Call function without optional arguments if they are None

Question

There's a function which takes optional arguments.

def alpha(p1="foo", p2="bar"):
     print('{0},{1}'.format(p1, p2))

Let me iterate over what happens when we use that function in different ways:

>>> alpha()
foo,bar
>>> alpha("FOO")
FOO,bar
>>> alpha(p2="BAR")
foo,BAR
>>> alpha(p1="FOO", p2=None)
FOO,None

Now consider the case where I want to call it like alpha("FOO", myp2) and myp2 will either contain a value to be passed, or be None. But even though the function handles p2=None, I want it to use its default value "bar" instead.
Maybe that's worded confusingly, so let me reword that:

If myp2 is None, call alpha("FOO"). Else, call alpha("FOO", myp2).

The distinction is relevant because alpha("FOO", None) has a different result than alpha("FOO").

How can I concisely (but readably) make this distinction?

One possibility would usually be to check for None within alpha, which would be encouraged because that would make the code safer. But assume that alpha is used in other places where it is actually supposed to handle None as it does.

I'd like to handle that on the caller-side.

One possibility is to do a case distinction:

if myp2 is None:
    alpha("FOO")
else:
    alpha("FOO", myp2)

But that can quickly become much code when there are multiple such arguments. (exponentially, 2^n)

Another possibility is to simply do alpha("FOO", myp2 or "bar"), but that requires us to know the default value. Usually, I'd probably go with this approach, but I might later change the default values for alpha and this call would then need to be updated manually in order to still call it with the (new) default value.

I am using python 3.4 but it would be best if your answers can provide a good way that works in any python version.

---

The question is technically finished here, but I reword some requirement again, since the first answer did gloss over that:
I want the behaviour of alpha with its default values "foo", "bar" preserved in general, so it is (probably) not an option to change alpha itself.
In yet again other words, assume that alpha is being used somewhere else as alpha("FOO", None) where the output FOO,None is expected behaviour.

Jetport

"But assume that alpha is used in other places where it is actually supposed to handle None as it does." -- What do you mean? You still want to be able to pass None as a parameter? – Sparge

@Sparge Does my edit at the end of the question help? (I think deceze's answer is what I was looking for. I have to try it yet though) – Jetport

Answer 1

Pass the arguments as keyword arguments from a dictionary using argument unpacking. Keyword arguments can be passed as a dict using the ** operator. To filter out the arguments with None as their value use dictionary comprehension.

kwargs = dict(p1='FOO', p2=None)

alpha(**{k: v for k, v in kwargs.items() if v is not None})

Premiere

This treats p1 = None the same of course, possibly what is wanted (or irrelevant) but worth noting perhaps – Cofield

This works perfectly! Thanks! Would you mind adding a short explanation or a good link? I think I understand what it does and could google the rest by myself, but it would also be useful for future visitors – Jetport

It simply uses argument unpacking and dictionary comprehensions. – Premiere

Answer 2

But assume that alpha is used in other places where it is actually supposed to handle None as it does.

To respond to this concern, I have been known to have a None-like value which isn't actually None for this exact purpose.

_novalue = object()

def alpha(p1=_novalue, p2=_novalue):
    if p1 is _novalue:
        p1 = "foo"
    if p2 is _novalue:
        p2 = "bar"
    print('{0},{1}'.format(p1, p2))

Now the arguments are still optional, so you can neglect to pass either of them. And the function handles None correctly. If you ever want to explicitly not pass an argument, you can pass _novalue.

>>> alpha(p1="FOO", p2=None)
FOO,None
>>> alpha(p1="FOO")
FOO,bar
>>> alpha(p1="FOO", p2=_novalue)
FOO,bar

and since _novalue is a special made-up value created for this express purpose, anyone who passes _novalue is certainly intending the "default argument" behavior, as opposed to someone who passes None who might intend that the value be interpreted as literal None.

