[Python-Dev] PEP 515: Underscores in Numeric Literals

[Steven D'Aprano]

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