optional arguments in initializer of python class

You can set default parameters:

class OpticalTransition(object):
    def __init__(self, chemical, i, j=None, k=0):
        self.chemical = chemical
        self.i = i
        self.k = k
        self.j = j if j is not None else i

If you don’t explicitly call the class with j and k, your instance will use the defaults you defined in the init parameters. So when you create an instance of this object, you can use all four parameters as normal: OpticalTransition('sodium', 5, 100, 27)

Or you can omit the parameters with defaults with OpticalTransition('sodium', 5), which would be interpreted as OpticalTransition('sodium', 5, None, 0)

You can use some default values but not all of them as well, by referencing the name of the parameter: OpticalTransition('sodium', 5, k=27) uses j‘s default but not k‘s.

Python won’t allow you to do j=i as a default parameter (i isn’t an existing object that the class definition can see), so the self.j line handles this with an if statement that in effect does the same thing.