[Python-Dev] PEP 515: Underscores in Numeric Literals (revision 3) (original) (raw)
Georg Brandl g.brandl at gmx.net
Sat Feb 13 03:48:49 EST 2016
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Hi all,
after talking to Guido and Serhiy we present the next revision of this PEP. It is a compromise that we are all happy with, and a relatively restricted rule that makes additions to PEP 8 basically unnecessary.
I think the discussion has shown that supporting underscores in the from-string constructors is valuable, therefore this is now added to the specification section.
The remaining open question is about the reverse direction: do we want a string formatting modifier that adds underscores as thousands separators?
cheers, Georg
PEP: 515 Title: Underscores in Numeric Literals Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Georg Brandl, Serhiy Storchaka Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 10-Feb-2016 Python-Version: 3.6 Post-History: 10-Feb-2016, 11-Feb-2016
Abstract and Rationale
This PEP proposes to extend Python's syntax and number-from-string constructors so that underscores can be used as visual separators for digit grouping purposes in integral, floating-point and complex number literals.
This is a common feature of other modern languages, and can aid readability of long literals, or literals whose value should clearly separate into parts, such as bytes or words in hexadecimal notation.
Examples::
# grouping decimal numbers by thousands
amount = 10_000_000.0
# grouping hexadecimal addresses by words
addr = 0xDEAD_BEEF
# grouping bits into nibbles in a binary literal
flags = 0b_0011_1111_0100_1110
# same, for string conversions
flags = int('0b_1111_0000', 2)Specification
The current proposal is to allow one underscore between digits, and after base specifiers in numeric literals. The underscores have no semantic meaning, and literals are parsed as if the underscores were absent.
Literal Grammar
The production list for integer literals would therefore look like this::
integer: decinteger | bininteger | octinteger | hexinteger decinteger: nonzerodigit ([""] digit)* | "0" ([""] "0")* bininteger: "0" ("b" | "B") ([""] bindigit)+ octinteger: "0" ("o" | "O") ([""] octdigit)+ hexinteger: "0" ("x" | "X") (["_"] hexdigit)+ nonzerodigit: "1"..."9" digit: "0"..."9" bindigit: "0" | "1" octdigit: "0"..."7" hexdigit: digit | "a"..."f" | "A"..."F"
For floating-point and complex literals::
floatnumber: pointfloat | exponentfloat pointfloat: [digitpart] fraction | digitpart "." exponentfloat: (digitpart | pointfloat) exponent digitpart: digit (["_"] digit)* fraction: "." digitpart exponent: ("e" | "E") ["+" | "-"] digitpart imagnumber: (floatnumber | digitpart) ("j" | "J")
Constructors
Following the same rules for placement, underscores will be allowed in the following constructors:
int()(with any base)float()complex()Decimal()
Prior Art
Those languages that do allow underscore grouping implement a large variety of rules for allowed placement of underscores. In cases where the language spec contradicts the actual behavior, the actual behavior is listed. ("single" or "multiple" refer to allowing runs of consecutive underscores.)
- Ada: single, only between digits [8]_
- C# (open proposal for 7.0): multiple, only between digits [6]_
- C++14: single, between digits (different separator chosen) [1]_
- D: multiple, anywhere, including trailing [2]_
- Java: multiple, only between digits [7]_
- Julia: single, only between digits (but not in float exponent parts) [9]_
- Perl 5: multiple, basically anywhere, although docs say it's restricted to one underscore between digits [3]_
- Ruby: single, only between digits (although docs say "anywhere") [10]_
- Rust: multiple, anywhere, except for between exponent "e" and digits [4]_
- Swift: multiple, between digits and trailing (although textual description says only "between digits") [5]_
Alternative Syntax
Underscore Placement Rules
Instead of the relatively strict rule specified above, the use of underscores could be limited. As we seen from other languages, common rules include:
- Only one consecutive underscore allowed, and only between digits.
- Multiple consecutive underscores allowed, but only between digits.
- Multiple consecutive underscores allowed, in most positions except for the start of the literal, or special positions like after a decimal point.
The syntax in this PEP has ultimately been selected because it covers the common use cases, and does not allow for syntax that would have to be discouraged in style guides anyway.
A less common rule would be to allow underscores only every N digits (where N could be 3 for decimal literals, or 4 for hexadecimal ones). This is unnecessarily restrictive, especially considering the separator placement is different in different cultures.
Different Separators
A proposed alternate syntax was to use whitespace for grouping. Although strings are a precedent for combining adjoining literals, the behavior can lead to unexpected effects which are not possible with underscores. Also, no other language is known to use this rule, except for languages that generally disregard any whitespace.
C++14 introduces apostrophes for grouping (because underscores introduce ambiguity with user-defined literals), which is not considered because of the use in Python's string literals. [1]_
Open Proposals
It has been proposed [11]_ to extend the number-to-string formatting
language to allow _ as a thousans separator, where currently only
, is supported. This could be used to easily generate code with
more readable literals.
Implementation
A preliminary patch that implements the specification given above has been posted to the issue tracker. [12]_
References
.. [1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3499.html
.. [2] http://dlang.org/spec/lex.html#integerliteral
.. [3] http://perldoc.perl.org/perldata.html#Scalar-value-constructors
.. [4] http://doc.rust-lang.org/reference.html#number-literals
.. [6] https://github.com/dotnet/roslyn/issues/216
.. [7] https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-literals.html
.. [8] http://archive.adaic.com/standards/83lrm/html/lrm-02-04.html#2.4
.. [9] http://docs.julialang.org/en/release-0.4/manual/integers-and-floating-point-numbers/
.. [10] http://ruby-doc.org/core-2.3.0/doc/syntax/literals_rdoc.html#label-Numbers
.. [11] https://mail.python.org/pipermail/python-dev/2016-February/143283.html
.. [12] http://bugs.python.org/issue26331
Copyright
This document has been placed in the public domain.
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