Question

I have the following code:

class random_walk:
    #ns: number of steps
    #np: number of particles
    #dimension : choose between 1D or 2D
    def __init__(self,ns,np,dimension=None):
        self.ns=ns
        self.np=np
        self.dimension=dimension

        if  self.dimension==2:
            #--- These represent the orientation of movement ------------
            step=random.choice([[0, 1], [1, 0], [0, -1], [-1, 0]])
            def step2d(self,ns):
                return [step for i in range(ns)]

    #steps=table with trajectories that constitute from m steps
    def steps(self,ns):    
        return 2*sc.random.random_integers(0,1,size=self.ns)-1

If I do rw=random_walk(ns,np) and then rw.steps(ns) it works ok.

But if I try rw=random_walk(ns,np,dimension) and then rw.step2d(ns) it gives me: random_walk instance has no attribute 'step2d'.

As far as I understand, it is because the step2d function is in the init method. Is there a way to access it?

Thank you!

Answer 1

Yes, you need to actually put the step2d() method on the class, not in the __init__() method. Python does let you add a method to your instance dynamically, so you could define it in __init__() like you have and then add it to the instance if the dimensionality is 2, but it is kind of hairy since you also need to create a bound method wrapper for it or it won’t work right. It is also a bad design and will be confusing to other programers who look at your code. So don’t do it that way.

Instead, either:

1. Have a distinct subclass for 2D objects that has the step2d() method. You can have a factory function that returns an instance of the proper class depending on the dimensionality (or implement the factory as part of the base class’s __new__()).

2. Simply define step2d() as another method; check self.dimension right at the top and raise TypeError if dimensionality is not 2. Like so:

   def step2d(self, ns):
       if self.dimension != 2:
           raise TypeError("dimension must be 2")
       # continue with what to do if the dimensionality is 2