I'm not Steven, but I think I'm the first one in the thread to use Einstein's phrase. As I understand your current semantics, that phrase is not the problem.

My initial impression of your intent was:

foo, bar = 42, 99

# ... a million lines ...

line = "123/"

# ... more lines ...

f"{foo}/{bar}" = line

# raises if bar wasn't previously set

# binds to prior value if it was set

But I think what you want is for the binding line never to raise, but also not to have any local means to know whether 'bar' is a name after that line. Or whether 'foo' is, for that matter.

Of course, you could test the name, e.g.:

try:

    bar

except:

    bar = remediate()

But that's cumbersome. Overall, I'd find the spooky action at a distance less troubling.

In contrast, this is straightforward:

line = "123/"

pat = "{foo}/{bar}"

dct = scan2dict(pat, line)

It's either going to create a dictionary of raise an exception. Or maybe a version of the function might make a dataclass or namedtuple instead.

I'm not sure exactly what the pattern language might look like, but whatever its details, everything is locally available.