Conditional collection literals

[blhsing (Ben Hsing)]

@sterliakov

I just grepped through some code I have written recently, and I found at least 16 cases of such *([...] if cond else []) unpacking in {list,tuple,dict} literals

That’s exactly how I’ve been relunctantly doing it myself. The proposed syntax is both more readable to me and more efficient.

@sterliakov

I see some problems with the current suggestion of stmt if cond:

1. It’s very similar to the ternary, so my eye immediately starts looking for else…
2. It’s utterly incomprehensible when put on one line. [1, "foo", "bar" if foo is not None, "qux"] already requires a lot of mental effort to parse, usually commas are not attracting attention.

I think we can remedy both concerns with a leading keyword and a forced indented block instead:

{
    'a': 1,
    if bar is not None:
        'b': 2,
        'c': 3,
    'd': 4,
}

This allows insertion of multiple items on one condition without the need for an unpacking operator too, although unpacking can still be allowed:

[
    '-l',
    if bar is not None:
        '--',
        *map(str, numbers),
]

The only downside I can think of is that an implementation will require a bigger change to the parser because currently there’s no indentation enforcement whatsoever inside brackets.