PEP 515: Underscores in Numeric Literals (revision 3)
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: $Revision$ Last-Modified: $Date$ 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 .. [5] https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift... .. [6] https://github.com/dotnet/roslyn/issues/216 .. [7] https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-... .. [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-... .. [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.
On 13.02.16 10:48, Georg Brandl wrote:
Following the same rules for placement, underscores will be allowed in the following constructors:
- ``int()`` (with any base) - ``float()`` - ``complex()`` - ``Decimal()``
What about float.fromhex()? Should underscores be allowed in it (I think no)?
On 02/13/2016 12:10 PM, Serhiy Storchaka wrote:
On 13.02.16 10:48, Georg Brandl wrote:
Following the same rules for placement, underscores will be allowed in the following constructors:
- ``int()`` (with any base) - ``float()`` - ``complex()`` - ``Decimal()``
What about float.fromhex()? Should underscores be allowed in it (I think no)?
Good question. It *does* accept a "0x" prefix, as does ``int(x, 16)``, so there is some precedent for literal-like interpretation of the input here as well. Georg
On Sat, Feb 13, 2016 at 09:48:49AM +0100, Georg Brandl wrote:
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.
What about Fraction? Currently this is legal: py> Fraction("1/1000000") Fraction(1, 1000000) I think the PEP should also support underscores in Fractions: Fraction("1/1_000_000")
The remaining open question is about the reverse direction: do we want a string formatting modifier that adds underscores as thousands separators?
Yes please.
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.
/s/thousans/thousands/ -- Steve
On 2/13/2016 12:48 AM, Georg Brandl wrote:
Instead of the relatively strict rule specified above, the use of underscores could be limited. This sentence doesn't really make sense.
Either s/limited/more limited/ or s/limited/further limited/ or s/limited/relaxed/ Maybe the whole section should be reworded.
On 02/13/2016 12:48 AM, Georg Brandl wrote:
The remaining open question is about the reverse direction: do we want a string formatting modifier that adds underscores as thousands separators?
+0 Would be nice, but also wouldn't make much sense in other groupings.
Instead of the relatively strict rule specified above, the use of underscores could be limited. As we seen from other languages, common rules include:
s/seen/see or s/we// -- ~Ethan~
On Sat, Feb 13, 2016, 00:49 Georg Brandl <g.brandl@gmx.net> wrote:
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.
+1 from me.
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?
+0 Brett
cheers, Georg
-----------------------------------------------------------------
PEP: 515 Title: Underscores in Numeric Literals Version: $Revision$ Last-Modified: $Date$ 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
.. [5]
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift...
.. [6] https://github.com/dotnet/roslyn/issues/216
.. [7]
https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-...
.. [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-...
.. [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.
_______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/brett%40python.org
Where did this PEP leave off? Anything blocking its acceptance? On Sat, 13 Feb 2016 at 00:49 Georg Brandl <g.brandl@gmx.net> wrote:
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: $Revision$ Last-Modified: $Date$ 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
.. [5]
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift...
.. [6] https://github.com/dotnet/roslyn/issues/216
.. [7]
https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-...
.. [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-...
.. [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.
_______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/brett%40python.org
I'm happy to accept this PEP as is stands, assuming the authors are ready for this news. I recommend also implementing the option from footnote [11] (extend the number-to-string formatting language to allow ``_`` as a thousans separator). On Thu, Mar 17, 2016 at 11:19 AM, Brett Cannon <brett@python.org> wrote:
Where did this PEP leave off? Anything blocking its acceptance?
On Sat, 13 Feb 2016 at 00:49 Georg Brandl <g.brandl@gmx.net> wrote:
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: $Revision$ Last-Modified: $Date$ 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
.. [5]
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift...
.. [6] https://github.com/dotnet/roslyn/issues/216
.. [7]
https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-...
.. [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-...
.. [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.
