How to define a method like int.from_bytes() in Python?

Question

In python there is a method on the type int, namely int.from_bytes. It's not a method on a particular int, instead it's a method on the type. E.g.

>>> int.from_bytes(b'\xee\xff',"big")
61183

>>> int.from_bytes
<built-in method from_bytes of type object at 0x107fdb388>

How do I define something like this? Let's say a class called "point" is defined, how do I define something like

>>> point.from_coordinates(3,5)
<__main__.point object at 0x10c0c9310>

>>> point.from_keys(b'\x12\x3e')
<__main__.point object at 0x10bed5890>

? (Assuming points are initialized by some different method.)

Answer 1

You want classmethod, which is normally used as a decorator:

class point(object):
    @classmethod
    def from_coordinates(cls, x, y):
        pt = cls()
        pt.x, pt.y = x, y
        return pt

---

This is sometimes called the "alternate constructor" idiom. If there are multiple different ways to construct your type, instead of putting them all into one __init__ method with a mess of optional parameters or varags, put them all into separate classmethods.

Answer 2

You can use a classmethod, such as the mocked up example whose init expects ints, but also provides a convenient from_hex that tries to take strings and convert them to integers...

class MyClass(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b
    @classmethod
    def from_hex(cls, a, b):
        return cls(int(a, 16), int(b, 16))

The from_hex knows which class it is associated with, so by calling cls(...) you're able to construct a type of MyClass as though you'd written MyClass(a, b) with valid integers yourself.