[Python-ideas] Possible method of distinguishing between set-literals, dict-literals, and odict-literals

[MRAB]

Carl Johnson wrote:
> Ron Adam wrote:
>> Ben Finney wrote:
>>> Carl Johnson writes:
>>>> we could introduce an empty set-literal and an odict-literal, and add
>>>> a more explicit form to replace the existing set literal.
>>> What do you mean by “more explicit”? The existing set literal syntax
>>> is quite explicit.
>> I think Carl is thinking that it is more concise and easier to read.
> 
> Yes, that's part of it. Thank you. What I mean by "explicit" in this
> case is something like "harder to accidentally misparse in your mind."
> So for example if you see {f(i) for i in my_data} obviously it is
> unambiguously a set-comprehension, but if you aren't paying close
> enough attention you might mistakenly think it was a
> dict-comprehension. On the other hand, s{f(i) for i in my_data} more
> or less screams, "I am a set-comprehension!" On the other hand, it's
> ugly, and the BDFL doesn't like it. So, it's probably a dead idea now…
> 
> Terry Reedy wrote:
>> Literals represent constants. They are sensibly interpreted into constants
>> sometime before runtime, even if at a later stage than I thought.
> 
> I'm not sure what you're getting at here. What about d = {} ? That's
> not immutable. Nor is l = [1, 2, 3]. Or by "constant" do you just mean
> something that will be the same at the *start* of every program, but
> it may or may not change before the program is over?
> 
> - - - -
> 
> I think ["a": 1, "b": 2] is an interesting syntax, and certainly more
> elegant than o{"a": 1, "b": 2} but I worry that it could be ambiguous.
> I can't think of any way to do it off the top of my head, but might
> there be some valid way of writing l[1:2] that could be ambiguous
> between the two? If nothing else, it would confuse newbies about what
> [:] means: slice or odict?
> 
What about l[2]? Is [2] an index or a list?