Steven D'Aprano writes:
> Ah, but that's because you're a Python programmer who has been seeped in
> the language for many, many years :-)
No, it's because I'm a Lisp programmer for twice as long, and know the
differences among #'=, #'string=, #'equal, #'eql, and #'eq (the last
is `is'`.
> To most people, "is" and "equals" are synonyms,
In Python, this is a *bug*, and if it's not fixed, they are going to
have a lot of trouble with mutable containers. Especially as default
values for function arguments.
> So for somebody who is expecting "is" to be a synonym for `==`, how do
> you explain what it actually is? You need to talk about the Python
> execution model,
I guess you need this if you want to explain how "x == None" can fail
to do what you want, but that's independent of None's uniqueness.
> how all values are objects, and that the reason that `x is None`
> always does the right thing
TOOWTDI violation. Python has both `==` and `is` *because* they do
different things. What each does is a right thing. The programmer's
problem is to learn what each does and when to use it.
> but `x is []` doesn't is that None is a singleton, so that there is
> only one object that is None, but the empty list is not, there are
> many, many distinct empty lists.
I would avoid use of "singleton" personally, since it's also used to
denote a *type* with only one member, and for many students (eg, all
of mine) you would have to define it. I don't see what "None is a
singleton, so that" adds to
You need to talk about the Python execution model, how all values
are objects, and that the reason that `x is None` always does the
right thing but `x is []` doesn't is that there is only one object
that is None, but the empty list is not, there are many, many
distinct empty lists.
> You might be able to avoid using the word "singleton",
I just did, or maybe I should say, you just did. ;-)
Steve
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