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.