Storing parameters in a class, and how to access them.

Question

I'm writing a program that randomly assembles mathematical expressions using the values stored in this class. The operators are stored in a dictionary along with the number of arguements they need. The arguements are stored in a list. (the four x's ensure that the x variable gets chosen often) depth, ratio, method and riddle are other values needed.

I put these in a class so they'd be in one place, where I can go to change them. Is this the best pythonic way to do this? It seems that I can't refer to them by Params.depth. This produces the error 'Params has no attribute 'depth'. I have to create an instance of Params() (p = Params()) and refer to them by p.depth.

I'm faily new to Python. Thanks

class Params(object):
    def __init__(self):
        object.__init__(self)
        self.atoms =['1.0','2.0','3.0','4.0','5.0','6.0','7.0','8.0','9.0','x','x','x','x']
        self.operators = {'+': 2, '-': 2, '*': 2, '/': 2,'+': 2, '-': 2, '*': 2, '/': 2, '**': 2, '%': 2}
        self.depth = 1
        self.ratio = .4
        self.method = ''
        self.riddle = '1 + np.sin(x)'

Answer 1

What you have there are object properties. You mean to use class variables:

class Params(object):
    atoms =['1.0','2.0','3.0','4.0','5.0','6.0','7.0','8.0','9.0','x','x','x','x']
    operators = {'+': 2, '-': 2, '*': 2, '/': 2,'+': 2, '-': 2, '*': 2, '/': 2, '**': 2, '%': 2}
    depth = 1
    ratio = .4
    method = ''
    riddle = '1 + np.sin(x)'

# This works fine:
Params.riddle

It's fairly common in Python to do this, since pretty much everyone agrees that Params.riddle is a lot nicer to type than Params['riddle']. If you find yourself doing this a lot you may want to use this recipe which makes things a bit easier and much clearer semantically.

Warning: if that Params class gets too big, an older, grumpier Pythonista may appear and tell you to just move all that crap into its own module.

Answer 2

You can do this in a class, but I'd prefer to just put it in a dictionary. Dictionaries are the all-purpose container type in Python, and they're great for this sort of thing.

params = {
    atoms: ['1.0','2.0','3.0','4.0','5.0','6.0','7.0','8.0','9.0','x','x','x','x'],
    operators: {'+': 2, '-': 2, '*': 2, '/': 2,'+': 2, '-': 2, '*': 2, '/': 2, '**': 2, '%': 2},
    depth: 1,
    ratio: .4,
    method: '',
    riddle = '1 + np.sin(x)'
}

Now you can do params['depth'] or params['atoms'][2]. Admittedly, this is slightly more verbose than the object form, but personally I think it's worth it to avoid polluting the namespace with unnecessary class definitions.