[Python-ideas] Add new `Symbol` type

[Steven D'Aprano]

On Thu, Jul 05, 2018 at 05:26:36PM -0700, Nathaniel Smith wrote:

> I think the name "symbol" here is pretty confusing. It comes
> originally from Lisp, where it's used to refer to an interned-string
> data type.

Even if Lisp was the first programming language to use the term for a 
data type (which seems likely), that doesn't give Lisp ownership of the 
concept. A number of languages have a Symbol data type, without needing 
to ape Lisp semantics or implementation.

https://en.wikipedia.org/wiki/Symbol_%28programming%29

See for example:

https://planspace.org/20141129-ruby_symbols_are_not_lisp_symbols/

Since "symbol" is a plain English word that doesn't necessarily have 
anything to do with Lisp, we're free to use it provided we determine 
what it will mean in Python. That may not be identical to Lisp's 
semantics, or Javascript semantics, or Ruby semantics. But that's okay, 
they're semantics aren't identical with each other either.


> It's a common source of confusion even there. Then it
> sounds like JS took that name, and it ended up drifting to mean
> something that's almost exactly the opposite of a Lisp symbol. In
> Lisp, symbols are always "global";

I don't think that's right, at least not in Common Lisp. Symbols may or 
may not be accessible from a particular namespace:

http://www.lispworks.com/documentation/HyperSpec/Body/t_symbol.htm#symbol

In Ruby, symbols are global, the opposite of Lisp symbols:

http://chneukirchen.org/blog/archive/2005/12/ruby-symbols-are-not-lisp-symbols.html


> the whole point is that if two
> different pieces of code use the same name for the same symbol then
> they end up with the same object. So this is *super* confusing.

Well, it's certainly super confusing to me *wink* since I'm not sure 
whether your "whole point" comment is referring to how symbols work in 
Javascript, Lisp, how you think they ought to work, or something else, 
or who "this" is super confusing to. (Javascript programmers? Lisp 
programmers? You?)


[...]
> The thing you're talking about is what Python devs call a "sentinel"
> object.

"Sentinel" describes the *purpose* of the object, not the "kind of 
thing" it is. It is a special value used to indicate the end of data.

https://en.wikipedia.org/wiki/Sentinel_value

By extension, any placeholder value indicating missing data can be 
called a sentinel:

    def function(value=None):

Here, None is being used as a sentinel in the second sense.


> If your proposal is to add a sentinel type to the stdlib, then
> your chance of success will be *much* higher if you use the word
> "sentinel" instead of "symbol". 

Oh I really, really hope not. Symbols aren't merely sentinels. They're 
also a kind of enumerated value: C enums are a kind of symbol. None 
and NotImplemented and Ellipsis are kinds of symbols. Symbols can be 
used for all sorts of reasons, not just as a sentinel.

For example, NotImplemented is returned by operator dunder methods to 
indicate "give the other operand a chance to respond".

None is not a sentinel, but it can be used as a sentinel. But for that 
matter, so could -1 or "Aardvark" be used as sentinels, under the right 
circumstances.

Arguably, True and False could be considered symbols. Sometimes I need a 
three-state flag, and I often use True, False and None, but often what 
I really want is True, False and Maybe. Or True, False, Mu.

https://en.wikipedia.org/wiki/Mu_(negative)#In_popular_culture


If we had Symbols first, we possibly wouldn't need enum.Enum; arguably, 
since we do have enum.Enum, perhaps we don't need Symbols. Or maybe 
there's still use for them both -- any potential PEP would have to sort 
out that question.


> People don't read mailing list threads
> carefully, so if you keep calling it "symbol" then you'll likely spend
> infinite time responding to people rushing in to critique your
> proposal based on some misconception about what you're trying to do,
> which is no fun at all.

As opposed to them critiquing the proposal based on the misuse of the 
term "sentinel" for something which isn't (just) used as a sentinel.



-- 
Steve