), 'val') | kwargs={}
>>> c[a=1] = 'val'
TypeError: __keyfn__ got unexpected keyword argument 'a'
[Above copied from
https://mail.python.org/archives/list/python-ideas@python.org/message/RNQFT4USRCTYTSLAUNPTWGMC6WLZEPKH/
]
I hope this helps.
--
Jonathan
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