[Python] Why do f-strings require parentheses around assignment expressions?

In Python (3.11) why does the use of an assignment expression (the "walrus operator") require wrapping in parentheses when used inside an f-string?

For example:

#!/usr/bin/env python

from pathlib import Path

import torch


DEVICE = torch.device("cuda" if torch.cuda.is_available() else "cpu")

ckpt_dir = Path("/home/path/to/checkpoints")

_ckpt = next(ckpt_dir.iterdir())
print(_ckpt)
sdict = torch.load(_ckpt, map_location=DEVICE)

model_dict = sdict["state_dict"]

for k, v in model_dict.items():
print(k)
print(type(v))
print(_shape := v.size())
print(f"{(_numel := v.numel())}")
print(_numel == torch.prod(torch.tensor(_shape)))


The code block above with print(f"{_numel := v.numel()}") instead does not parse.

What about the parsing / AST creation mandates this?

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