It just calls the repr of the value supplied.

It's usage is generally not really needed with f-strings since with them you can just do repr(self.radius) which is arguably more clear in its intent.

!r (repr), !s (str) and !a (ascii) were kept around just to ease compatibility with the str.format alternative, you don't need to use them with f-strings.

From "Formatted string literals" in the Python reference: f-strings invoke the "format() protocol", meaning that the __format__ magic method is called instead of __str__.

class Foo:
    def __repr__(self):
        return "Foo()"

    def __str__(self):
        return "A wild Foo"
    
    def __format__(self, format_spec):
        if not format_spec:
            return "A formatted Foo"
        return f"A formatted Foo, but also {format_spec}!"

>>> foo = Foo()
>>> repr(foo)
'Foo()'
>>> str(foo)
'A wild Foo'
>>> format(foo)
'A formatted Foo'
>>> f"{foo}"
'A formatted Foo'
>>> format(foo, "Bar")
'A formatted Foo, but also Bar!'
>>> f"{foo:Bar}"
'A formatted Foo, but also Bar!'

If you don't want __format__ to be called, you can specify !s (for str), !r (for repr) or !a (for ascii) after the expression:

>>> foo = Foo()
>>> f"{foo}"
'A formatted Foo'
>>> f"{foo!s}"
'A wild Foo'
>>> f"{foo!r}"
'Foo()'

This is occasionally useful with strings:

>>> key = 'something\n nasty!'
>>> error_message = f"Key not found: {key!r}"
>>> error_message
"Key not found: 'something\\n nasty!'"