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()

Further changes

The new-style number-to-string formatting language will be extended to allow _ as a thousands separator, where currently only , is supported. This can be used to easily generate code with more readable literals. [11]

The syntax would be the same as for the comma, e.g. {:10_} for a width of 10 with _ separator.

For the b, x and o format specifiers, _ will be allowed and group by 4 digits.

Underscore Placement Rules

Instead of the relatively strict rule specified above, the use of underscores could be less limited. As seen in 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]