Python assert isinstance() Vector

Question

I am trying to implement a Vector3 class in python. If I would write the Vector3 class in c++ or c# i would have X, Y, and Z members stored as floats but in python I read that ducktyping is the way to go. So according to my c++/c# knowledge I wrote something like this:

class Vector3:
    def __init__(self, x=0.0, y=0.0, z=0.0):
        assert (isinstance(x, float) or isinstance(x, int)) and (isinstance(y, float) or isinstance(y, int)) and \
               (isinstance(z, float) or isinstance(z, int))
        self.x = float(x)
        self.y = float(y)
        self.z = float(z)

The question is regarding the assert statements: Would you use them or not in this instance (a Vector3 implementation for math). I also used it for the operations like

def __add__(self, other):
    assert isinstance(other, Vector3)
    return Vector3(self.x + other.x, self.y + other.y, self.z + other.z)

Would you use assert in these instances or not? According to this website : https://wiki.python.org/moin/UsingAssertionsEffectively it should not be overused but for me as someone who used static typing all along, not checking for the same datatype is extremely weird.

Answer 1

assert is better used for debugging than left loafing around in production code. You can instead create properties for the vector attributes x, y and z, and raise ValueError when the passed values are not of the required type:

class Vector3:
    def __init__(self, x=0.0, y=0.0, z=0.0):
        self.x = x
        self.y = y
        self.z = z

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, val):
        if not isinstance(val, (int, float)):
            raise TypeError('Inappropriate type: {} for x whereas a float \
            or int is expected'.format(type(val)))
        self._x = float(val)

    ...

Notice how isinstance also takes a tuple of types.

In the __add__ operator, you'll want to raise TypeError also, including an appropriate message:

def __add__(self, other):
    if not isinstance(other, Vector3):
        raise TypeError('Object of type Vector3 expected, \
        however type {} was passed'.format(type(other)))
    ...