[Python-3000] Set literals - another try

Josiah Carlson jcarlson at uci.edu
Tue Aug 8 19:36:40 CEST 2006

Talin <talin at acm.org> wrote:
> Part 1: The concrete proposal part.
> I noticed that a lot of folks seemed to like the idea of making the 
> empty set resemble the greek letter Phi, using a combination of 
> parentheses and the vertical bar or forward slash character.
> So lets expand on this: slice Phi in half and say that (| and |) are 
> delimiters for a set literal, as follows:
>     (|)     # Empty set
>     (|a|)   # Set with 1 item
>     (|a,b|) # Set with 2 items
> The advantage of this proposal is that it maintains visual consistency 
> between the 0, 1, and N element cases.

That's quite a bit of punctuation to define a set literal.  In fact, for
1+ element sets, it's only 1 character shy of the set() punctuation,
while also being more difficult to type on at least US keyboards.

And if I remember my set math correctly, phi wasn't the character
generally used, it was usually a zero with a diagonal cross through it,
making (/) a better empty set literal.  But from there, the notation
devolves into a place I don't want to go.

> Part 2: The idle speculation part, not to be considered as a actual 
> proposal.
> I've often said that "whenever a programmer has the urge to invent a new 
> programming language, that they should lie down on the couch until the 
> feeling passes".

Presumably you again don't remember the source of this quote, but it is
still applicable.

> As a thought experiment, I decided to apply this idea to the Python set 
> literal case - i.e. if we were going to do a massive "do over" of 
> Python, how would we approach the problem of set literals?
> The syntax that comes to mind is something like this:
>     a = b|c

The pipe character/bitwise or operator doesn't say to me "make a set".

Knowing what I do about set math, the only literal that really makes
sense to me is:

With the empty set being:

Interstingly enough, the non-empty set case has already been proposed,
and if I remember correctly, was generally liked, except for the
somewhat ambiguity with regards to dictionary literals.

I personally don't see much of a use for set literals, considering that
there is a non-ambiguous spelling of it currently; set(...), whose only
cost above and beyond that of a set literal is a global name lookup.  It
is 'different' from some other first-class objects (tuple, list,
dictionary, string, unicode, ...), but other first-class objects also
require such spelling: bool, enumerate, iter, len, property, reduce.
Each of which may be used sufficiently often to make sense as having a
syntax for their operations, though perhaps only len having an obvious
syntax of |obj| -> len(obj), though |obj| could also mean abs(obj), but
presumably objects would only ever have __len__ or __abs__ and not both. 
I digress.

-.5 for a set literal syntax at all, -1 for offering your particular set
literal variant, -2 for your change propagating to dictionaries and

 - Josiah

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