Format the hex codes in Unicode/hex escape sequences (\U, \u, \x) in string literals

[Jackenmen]

Is your feature request related to a problem? Please describe.
Currently, one can write either "\U0001f977" or "\U0001F977" which are equivalent but they look differently. Similarly, one can write "\u200b" or "\u200B" which also are equivalent but they look differently (do mind that \U and \u are NOT equivalent though; unless we also want to talk about shortening "\U0000200b" to "\u200b" which I guess would make sense but is probably a separate issue).

Right now, I'm forced to think whether I should use uppercase or lowercase letters as Black doesn't enforce it.

Describe the solution you'd like
I think it would make sense to have these be consistent in some way, possibly in the same way as the numeric literals, although personally, I think it would make more sense to have it all uppercase in case of \U0001F977 and all lowercase in case of \u200b. I definitely think that the \u200b should not be \u200B but I don't have a strong opinion on the \U0001f977 vs \U0001F977.

Describe alternatives you've considered
Alternatives were not considered.

Additional context
None.

[JelleZijlstra]

I agree that Black should make this consistent but have no real view on which direction the consistency should go. Some things that may help a decision:

• Are there existing style guides that mandate one or the other?
• Does the Python documentation prefer one over the other?
• Does either output appear in repr() somehow? If so, I'd prefer we match it.

[Jackenmen]

• Are there existing style guides that mandate one or the other?

None that I know of.

• Does the Python documentation prefer one over the other?

Most occurrences seem to use the lowercased representation, though there's not that much usage of it within the Python documentation. As for CPython's Python code, it seems to use a lowercased version more often, but I don't think it's that consistent there.

• Does either output appear in repr() somehow? If so, I'd prefer we match it.

Python's repr() shows lowercased representation:

>>> "\x1B"
'\x1b'
>>> "\u200B"
'\u200b'
>>> "\U0001F977"
'\U0001f977'

[JelleZijlstra]

Thanks! That would make me lean towards using all lowercase in Black too.

[TomFryers]

I think it may be relevant that Black currently formats hex literals 0xf to upper case 0xF, but Python’s built-in hex outputs lower case.

[JelleZijlstra]

This also affects \x escapes. (#2828 is about \N escapes, where I think there's a pretty clear norm to use uppercase.)

I'm leaning towards keeping \U, \u, \x all lowercase but happy to change if people have a different preference.

[JelleZijlstra]

Also see https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals for reference on how these work.