`frozenset` and `frozendict` comprehensions

[sobolevn (Nikita Sobolev)]

I implemented both versions with f and $ as prefixes for {

• f: Comparing main...c9004e5977a05d5ac644e1741adc446af96d0f29 · python/cpython · GitHub
• $: https://github.com/python/cpython/pull/152820/commits/657fff2f18a84601a180f1be74876c537fc9b7eb

Now, we would have to decide which one is better. There are several concerns for each of them:

• f
• Looks like fstring: f{0} and f'{0}', but does a completely different thing: Frozen vs Format
• In fstring you can use both F and f, it would be strange to allow the same for f{. So, it would be incosinstent
• It is rather hard to read when these things are nested: f{f'{user}', f'{task}'}
• It does not scale for other possible similar syntax (which is not a part of the proposal, but just a possibiliy for the future). Because f() and f[] are already valid as a syntax

Now, the second option:

• $
• It is a new unfamiliar thing, that people might associate with PHP or BASH
• It one of the last unused symbols in the syntax

Why not some other options?

• No identifiers apply because of the scalability concerns
• No suffixes apply, because it is hard to read
• No chars that are used in operators apply, because they create ambiguity when parsing
• No existing valid syntax apply, we can’t break stuff. So, things like {{}} won’t work

We are left with very little options, which are reasonable. With this in mind: please, vote

• ${}
• f{}