Okay, I get it. Someone might be using two braces in the format specifier because they found that it is a way to both evaluate a sub-expression and get braces in the formatted result. I was thinking that they would just use three braces, but that does not appear to work, though I cannot understand the resulting error message.

    >>> x = 42
    >>> import datetime
    >>> now = datetime.datetime.now()

    >>> f'{now:x{x}x}'
    'x42x'

    >>> f'{now:x{{x}}x}'
    'x{42}x'

    >>> f'{now:x{{{x}}}x}'
    Traceback (most recent call last):
    ...
    TypeError: unhashable type: 'set'

I think you are right. This particular ship may have already sailed away.