[Python] `__str__` of a subclass is not called in an f-string

I wrote a subclass of Decimal to represent an amount of money. I wrote a custom __str__ to display the currency along with a sign format. My method works when calling str() but in a f-string somehow my custom __str__ is not used. What is happening here?

More than workarounds, I'm interested in understanding what is happening here. Something is going on that defies my current understanding of __str__, __format__ and inheritance. I thought the default behavior of format with no format specified was to delegate to str, but here it sends it to str of the parent class instead.

Here is the minimal code to reproduce:

from decimal import Decimal


class Money(Decimal):
CURRENCY = "€"

def __new__(cls, number):
return super().__new__(cls, Decimal(number).quantize(Decimal("0.01")))

def __str__(self):
return f"{self:+}{self.CURRENCY}"


m = Money(10)
print("Test 1 - str():", str(m))
print("Test 2 - print():", m)
print(f"Test 3 - f-string: {m}")
print("Test 4 - f-string str():", f"{str(m)}")


This results in

Test 1 - str(): +10.00€
Test 2 - print(): +10.00€
Test 3 - f-string: 10.00 # only here is my custom str not called
Test 4 - f-string str(): +10.00€


Python version is 3.13.7

Edit:

The solution I found that work is the following:

def __str__(self):
return super().__format__("+") + self.CURRENCY

def __format__(self, _):
return str(self)


Yet I don't fully understand why this work and not my original solution.

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