@classmethod decorator

https://docs.python.org/3/library/functions.html

INTRODUCTION

The @classmethod decorator transforms a method into a class method. A class method receives the class as implicit first argument, just like an instance method receives the instance. Class methods are useful for creating alternate class constructors.

# Syntax.

class C:
    @classmethod
    def f(cls, arg1, arg2, ...):
        statements

# Calling: C.f() or C().f() [the instance is ignored except for its class]

POLYNOMIALS

class Poly:

    def __init__(self, c=0, n=0):
        self.data = (n+1) * [0]
        self.data[-1] = c

    @classmethod
    def fromiterable(cls, iterable):
        new_poly = cls()
        new_poly.data = list(iterable)
        return new_poly

poly1 = Poly.fromiterable((3, 5, 7))
assert poly1.data == [3, 5, 7]
assert isinstance(poly1, Poly)

class BetterPoly(Poly): pass

poly2 = BetterPoly.fromiterable([4, 3, 2])
assert isinstance(poly2, BetterPoly)