An unexpected behavior lookup:

>>> ast.FormattedValue(ast.Str('ss')).conversion
>>> ast.unparse(ast.FormattedValue(ast.Str('ss')))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/ast.py", line 1564, in unparse
    return unparser.visit(ast_obj)
  File "/usr/local/lib/python3.9/ast.py", line 801, in visit
    self.traverse(node)
  File "/usr/local/lib/python3.9/ast.py", line 795, in traverse
    super().visit(node)
  File "/usr/local/lib/python3.9/ast.py", line 407, in visit
    return visitor(node)
  File "/usr/local/lib/python3.9/ast.py", line 1153, in visit_FormattedValue
    self._fstring_FormattedValue(node, self.buffer_writer)
  File "/usr/local/lib/python3.9/ast.py", line 1178, in _fstring_FormattedValue
    conversion = chr(node.conversion)
TypeError: an integer is required (got type NoneType)
>>> ast.unparse(ast.FormattedValue(ast.Str('ss'), -1))
'f"{\'ss\'}"'

ast.FormattedValue conversion's default value is expected to be -1 but not None

See: https://docs.python.org/3/library/ast.html#ast.FormattedValue

Other:

If certainly, it's also a bug on typeshed.

ASDL technically allows it to be None though neither compiler nor ast.unparse can work with it at this moment.

FormattedValue(expr value, int? conversion, expr? format_spec)

cpython/Parser/Python.asdl

Line 78 in b127e70

| FormattedValue(expr value, int? conversion, expr? format_spec)

>>> import ast
>>> tree = ast.parse("f'{x + 1}'")
>>> tree.body[0].value.values[0].conversion = None
>>> compile(tree, "<test>", "exec")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: Unrecognized conversion character 0

We can either:
A) change ASDL to reflect it is an integer (since it always has been treated that way)
B) Support this on both the compiler as well as in the ast.unparse

I'd say A, since this was always broken. @pablogsal @lys.nikolaou WDYT?

I agree that A is probably the way to go.

I think A is the best option

New changeset d382f7e by Batuhan Taskaya in branch 'main':
bpo-46289: Make conversion of FormattedValue not optional on ASDL (GH-30467)
d382f7e

Should we backport this?

On Sat, Jan 8, 2022, 12:05 AM miss-islington <report@bugs.python.org> wrote:

miss-islington <mariatta.wijaya+miss-islington@gmail.com> added the
comment:

New changeset d382f7e by Batuhan Taskaya
in branch 'main':
bpo-46289: Make conversion of FormattedValue not optional on ASDL
(GH-30467)

d382f7e

----------
nosy: +miss-islington

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue46289\>

---

I would say yes, for consistency. It doesn't have any effects on user code that I am aware

New changeset bea3f42 by Miss Islington (bot) in branch '3.10':
bpo-46289: Make conversion of FormattedValue not optional on ASDL (GH-30467)
bea3f42