`f'"{var:}"'` adds redundant empty string literal in AST in 3.12

[sobolevn]

3.11:

>>> import ast
>>> print(ast.dump(ast.parse("""f'"{var:}"'"""), indent=2))
Module(
  body=[
    Expr(
      value=JoinedStr(
        values=[
          Constant(value='"'),
          FormattedValue(
            value=Name(id='var', ctx=Load()),
            conversion=-1,
            format_spec=JoinedStr(values=[])),
          Constant(value='"')]))],
  type_ignores=[])

3.12:

>>> import ast
>>> print(ast.dump(ast.parse("""f'"{var:}"'"""), indent=2))
Module(
  body=[
    Expr(
      value=JoinedStr(
        values=[
          Constant(value='"'),
          FormattedValue(
            value=Name(id='var', ctx=Load()),
            conversion=-1,
            format_spec=JoinedStr(
              values=[
                Constant(value='')])),
          Constant(value='"')]))],
  type_ignores=[])

This affects several linters:

1. Fix parse test case on Python 3.12 mypy#15577
2. B907 crashes when bugbear is run on Python 3.12 PyCQA/flake8-bugbear#393

Is this a bug? Or is it a feature?

Linked PRs

• gh-106396: Special-case empty format spec to gen empty JoinedStr node #106401
• [3.12] gh-106396: Special-case empty format spec to gen empty JoinedStr node (GH-106401) #106416

[sunmy2019]

First of all, it is not wrong. We do have an empty format spec string here.

Fixing it for compatibility reasons is also acceptable to me. I am 60-40 towards fixing it.

[lysnikolaou]

I'd say let's fix it both for compatibility and for consistency with things like an empty f-string, which is also a JoinedStr with an empty values array.

cpython on main via C v14.0.3-clang via 🐍 pyenv 3.11.3 
❯ ./python.exe           
Python 3.13.0a0 (heads/main:6c60684bf5, Jul  4 2023, 11:09:56) [Clang 14.0.3 (clang-1403.0.22.14.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ast
>>> print(ast.dump(ast.parse("""f''"""), indent=2))
Module(
  body=[
    Expr(
      value=JoinedStr(values=[]))],
  type_ignores=[])
>>> print(ast.dump(ast.parse("""f'"{var:}"'"""), indent=2))
Module(
  body=[
    Expr(
      value=JoinedStr(
        values=[
          Constant(value='"'),
          FormattedValue(
            value=Name(id='var', ctx=Load()),
            conversion=-1,
            format_spec=JoinedStr(
              values=[
                Constant(value='')])),
          Constant(value='"')]))],
  type_ignores=[])