Pythonic way to have a choice of 2-3 options as an

Question

I have a Python function which requires a number of parameters, one of which is the type of simulation to perform. For example, the options could be "solar", "view" or "both.

What is a Pythonic way to allow the user to set these?

I can see various options:

1. Use a string variable and check it - so it would be func(a, b, c, type='solar')

2. Set some constants in the class and use func(a, b, c, type=classname.SOLAR)

3. If there are only two options (as there are for some of my functions) force it into a True/False argument, by using something like func(a, b, c, do_solar=False) to get it to use the 'view' option.

Any preferences (or other ideas) for Pythonic ways of doing this?

Answer 1

Since functions are objects in python, you could actually process *args as a list of methods and pass the types of simulations as arbitratry args at the end. This would have the benefit of allowing you to define new simulations in the future without having to refactor this code.

def func(a, b, c, *args):
    for arg in args:
        arg(a, b, c)

def foosim(a, b, c):
    print 'foosim %d' % (a + b + c)

def barsim(a, b, c):
    print 'barsim %d' % (a * b * c)

Use:

func(2, 2, 3, foosim)
func(2, 2, 3, barsim)
func(2, 2, 3, foosim, barsim)

Output:

foosim 7
barsim 12
foosim 7
barsim 12

Answer 2

You can use optional (keyword) arguments like this

def func(a, b, c, **kw):
    if kw.get('do_solar'):
        # Do Solar
    if kw.get('do_view'):
        # Do view

Answer 3

If the point Niklas' makes in his answer doesn't hold, I would use a string argument. There are Python modules in the standard library that use similar arguments. For example csv.reader().

sim_func(a, b, c, sim_type='solar')

Remember to give a reasonable error inside the function, that helps people out if they type in the wrong thing.

def sim_func(a, b, c, sim_type='solar'):
    sim_types = ['solar', 'view', 'both']
    if sim_type not in sim_types:
        raise ValueError("Invalid sim type. Expected one of: %s" % sim_types)
    ...

Answer 4

I don't like any of those options.

I'd define two different functions, perform_solar(a, b, c) and perform_view(a, b, c) and let the caller decide which ones he wants to use, in which order and with which arguments.

If the reason why you thought you'd have to pack these into one single function is that they share state, you should share that state in an object and define the functions as methods.

Answer 5

You can use the assert statement like this:

assert sim_types in ['solar', 'view', 'both'], 'sim type parameter must be solar, view or both'

If sim_types is not in the list, python will raise an Assertion Error