[Python-Dev] PEP 515: Underscores in Numeric Literals (original) (raw)

Steven D'Aprano steve at pearwood.info
Wed Feb 10 19:04:27 EST 2016

On Wed, Feb 10, 2016 at 11:20:38PM +0100, Georg Brandl wrote:

This came up in python-ideas, and has met mostly positive comments, although the exact syntax rules are up for discussion.

Nicely done. But I would change the restrictions to a simpler version. Instead of five rules to learn:

The current proposal is to allow underscores anywhere in numeric literals, with these exceptions:

* Leading underscores cannot be allowed, since they already introduce identifiers. * Trailing underscores are not allowed, because they look confusing and don't contribute much to readability. * The number base prefixes 0x, 0o, and 0b cannot be split up, because they are fixed strings and not logically part of the number. * No underscore allowed after a sign in an exponent (1e-5), because underscores can also not be used after the signs in front of the number (-1e5). * No underscore allowed after a decimal point, because this leads to ambiguity with attribute access (the lexer cannot know that there is no number literal in foo.5).

change to a single rule "one or more underscores may appear between two (hex)digits, but otherwise nowhere else". That's much simpler to understand than a series of restrictions as given above.

That would be your second restrictive rule:

"Multiple consecutive underscore allowed, but only between digits."

That forbids leading and trailing underscores, underscores inside or immediately after the leading number base (since x, o and b aren't digits), and immediately before or after the sign, decimal point or e|E exponent symbol.

There appears to be no reason to restrict the use of underscores otherwise.

I don't like underscores immediately before the . or e|E in floats either: 123_.000_456

The dot is already visually distinctive enough, as is the e|E, and placing an underscore immediately before them doesn't aid in grouping the digits.

Instead of the liberal rule specified above, the use of underscores could be limited. Common rules are (see the "other languages" section):

* Only one consecutive underscore allowed, and only between digits. * Multiple consecutive underscore allowed, but only between digits.

I don't think there is any need to restrict it to only a single underscore. There are uses for more than one:

Fraction(3__141_592_654, 1_000_000_000)

hints that the 3 is somewhat special (for obvious reasons).

-- Steve

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