trying to learn f-strings - puzzled by format() protocol

[DeaD_EyE]

It's formatted one time. Here the proof of it:

# explaining with code
class MyFormat(str):
    def __format__(self, fmt_str):
        print('Magic method __format__ was called.')
        fmt_str = fmt_str.strip()
        if 'UPPER' == fmt_str:
            return self.upper()
        elif 'LOWER' == fmt_str:
            return self.lower()
        else:
            return self




my_str = MyFormat('Hello World')

# 3 calls, 3 times formatting
print(f'{my_str: UPPER }')
print(f'{my_str: LOWER }')
print(f'{my_str}')


# the same with the format method
# 3 calls, 3 times formatting
print('{:UPPER}'.format(my_str))
print('{:LOWER }'.format(my_str))
print('{}'.format(my_str))

Output:
Magic method __format__ was called.
HELLO WORLD
Magic method __format__ was called.
hello world
Magic method __format__ was called.
Hello World
Magic method __format__ was called.
HELLO WORLD
Magic method __format__ was called.
hello world
Magic method __format__ was called.
Hello World