empty string constant in f-string format_spec

[15r10nk]

Bug report

Bug description:

The f-string f'{1:{name}}' has a extra Constant(value='') in the format_spec part in 3.12, which is not part of the AST in 3.11. This has also an effect on the generated bytecode.

import ast
import dis

print(ast.dump(ast.parse("f'{1:{name}}'"),indent=2))

dis.dis(compile("f'{1:{name}}'","<string>","exec"))

output (Python 3.12.0):

Module(
  body=[
    Expr(
      value=JoinedStr(
        values=[
          FormattedValue(
            value=Constant(value=1),
            conversion=-1,
            format_spec=JoinedStr(
              values=[
                FormattedValue(
                  value=Name(id='name', ctx=Load()),
                  conversion=-1),
                Constant(value='')]))]))],            # <-- extra Constant
  type_ignores=[])
  0           0 RESUME                   0

  1           2 LOAD_CONST               0 (1)
              4 LOAD_NAME                0 (name)
              6 FORMAT_VALUE             0
              8 LOAD_CONST               1 ('')        # <-- extra LOAD_CONST
             10 BUILD_STRING             2             # <-- extra BUILD_STRING
             12 FORMAT_VALUE             4 (with format)
             14 POP_TOP
             16 RETURN_CONST             2 (None)

output (Python 3.11.3):

Module(
  body=[
    Expr(
      value=JoinedStr(
        values=[
          FormattedValue(
            value=Constant(value=1),
            conversion=-1,
            format_spec=JoinedStr(
              values=[
                FormattedValue(
                  value=Name(id='name', ctx=Load()),
                  conversion=-1)]))]))],
  type_ignores=[])
  0           0 RESUME                   0

  1           2 LOAD_CONST               0 (1)
              4 LOAD_NAME                0 (name)
              6 FORMAT_VALUE             0
              8 FORMAT_VALUE             4 (with format)
             10 POP_TOP
             12 LOAD_CONST               1 (None)
             14 RETURN_VALUE

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-110309: prune empty constant in format specs #110320

[sunmy2019]

Confirmed also on main: 8d92b6e

[pablogsal]

@lysnikolaou @isidentical can you take a look? If you are not available I can take a look at this next week

[sunmy2019]

I proposed a fix: apply the same empty constant prune algorithm here.