Random numbers for Python class init function

I'm generating objects from a class. One of the, optional, attributes is a number. When no number is passed, I want to generate a random one. Unfortunately, when using numpy rand it seems a single random number is chosen and used for all future objects.

Below there's an example.

import numpy as np
class Class_1:
     def __init__(self, num=np.random.rand()):
         self.num = num

obj_1 = Class_1()
obj_2 = Class_1()
obj_3 = Class_1()

>>> obj_1.num
0.4664677766265517
>>> obj_2.num
0.4664677766265517
>>> obj_3.num
0.4664677766265517

I could use

obj_1 = Class_1(np.random.rand())
obj_2 = Class_1(np.random.rand())
obj_3 = Class_1(np.random.rand())

but that's not very pretty.