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

[Skaperen]

there seems to be a more advanced level of understanding needed to be able to interpret all kings of f-string.

You dont really need to know the inner workings of format to use f-string over format. F-strings are suppose to be more easily understandable as well as less code to type.

i need to, at least, understand what the documentation is trying to say. it describes some formatting, then it says the result is passed to __format__(). is it just the string result that is being passed? what would __format__() do with it, like that?

---

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

as described in the documentation, calling __format__() with the result would be the 2nd step.

from your example, it is calling there, so i need to understand how that works (but i will read over your code a few times to see if i can pick up on it).

---

I don't think it saying its formatted twice, the f string is passed to format() to do the actual formatting.

the documentation describes two steps, the 2nd being the call to __format__(). maybe they worded it badly?