On Tue, 24 Aug 2021 at 12:07, Tim Hoffmann via Python-ideas
<python-ideas@python.org> wrote:
> Just like length is. It's a basic concept and like __bool__ and __len__ it
> should be upon the objects to specify what empty means.
It feels like these arguments in the abstract are mostly going round
in circles. It's possible something has been mentioned earlier in this
thread, but I don't recall if so - but is there any actual real-world
code that would be substantially improved if we had built into the
language a protocol that users could override in their classes to
explicitly define what "is empty" meant for that class?
Some things to consider:
1. It would have to be a case where neither len(x) == 0 or bool(x) did
the right thing.
2. We can discount classes that maliciously have bizarre behaviour,
I'm asking for *real world* use cases.
3. It would need to have demonstrable benefits over a user-defined
"isempty" function (see below).
4. Use cases that *don't* involve numpy/pandas would be ideal - the
scientific/data science community have deliberately chosen to use
container objects that are incompatible in many ways with "standard"
containers. Those incompatibilities are deeply rooted in the needs and
practices of that ecosystem, and frankly, anyone working with those
objects should be both well aware of, and comfortable with, the amount
of special-casing they need.
To illustrate the third point, we can right now do the following:
from functools import singledispatch
@singledispatch
def isempty(container):
return len(container) == 0
# If you are particularly wedded to special methods, you could even do
#
# @singledispatch
# def isempty(container):
# if hasattr(container, "__isempty__"):
# return container.__isempty()
# return len(container) == 0
#
# But frankly I think this is unnecessary. I may be in a minority here, though.
@isempty.register
def _(arr: numpy.ndarray):
return len(arr.ravel()) == 0
So any protocol built into the language needs to be somehow better than that.
If someone wanted to propose that the above (default) definition of
isempty were added to the stdlib somewhere, so that people could
register specialisations for their own code, then that might be more
plausible - at least it wouldn't have to achieve the extremely high
bar of usefulness to warrant being part of the language itself. I
still don't think it's sufficiently useful to be worth having in the
stdlib, but you're welcome to have a go at making the case...
Paul
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