On Sep 2, 2019, at 23:50, Philippe Prados <pyt...@prados.fr> wrote:
>
> Add a new operator for `Union[type1|type2]` ?
Hold on. Are you proposing `Union[t1 | t2]` as a new spelling for `Union[t1,
t2]`? That seems pointless. I thought you were proposing just `t1 | t2`, which
seems a whole lot more useful (and no more disruptive)?
I’m going to assume this is just a typo?
Also, as far as the runtime-vs.-static, etc., issues of `int|str` vs.
`(int,str)` vs. `{int,str}`, I think it’s worth getting them all down in one
place.
* `(int, str)`
* Not just valid syntax, but a legal value in every version of Python, so
you can use it in annotations and still run your code in 3.5 or something (you
can’t type-check it as 3.5 code, but that rarely matters).
* The same value can be passed to `isinstance` and will work as intended in
every version of Python, unlike `Union[int, str]`, which fails because `Union`
(like all generic types) refuses to allow its instantiated types in
`isinstance`. Whether this is a good or bad thing is an open question, but it
seems good at first glance.
* Potentially confusing, implying `Tuple` rather than `Union`, but maybe
that’s just something for people to get used to
* `{int, str}`
* Also a legal value in all existing Pythons.
* Not a legal argument to `isinstance`. Or `except` statements. This could
be changed very easily and without much disruption, although of course only for
3.9+. It also opens the question of whether other Spam-or-tuple-of-Spam
functions (like all the str methods) should also change to also accept
-or-set-of-Spam, but Guido suggested that this could be left open until people
either do or do not file bug reports about trying to pass a frozenset of
suffixes to endswith.
* Not confusing; it’s hard to imagine what it could mean other than “must be
one of this set of types”.
* `int|str`
* Valid syntax in 3.8, but raises a TypeError when evaluated, so it can’t
even be used in annotations without quoting it.
* Can add `type.__or__` and `__ror__` methods in 3.9, so it _can_ be used
unquoted, and can be inspected as a meaningful value at runtime. That means
code they used it won’t compile in 3.8, but that’s nothing unprecedented, and
might be worth it. And it’s not that major of a change.
* Can’t be passed to `isinstance`. This would require an entirely
independent change (which might be a good one, but should probably be argued
out separately) to special-case `Union` as not like other generic types, and
usable in `isinstance` calls.
* Unlike the other options, this one has an obvious extension to `Optional`
via a unary operator like `~`.
* Close enough to similar meanings in other languages that it would be an
aid to memory rather than a source of confusion. Plus, even if you don’t know
any of those other languages, and don’t know much about typing, and read it as
“int or string”, you still get the right idea.
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