_______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/brett%40python.org
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-- --Guido van Rossum (python.org/~guido)
I'll update the text so that the format() gets promoted from optional to specified. There was one point of discussion in the tracker issue that should be resolved before acceptance: the Decimal constructor is listed as getting updated to allow underscores, but its syntax is specified in the Decimal spec: http://speleotrove.com/decimal/daconvs.html Acccepting underscores would be an extension to the spec, which may not be what we want to do as otherwise Decimal follows that spec closely. On the other hand, assuming decimal literals are introduced at some point, they would almost definitely need to support underscores. Of course, the decision whether to modify the Decimal constructor can be postponed until that time. cheers, Georg On 03/19/2016 01:02 AM, Guido van Rossum wrote:
I'm happy to accept this PEP as is stands, assuming the authors are ready for this news. I recommend also implementing the option from footnote [11] (extend the number-to-string formatting language to allow ``_`` as a thousans separator).
On Thu, Mar 17, 2016 at 11:19 AM, Brett Cannon <brett@python.org> wrote:
Where did this PEP leave off? Anything blocking its acceptance?
On Sat, 13 Feb 2016 at 00:49 Georg Brandl <g.brandl@gmx.net> wrote:
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: $Revision$ Last-Modified: $Date$ 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
.. [5]
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift...
.. [6] https://github.com/dotnet/roslyn/issues/216
.. [7]
https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-...
.. [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-...
.. [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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On 19 March 2016 at 16:44, Georg Brandl <g.brandl@gmx.net> wrote:
On the other hand, assuming decimal literals are introduced at some point, they would almost definitely need to support underscores. Of course, the decision whether to modify the Decimal constructor can be postponed until that time.
The idea of Decimal literals is complicated significantly by their current context dependent behaviour (especially when it comes to rounding), so I'd suggest leaving them alone in the context of this PEP. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia
I don't care too much either way, but I think passing underscores to the constructor shouldn't be affected by the context -- the underscores are just removed before parsing the number. But if it's too complicated to implement I'm fine with punting. --Guido (mobile) On Mar 19, 2016 6:24 AM, "Nick Coghlan" <ncoghlan@gmail.com> wrote:
On 19 March 2016 at 16:44, Georg Brandl <g.brandl@gmx.net> wrote:
On the other hand, assuming decimal literals are introduced at some point, they would almost definitely need to support underscores. Of course, the decision whether to modify the Decimal constructor can be postponed until that time.
The idea of Decimal literals is complicated significantly by their current context dependent behaviour (especially when it comes to rounding), so I'd suggest leaving them alone in the context of this PEP.
Cheers, Nick.
-- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/guido%40python.org
Guido van Rossum <guido <at> python.org> writes:
I don't care too much either way, but I think passing underscores to the constructor shouldn't be affected by the context -- the underscores are just removed before parsing the number. But if it's too complicated to implement I'm fine with punting.
Just removing the underscores would be fine. The problem is that per the PEP the conversion should happen according the Python float grammar but the actual decimal grammar is the one from the IBM specification. I'd much rather express the problem like you did above: A preprocessing step followed by the IBM specification grammar. Stefan Krah
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)? Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important. On Sat, Mar 19, 2016 at 10:24 AM, Stefan Krah <stefan@bytereef.org> wrote:
Guido van Rossum <guido <at> python.org> writes:
I don't care too much either way, but I think passing underscores to the constructor shouldn't be affected by the context -- the underscores are just removed before parsing the number. But if it's too complicated to implement I'm fine with punting.
Just removing the underscores would be fine. The problem is that per the PEP the conversion should happen according the Python float grammar but the actual decimal grammar is the one from the IBM specification.
I'd much rather express the problem like you did above: A preprocessing step followed by the IBM specification grammar.
Stefan Krah
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-- --Guido van Rossum (python.org/~guido)
Guido van Rossum <guido <at> python.org> writes:
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)?
Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important.
I'd keep it simple for Decimal: Remove left and right whitespace (we're already doing this), then remove underscores from the remaining string (which must not contain any further whitespace), then use the IBM grammar. We could add a clause to the PEP that only those strings that follow the spirit of the PEP are guaranteed to be accepted in the future. One reason for keeping it simple is that I would not like to slow down string conversion, but thinking about two grammars is also a problem -- part of the string conversion in libmpdec is modeled in ACL2, which would be invalidated or at least complicated with two grammars. Stefan Krah
All that sounds fine! On Sat, Mar 19, 2016 at 11:28 AM, Stefan Krah <stefan@bytereef.org> wrote:
Guido van Rossum <guido <at> python.org> writes:
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)?
Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important.
I'd keep it simple for Decimal: Remove left and right whitespace (we're already doing this), then remove underscores from the remaining string (which must not contain any further whitespace), then use the IBM grammar.
We could add a clause to the PEP that only those strings that follow the spirit of the PEP are guaranteed to be accepted in the future.
One reason for keeping it simple is that I would not like to slow down string conversion, but thinking about two grammars is also a problem -- part of the string conversion in libmpdec is modeled in ACL2, which would be invalidated or at least complicated with two grammars.
Stefan Krah
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-- --Guido van Rossum (python.org/~guido)
Is there anything holding up PEP 515 at this point in terms of acceptance or implementation? On Sat, 19 Mar 2016 at 11:56 Guido van Rossum <guido@python.org> wrote:
All that sounds fine!
On Sat, Mar 19, 2016 at 11:28 AM, Stefan Krah <stefan@bytereef.org> wrote:
Guido van Rossum <guido <at> python.org> writes:
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)?
Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important.
I'd keep it simple for Decimal: Remove left and right whitespace (we're already doing this), then remove underscores from the remaining string (which must not contain any further whitespace), then use the IBM grammar.
We could add a clause to the PEP that only those strings that follow the spirit of the PEP are guaranteed to be accepted in the future.
One reason for keeping it simple is that I would not like to slow down string conversion, but thinking about two grammars is also a problem -- part of the string conversion in libmpdec is modeled in ACL2, which would be invalidated or at least complicated with two grammars.
Stefan Krah
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If the authors are happy I'll accept it right away. (I vaguely recall there's another PEP that's ready for pronouncement -- but which one?) On Wed, May 11, 2016 at 9:34 AM, Brett Cannon <brett@python.org> wrote:
Is there anything holding up PEP 515 at this point in terms of acceptance or implementation?
On Sat, 19 Mar 2016 at 11:56 Guido van Rossum <guido@python.org> wrote:
All that sounds fine!
On Sat, Mar 19, 2016 at 11:28 AM, Stefan Krah <stefan@bytereef.org> wrote:
Guido van Rossum <guido <at> python.org> writes:
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)?
Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important.
I'd keep it simple for Decimal: Remove left and right whitespace (we're already doing this), then remove underscores from the remaining string (which must not contain any further whitespace), then use the IBM grammar.
We could add a clause to the PEP that only those strings that follow the spirit of the PEP are guaranteed to be accepted in the future.
One reason for keeping it simple is that I would not like to slow down string conversion, but thinking about two grammars is also a problem -- part of the string conversion in libmpdec is modeled in ACL2, which would be invalidated or at least complicated with two grammars.
Stefan Krah
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-- --Guido van Rossum (python.org/~guido)
On Wed, 11 May 2016 at 09:47 Guido van Rossum <guido@python.org> wrote:
If the authors are happy I'll accept it right away.
(I vaguely recall there's another PEP that's ready for pronouncement -- but which one?)
PEP 509 is the only one I can think of. -Brett
On Wed, May 11, 2016 at 9:34 AM, Brett Cannon <brett@python.org> wrote:
Is there anything holding up PEP 515 at this point in terms of acceptance or implementation?
On Sat, 19 Mar 2016 at 11:56 Guido van Rossum <guido@python.org> wrote:
All that sounds fine!
On Sat, Mar 19, 2016 at 11:28 AM, Stefan Krah <stefan@bytereef.org> wrote:
Guido van Rossum <guido <at> python.org> writes:
So should the preprocessing step just be s.replace('_', ''), or should it reject underscores that don't follow the rules from the PEP (perhaps augmented so they follow the spirit of the PEP and the letter of the IBM spec)?
Honestly I think it's also fine if specifying this exactly is left out of the PEP, and handled by whoever adds this to Decimal. Having a PEP to work from for the language spec and core builtins (int(), float() complex()) is more important.
I'd keep it simple for Decimal: Remove left and right whitespace (we're already doing this), then remove underscores from the remaining string (which must not contain any further whitespace), then use the IBM grammar.
