Black: Numeric formatting

[JelleZijlstra]

I recently landed a diff adding normalization of numeric literals (#454, #464), but I want to make sure we all agree on the behavior before this goes into a release. It would be bad for user trust in Black if we change formatting in a release, then change our mind again in another release, so I'd rather discuss these now before they're locked into a release.

Here are the formatting rules included in my diff:

• 123456789 -> 123_456_789
• 0.12345678 -> 0.12_345_678
  
  
  
  • Should this be 0.123_456_78 instead? Or perhaps we don't need underscores at all in the fractional part of floats; my code adds it to any part of a numeric literal that consists of multiple digits.
  
  
• 1E10 -> 1e10, and similarly 2J -> 2j and (Python 2) 2L -> 2l
• 1e+10 -> 1e10
• 0XABCD -> 0xabcd (so generally, all letters are lowercased)
• .1 -> 0.1 and 1. -> 1.0
  
  
  
  • Not too sure about whether this is a good idea
  
  
• 0xabcdefgh stays unchanged, instead of changing to 0x_abcd_efgh or similar. Similarly, no underscores in octal or binary literals. PEP 515 (https://www.python.org/dev/peps/pep-0515/#id26) suggests splitting hex literals by words and binary literals by nybbles in examples. The current code also doesn't remove any underscores in hex/oct/bin literals.

Let me know if you disagree with any of these. If nobody feels strongly, we can just stick with the current behavior.

[jgirardet]

Hi,
0.12_345_678 has no sense for me (I'm french, regional choice ?), 0.123_456_78 is far better but seems unusual.
The others seems ok to me

[zsol]

(Python 2) 2L -> 2l

I realize this would make the rules less consistent, but I think this deserves a special case. In most fonts, 2l is hard to distinguish from 21 so I usually prefer 2L to enhance readability.

0.12345678 -> 0.12_345_678

I also think 0.123_456_78 is easier to read because you don't have to remember the initial "offset" all the way while reading from left to right. I think it's helpful to use underscores in this case because of the same reasons as with integers.

.1 -> 0.1 and 1. -> 1.0

I think the second one is actually really helpful because it's easy to miss an extra dot at the end of a line. I don't have as strong of an opinion on .1 probably because I've gotten used to it.

[JelleZijlstra]

OK, I'm going to go with these two changes (capitalize 2L and change underscores in the fractional part of floats). Thanks for the responses!