Implement JavaScript's style of optional parameter in Python

Question

JavaScript style

Suppose I have a function takes two objects as optional parameters input, and some of the args in the objects has default value, in JavaScript, I may implement it like this:

function func(setting1, setting2){
    arga = setting1.a || 1;
    argb = setting2.b || 2;
    argc = setting2.c;
    ...
}

Python implementation

There's some ideas comes to my mind, but i think none of them is actually "good".

Here's the first one:

params = {"arga":2, "argb":3}
def func(argc, arga=1, argb=2):
    ...
func(**params)

The question is, this type of implementation cannot support the semantic i want, that is, the setting1 and setting2 need two be separated since they serve for different sub-procedures. Also, It need to manually choose the default parameters's position carefully.

Here's the second one:

def func(setting1, setting2):
    try:
        arga = setting1.a
    except KeyError:
        arga = 1
    ...

I think it's quite ugly.

Maybe this is the best one:

def func(setting1, setting2):
    arga = setting1.get("a") or 1
    argb = setting1.get("b") or 2
    ...

Is there any good way to implement this in python?

ForceBru · Accepted Answer · 2018-07-29 11:27:00Z

1

If settingN is a dict, you can do this:

argZ = settingN.get('z', default_value)

So, if settingN doesn't have the key 'z', default_value will be returned.

answered Jul 29, 2018 at 11:27

ForceBru

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kabanus · Accepted Answer · 2018-07-29 11:35:06Z

1

Not sure what you mean by ugly, but this is closest to what you want I think:

def func(setting1 = None, setting2 = None):
    setting1 = "a" if setting1 is None else setting1 
    setting2 = "b" if setting2 is None else setting2

Note there is no real point to this in Python, since it supports default parameters and keyed calls, so

def func(setting1 = "A", setting2 = "B"):

called with

func(setting2="C")

will set only the second setting, and use default for the first. Of course, if the default involves a function call then you have to use your solution.

edited Jul 29, 2018 at 11:35

answered Jul 29, 2018 at 11:24

kabanus

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3 Comments

user8510613 Over a year ago

Since the parameters could be long

kabanus Over a year ago

@user8510613 I did not understand the comment. In any case, the first solution is the most JS similar solution I know of, requiring no changes on the caller side (like requiring a dictionary).

user8510613 Over a year ago

You're right, i think i could use the ** operator to unpack the two parameter dict to different sub-procedure.

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