Python expressions in f-strings do not return same result as str() conversion for subclasses?

Question

For example, we override the __str__ method in Decimal then call it two different ways

from decimal import Decimal
class D(Decimal):
    def __str__(self):
        return super().normalize().__str__()
num = D('1.0')
print(f"{num}")
print(f"{str(num)}")

outputs

1.0
1

But they should both output 1. It seems like the first print is calling Decimal.__str__ instead of D.__str__ for some reason. Why does this happen and how do I fix the behavior?

Answer 1

You'll need to override __format__ too (see the f-string PEP, search for __format__).

from decimal import Decimal


class D(Decimal):
    def __str__(self):
        return super().normalize().__str__()

    def __format__(self, format_spec):
        return super().normalize().__format__(format_spec)


num = D('1.0')
print(f"{num}")
print(f"{str(num)}")

prints out

1
1