Avi
Haven't noticed an answer to this. Did I miss anything?
On 20/01/19 11:07 AM, Avi Gross wrote:
Short question. Checking if a protocol is set up?
=do you mean that to check/require that a class exhibits a particular
protocol we should use abstract classes - will not instantiate unless
all the required components are coded?
Many python improvements are changes to classes that implement a protocol.
There are things you can do to make your own classes work with the protocol
by setting various dunder variables like __iter__, __next__ and writing
appropriate ode including throwing the right error class when done.
Similarly the "with" statement works with objects that implement __enter__
and __exit__. There can be plenty of others like this and more can be
anticipated in the future.
So, several related questions. Tools that help a developer add appropriate
things to an object to implement the protocol or to test if it was done
right. Perhaps a function with a name like is_iterable() that tells if the
protocol can be applied. For the specific case of an iterable, I found
something that seems to work for at least some cases:
from collections import Iterable
item = [1, 2, 3, 4]
isinstance(item, Iterable)
Not sure if it would work on one I created that did the right things or what
it checks.
=your code should be 'approved' if it implements the next() method, etc.
Did you try such?
I am interested in a pointer to something that describes many of the known
protocols or extensions and maybe to modules designed sort of as I said
above. I am aware some protocols may be not-quite standard with parts of the
protocol embedded in different objects like wrappers or objects returned
upon a request to have a proxy and many other techniques that seem to abound
and allow multiple layers of indirection or seemingly almost magical as in
multiple inheritance drop-ins and so on. That is what may make these things
harder if someone uses something like __getattr__ or descriptors to
intercept calls and provide the functionality without any actual sign of the
dunder key normally expected.
=Questioning similarly, I recall finding one of these - but do you think
that I can re-find it now? Apologies.
I (too) think it would be handy to have such a list. There are many for
the 'magic methods' themselves, in all the better Py3 texts.
Yesterday I needed to add __LT__() to allow a list of class instances to
be sorted, __EQ__ to enable a list of (other) instances to be searched
(if element in list_of_instances), and made a class callable
(__call__()). Each time I wondered: is this the best way to accomplish
or is there already a mechanism I could be employing/not 'reinventing
the wheel'. (perhaps incompletely!)
For your further reading pleasure (maybe):
On the this topic, one of many references is Interfaces in Python:
Protocols and ABCs
http://masnun.rocks/2017/04/15/interfaces-in-python-protocols-and-abcs/
Which reminded me of the amusingly titled: Duck Typing vs. Goose Typing,
Pythonic Interfaces
https://dgkim5360.github.io/blog/python/2017/07/duck-typing-vs-goose-typing-pythonic-interfaces/
Diving into the docs (although not finding exactly what we seek):
https://docs.python.org/3/library/abc.html
- didn't include all, eg context managers:
https://docs.python.org/3/library/contextlib.html
At a deeper level:
https://docs.python.org/3/c-api/abstract.html?highlight=abstract
and
https://docs.python.org/3.6/c-api/object.html
- again, incomplete in the sense (I gained) of this enquiry.
Hopefully there's something to keep your mind occupied...
--
Regards =dn
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