Steve D'Aprano wrote:
Circle
didn't use any setters, but I could have let you set the diameter, which in
turn would set the radius:
circle.radius = 2
assert circle.diameter == 4
circle.diameter == 2 # requires a setter
assert circle.radius == 1
Getting that to work is left as an exercise :-)
I may start that exercise in a few minutes!
But most commonly, computed attributes need to store some data aside, somewhere.
You could use a global variable, or write it to a file, or stick it in a list.
All of these things have obvious problems, so the most sensible approach it to
stick the data in a private attribute.
The interpreter doesn't enforce notions of private/public when it comes to
Python classes, but there's a very strong convention that anything starting
with a single underscore is private.
[1] Technically, the interpreter knows nothing about properties. What it cares
about is *descriptors*. Properties are just one kind of descriptor, as are
methods. But I'm intentionally not talking about the gory details of
descriptors. Feel free to ask if you care, but honestly, you don't need to care
unless you are writing your own descriptor class.
Thank you, and everyone else who has contributed to this thread, for
helping me. Each contribution I read helped me to get further ahead!
I watched an example on YouTube where someone wrote a simple descriptor
("@Time_it) to output the amount of time that it took ordinary functions
to complete. To be honest, I AM interested in descriptors. I may
reexamine whether David Beazley has more to say about them in his book
"Python: Essential Reference", which I have been reading. Obviously, I
still have some gaps in my understanding after my first reading.
If you were going to show non-Python users, say science undergraduates
and faculty, that Python is an interesting tool (in 45 minutes), would
one delve into descriptors? I am thinking maybe. Here is what I am
thinking at this moment: trivial applications (probably), list
comprehensions (definitely), generators (maybe briefly). Whatever I
would discuss, I think ending with descriptors could be a strong finish.
But I'm certainly not merely interested for the sake of my talk, I
obtain some satisfaction in learning how things work. If you can
suggest any references for descriptors which you think are good, I would
be interested.
Thanks,
Bill
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