We could add a clause to the PEP that only those strings that follow the spirit of the PEP are guaranteed to be accepted in the future.
One reason for keeping it simple is that I would not like to slow down string conversion, but thinking about two grammars is also a problem -- part of the string conversion in libmpdec is modeled in ACL2, which would be invalidated or at least complicated with two grammars.
Stefan Krah
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On Wed, May 11, 2016 at 10:28 AM, Brett Cannon <brett@python.org> wrote:
On Wed, 11 May 2016 at 09:47 Guido van Rossum <guido@python.org> wrote:
If the authors are happy I'll accept it right away.
(I vaguely recall there's another PEP that's ready for pronouncement -- but which one?)
PEP 509 is the only one I can think of.
That's in limbo pending conclusive proof (through benchmarks) that at least one of Yury's patches that depends on it makes a big enough difference. Which IIUC itself is in limbo pending the wordcode changes (we're doing that right?). -- --Guido van Rossum (python.org/~guido)
On Wed, 11 May 2016 at 10:49 Guido van Rossum <guido@python.org> wrote:
On Wed, May 11, 2016 at 10:28 AM, Brett Cannon <brett@python.org> wrote:
On Wed, 11 May 2016 at 09:47 Guido van Rossum <guido@python.org> wrote:
If the authors are happy I'll accept it right away.
(I vaguely recall there's another PEP that's ready for pronouncement -- but which one?)
PEP 509 is the only one I can think of.
That's in limbo pending conclusive proof (through benchmarks) that at least one of Yury's patches that depends on it makes a big enough difference.
Which IIUC itself is in limbo pending the wordcode changes (we're doing that right?).
Yes. Last I checked the author of the patch was waiting on a review from Serhiy. -Brett
-- --Guido van Rossum (python.org/~guido)
I'm happy with the latest version. Georg On 05/11/2016 06:46 PM, Guido van Rossum wrote:
If the authors are happy I'll accept it right away.
(I vaguely recall there's another PEP that's ready for pronouncement -- but which one?)
On Wed, May 11, 2016 at 9:34 AM, Brett Cannon <brett@python.org <mailto:brett@python.org>> wrote:
Is there anything holding up PEP 515 at this point in terms of acceptance or implementation?
On Sat, 19 Mar 2016 at 11:56 Guido van Rossum <guido@python.org <mailto:guido@python.org>> wrote:
All that sounds fine!
On Sat, Mar 19, 2016 at 11:28 AM, Stefan Krah <stefan@bytereef.org <mailto:stefan@bytereef.org>> wrote: > Guido van Rossum <guido <at> python.org <http://python.org>> writes: >> So should the preprocessing step just be s.replace('_', ''), or should >> it reject underscores that don't follow the rules from the PEP >> (perhaps augmented so they follow the spirit of the PEP and the letter >> of the IBM spec)? >> >> Honestly I think it's also fine if specifying this exactly is left out >> of the PEP, and handled by whoever adds this to Decimal. Having a PEP >> to work from for the language spec and core builtins (int(), float() >> complex()) is more important. > > I'd keep it simple for Decimal: Remove left and right whitespace (we're > already doing this), then remove underscores from the remaining string > (which must not contain any further whitespace), then use the IBM grammar. > > > We could add a clause to the PEP that only those strings that follow > the spirit of the PEP are guaranteed to be accepted in the future. > > > One reason for keeping it simple is that I would not like to slow down > string conversion, but thinking about two grammars is also a problem -- > part of the string conversion in libmpdec is modeled in ACL2, which > would be invalidated or at least complicated with two grammars. > > > > Stefan Krah > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org <mailto:Python-Dev@python.org> > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: https://mail.python.org/mailman/options/python-dev/guido%40python.org
-- --Guido van Rossum (python.org/~guido <http://python.org/~guido>) _______________________________________________ Python-Dev mailing list Python-Dev@python.org <mailto:Python-Dev@python.org> https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/brett%40python.org
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participants (9)
-
Brett Cannon -
Ethan Furman -
Georg Brandl -
Glenn Linderman -
Guido van Rossum -
Nick Coghlan -
Serhiy Storchaka -
Stefan Krah -
Steven D'Aprano