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.

Solution

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)))
    ...

Comments

[Stefan B]

So You would also use isinstance() in that example and not just expect the right type right away?

[Moses Koledoye]

@StefanB You can, to prevent users from passing other numeric types such as complex types.