When to use keyword arguments for class initialisation?

Question

When creating my own classes I can't work out when I should or shouldn't use keyword arguments.

Are there any specific circumstances or general rules of thumb on this?

As far as I can tell:

Pros:

1. Explicit definition so makes it harder for variables to get put in the wrong order
2. Makes the code more readable

Cons:

1. By having a keyword argument you need to also supply a default value, which means instances can be initialised without any arguments being supplied.

Please see below MyClassA and MyClassB which illustrate these two points:

class MyClassA():

    def __init__(self,name,city):

        self.name = name
        self.city = city

x = MyClassA('Andy','New York')



class MyClassB():

    def __init__(self,name=None,city=None):

        self.name = name
        self.city = city

 y = MyClassB(city='New York',name='Andy')

Answer 1

Actually, in both cases, you can pass arguments as keyword arguments:

x = MyClassA(city='New York', name='Andy')

would also work. And this:

x = MyClassA(name='Andy', city='New York')

would also work.

The general rule of thumb (IMHO): if it is obvious at the call site what are the arguments - no need in keywords, otherwise - Explicit is better than implicit.

And just FYI, you can force usage of keyword args like so:

def fun(positional_arg1, *, keyword_only_arg1, keyword_only_arg2): ...

which means that you can't call fun with positional arguments:

>>> fun(1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: fun() takes 1 positional argument but 3 were given

and you have to provide all arguments:

>>> fun(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: fun() missing 2 required keyword-only arguments: 
'keyword_only_arg1' and 'keyword_only_arg2'

So the only way to call fun is:

fun(1, keyword_only_arg1=1, keyword_only_arg2=3)