Given this code:

print(f"Here is that pesky {xxx/2:.3f} again")

The traceback prints:

Traceback (most recent call last):
  File "/home/pablogsal/github/python/main/lel.py", line 1, in <module>
    print(f"Here is that pesky {xxx/2:.3f} again")
           ^^^
NameError: name 'xxx' is not defined

Removing the formatting part ":.3f" makes it work as expected

(I originally reported this.)

The problem lies here:

https://github.com/python/cpython/blob/f66d00fdd7e9a333accc6bf0e37173051aaa55d0/Parser/string_parser.c#L374-L389

The problem is that the strstr call will fail because the string containing the expression doesn't have the formatting part in it, so the offsets never get corrected.

Actually, this has even more problems. Because we are using strstr to find the start of the expression in the parent string, if the expression is repeated the offsets are incorrectly generated:

For example:

print(f"Here is that {xxx} pesky {xxx} again")

This produces:

...
                     FormattedValue(
                        value=Name(
                           id='xxx',
                           ctx=Load(),
                           lineno=1,
                           col_offset=22,
                           end_lineno=1,
                           end_col_offset=25),
...
                     FormattedValue(
                        value=Name(
                           id='xxxx',
                           ctx=Load(),
                           lineno=1,
                           col_offset=22,
                           end_lineno=1,
                           end_col_offset=25),
...

while

print(f"Here is that {xxx} pesky {xxxx} again")

(different variables) produces:

...
                     FormattedValue(
                        value=Name(
                           id='xxx',
                           ctx=Load(),
                           lineno=1,
                           col_offset=22,
                           end_lineno=1,
                           end_col_offset=25),
...
                     FormattedValue(
                        value=Name(
                           id='xxxx',
                           ctx=Load(),
                           lineno=1,
                           col_offset=34,
                           end_lineno=1,
                           end_col_offset=38),
...

Ha, we have test that knows is broken:

https://github.com/python/cpython/blob/f66d00fdd7e9a333accc6bf0e37173051aaa55d0/Lib/test/test_fstring.py#L217

New changeset 8e832fb2a2cb54d7262148b6ec15563dffb48d63 by Pablo Galindo Salgado in branch 'main':
bpo-44885: Correct the ast locations of f-strings with format specs and repeated expressions (GH-27729)
https://github.com/python/cpython/commit/8e832fb2a2cb54d7262148b6ec15563dffb48d63

New changeset c28c2e1cb0dd5fc8b17514e2c4ee17415af733a4 by Pablo Galindo Salgado in branch '3.10':
[3.10] bpo-44885: Correct the ast locations of f-strings with format specs and repeated expressions (GH-27729) (GH-27743)
https://github.com/python/cpython/commit/c28c2e1cb0dd5fc8b17514e2c4ee17415af733a4

New changeset 4b86c9c5146c339c689830619be9d29b8f7bf417 by Pablo Galindo Salgado in branch '3.9':
[3.9] bpo-44885: Correct the ast locations of f-strings with format specs and repeated expressions (GH-27729) (GH-27744)
https://github.com/python/cpython/commit/4b86c9c5146c339c689830619be9d29b8f7bf417