How do I overload `float()` for a custom class?

Question

How do I implement a method that overloads float(), i.e. it is called when an instance of a class I've written is passed to float()?

I am starting to write my own Fraction class:

class Fraction:
    def __init__(self, num = 0, denom = 1):
        self.num = float(num)
        self.denom = float(denom)

Now I want to be able to do this:

float(Fraction())

I tried using the below method, but that didn't work.

def float(self):
    return self.num / self.denom

Answer 1

Define the __float__() special method on your class.

class MyClass(object):
    def __float__(self):
         return 0.0

float(MyClass())   # 0.0

Note that this method must return a float! The calculation self.num / self.denom, returns an int by default in versions of Python prior to 3.0 assuming both operands are integers. In this case you'd just make sure one of the operands is a float: float(self.num) / self.denom for example.