JoinedStr

>>> print(ast.dump(ast.parse('f"sin({a}) is {sin(a):.3}"', mode='eval'), indent=4))
Expression(
    body=JoinedStr(
        values=[
            Constant(value='sin('),
            FormattedValue(
                value=Name(id='a'),
                conversion=-1),
            Constant(value=') is '),
            FormattedValue(
                value=Call(
                    func=Name(id='sin'),
                    args=[
                        Name(id='a')]),
                conversion=-1,
                format_spec=JoinedStr(
                    values=[
                        Constant(value='.3')]))]))