Optional arguments in initializer of Python class [duplicate]

Question

I was wondering if anyone had any good ways of quickly explaining how to efficiently and pythonically create user defined objects with optional arguments. For instance, I want to create this object:

class Object:
    def __init__(self, some_other_object, i, *j, *k):
        self.some_other_object = some_other_object
        self.i = i
        # If j is specified, assume it is = i
        if(j==None):
            self.j = i
        else:
            self.j = j
        # If k is given, assume 0
        if(k==None):
            self.k = 0
        else:
            self.k = k

Is there a better way to do this?

This isn't valid, you can't have multiple * arguments. The argument prefixed with a * becomes a list of all non-keyword arguments that come after that point. — Gareth Latty
– Gareth Latty, Commented Mar 20, 2013 at 22:12
Thanks @Lattyware , that helps. I saw the *args and *keywordsargs, etc. and thought they may have been referring to different arguments passed. — chase
– chase, Commented Mar 20, 2013 at 22:17
toxotes's answer is what you want, it just has one major flaw (see my comment on it). — Gareth Latty
– Gareth Latty, Commented Mar 20, 2013 at 22:17

toxotes · Accepted Answer · 2019-07-05 23:23:15Z

64

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.

edited Jul 5, 2019 at 23:23

answered Mar 20, 2013 at 22:14

toxotes

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8 Comments

Gareth Latty Over a year ago

Generally, by the way, don't bother with if x == None, just do if x. is bad advice. Many values can evaluate to False - if you want to check for None, do if x is None:, not if x:. In this example, if j is 0, it will do something unexpected. -1, until this is fixed.

toxotes Over a year ago

You're right -- I was thinking the OP was doing that to see if it existed at all, not necessarily to see if they had the value of None.

chase Over a year ago

Thanks @toxotes , this makes sense and looks very simple.

chase Over a year ago

@Lattyware Shouldn't it be 'if j is not None:'? since it is already 'None' by default?

Gareth Latty Over a year ago

@chase Good catch, fixed - it was actually a little more complicated a fault than that, but should be good now. I felt the ternary operator is a good choice here.

|

mertzkeaton · Accepted Answer · 2020-04-26 23:33:14Z

5

I am too new to stackoverflow to up-vote or comment, but I approve of ronak's answer. Gareth Latty's comment about default parameters is addressed by using the [default] option of the get method.

dictionary.get(key[, default])

using **args allows for less/ more readable code when there are many optional variables to be passed. It is simply passing passing dictionary key-value pairs as parameters.

answered Apr 26, 2020 at 23:33

mertzkeaton

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