On 1/24/2020 5:29 PM, Tim Peters wrote:
[Terry Reedy <tjre...@udel.edu>]
...
It is, in the section on how to understand and use value comparison
*operators* ('==', etc.).
https://docs.python.org/3/reference/expressions.html#value-comparisons
First "The default behavior for equality comparison (== and !=) is based
on the identity of the objects."
Then in particular, "The built-in containers typically assume identical
objects are equal to themselves. That lets them bypass equality tests
for identical objects to improve performance and to maintain their
internal invariants."
This intentionally dropped 'reflexive' (a technical term not familiar to
many python beginners) and 'nan' (something beginners may not know about
and almost never actually use) to focus on who does what and why.
Cool! I didn't find that, and I assume Victor didn't either. It's good!
But clearly insufficiently discoverable. And the use of 'is' is not
limited to sequences, nor even to built-in containers (as shown below).
We might try moving these sentences up under the general paragraph on
equality comparison.
I think it needs more words, though, to flesh out what about this is
allowed by the language (as opposed to what CPython happens to do),
and to get closer to what Guido is trying to get at with his
"*implicit* calls".
I am thinking about how to do this with minimal words.
For example, it's at work here, but there's not a
built-in container in sight:
'in' is a built-in operation whose second argument can be anything that
can be seen as a 'container': iterables plus objects with __contains__.
import math
def f():
... while True:
... yield math.nan
math.nan in f()
True
> f().__iter__
<method-wrapper '__iter__' of generator object at 0x...>
Covered by "For user-defined classes which do not define __contains__()
but do define __iter__(), x in y is True if some value z, for which the
expression x is z or x == z is true, is produced while iterating over y.
" in
https://docs.python.org/3/reference/expressions.html#membership-test-operations
For example, in __eq__() method documentation:
https://docs.python.org/dev/reference/datamodel.html#object.__eq
The text that follows discusses rich comparison *special methods* and
how to write them. It should refer people to the value-comparisons
section for information on specific classes, as in the second quote
above. It would not hurt if the operator section referred people back
to special method discussion. I think you should go ahead and add one
or both links.
Agreed. If I want to know why my __eq__ (or __ne__) isn't being
called, it's __eq__/__ne__ docs I'm going to look at first. And
probably last, unless I'm nudged in the right direction.
I will try to work on a draft PR.
--
Terry Jan Reedy
--
Terry Jan Reedy
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