On Thu, Feb 11, 2016 at 10:15 AM, Andrew Barnert via Python-Dev < python-dev@python.org> wrote:
> On Feb 11, 2016, at 09:39, Terry Reedy <tjre...@udel.edu> wrote: > > > > If trailing _ is allowed, to simplify the implementation, I would like > PEP 8, while on the subject, to say something like "While trailing _s on > numbers are allowed, to simplify the implementation, they serve no purpose > and are strongly discouraged". > > That's a good point: we need style rules for PEP 8. > > But I think everything that's just obviously pointless (like putting an > underscore between every pair of digits, or sprinkling underscores all over > a huge number to make ASCII art), or already handled by other guidelines > (e.g., using a ton of underscores to "line up a table" is the same as using > a ton of spaces, which is already discouraged) doesn't really need to be > covered. And I think trailing underscores probably fall into that category. > > It might be simpler to write a "whitelist" than a "blacklist" of all the > ugly things people might come up with, and then just give a bunch of > examples instead of a bunch of rules. Something like this: > > While underscores can legally appear anywhere in the digit string, you > should never use them for purposes other than visually separating > meaningful digit groups like thousands, bytes, and the like. > > 123456_789012: ok (millions are groups, but thousands are more common, > and 6-digit groups are readable, but on the edge) > 123_456_789_012: better > 123_456_789_012_: bad (trailing) > 1_2_3_4_5_6: bad (too many) > 1234_5678: ok if code is intended to deal with east-Asian numerals > (where 10000 is a standard grouping), bad otherwise > 3__141_592_654: ok if this represents a fixed-point fraction > (obviously bad otherwise) > 123.456_789e123: good > 123.456_789e1_23: bad (never useful in exponent) > 0x1234_5678: good > 0o123_456: good > 0x123_456_789: bad (3 hex digits is usually not a meaningful group) > > The one case that seems contentious is "123_456_j". Honestly, I don't care > which way that goes, and I'd be fine if the PEP left out any mention of it, > but if people feel strongly one way or the other, the PEP could just give > it as a good or a bad example and that would be enough to clarify the > intention. > I imagine that for whatever "bad" grouping you can suggest, someone, somewhere, has a legitimate reason to use it. Any rule more complex than "Use underscores in numeric literals only when the improve clarity" is unnecessarily prescriptive. - Jeff
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