Jello

+1 , this satisfies all the conditions of OP ! I think some alternate examples of _novalue can be : passing -1 for a positive numeric argument , or passing 0.5 for a boolean argument , or passing "" where "" is an invalid argument. – Dispel

I’ve also done to differentiate None from not-set-yet on properties. It works well, but you have to be careful to always use the same _novalue. it’s easy to sprinkle around different ‘_novalue = object()’ and end up w extremely subtle bugs. – Cistern

Answer 3

although ** is definitely a language feature, it's surely not created for solving this particular problem. Your suggestion works, so does mine. Which one works better depends on the rest of the OP's code. However, there is still no way to write f(x or dont_pass_it_at_all) - blue_note

Thanks to your great answers, I thought I'd try to do just that:

# gen.py
def callWithNonNoneArgs(f, *args, **kwargs):
    kwargsNotNone = {k: v for k, v in kwargs.items() if v is not None}
    return f(*args, **kwargsNotNone)

 

# python interpreter
>>> import gen
>>> def alpha(p1="foo", p2="bar"):
...     print('{0},{1}'.format(p1,p2))
...
>>> gen.callWithNonNoneArgs(alpha, p1="FOO", p2=None)
FOO,bar
>>> def beta(ree, p1="foo", p2="bar"):
...     print('{0},{1},{2}'.format(ree,p1,p2))
...
>>> beta('hello', p2="world")
hello,foo,world
>>> beta('hello', p2=None)
hello,foo,None
>>> gen.callWithNonNoneArgs(beta, 'hello', p2=None)
hello,foo,bar

This is probably not perfect, but it seems to work: It's a function that you can call with another function and it's arguments, and it applies deceze's answer to filter out the arguments that are None.

Jetport

I like this answer better than the highest one because the hairy code is moved into a function rather than being inline. – Keats

Answer 4

You could inspect the default values via alpha.__defaults__ and then use them instead of None. That way you circumvent the hard-coding of default values:

>>> args = [None]
>>> alpha('FOO', *[x if x is not None else y for x, y in zip(args, alpha.__defaults__[1:])])

Whine

Feels ugly and elegant at the same time, but I don't see why not to use this answer. Why do you not recomment this way? – Jetport

@Jetport Well okay, technically there's nothing wrong with it and when you have many arguments it saves you from explicitly writing down the arguments' names. It's just when you want to use a default value normally you just don't pass the argument. But yes, there might be actually uses cases for that, I think it depends on the judgement of the reader. I'll remove rephrase that statement. – Whine

Answer 5

I had the same problem when calling some Swagger generated client code, which I couldn't modify, where None could end up in the query string if I didn't clean up the arguments before calling the generated methods. I ended up creating a simple helper function:

def defined_kwargs(**kwargs):
    return {k: v for k, v in kwargs.items() if v is not None}

>>> alpha(**defined_kwargs(p1="FOO", p2=None))
FOO,bar

It keeps things quite readable for more complex invocations:

def beta(a, b, p1="foo", p2="bar"):
     print('{0},{1},{2},{3}'.format(a, b, p1, p2,))

p1_value = "foo"
p2_value = None

>>> beta("hello",
         "world",
         **defined_kwargs(
             p1=p1_value, 
             p2=p2_value))

hello,world,FOO,bar

Lumbering

Answer 6

I'm surprised nobody brought this up

def f(p1="foo", p2=None):
    p2 = "bar" if p2 is None else p2
    print(p1+p2)

You assign None to p2 as standart (or don't, but this way you have the true standart at one point in your code) and use an inline if. Imo the most pythonic answer. Another thing that comes to mind is using a wrapper, but that would be way less readable.

EDIT: What I'd probably do is use a dummy as standart value and check for that. So something like this:

class dummy():
    pass

def alpha(p1="foo", p2=dummy()):
    if isinstance(p2, dummy):
        p2 = "bar"
    print("{0},{1}".format(p1, p2))

alpha()
alpha("a","b")
alpha(p2=None)

produces:

foo,bar
a,b
foo,None

Psychopathology

This does not preserve the original behavior for other calls. (I.e. None should usually print None, just not in this specific call). I am aware that my requirements imply bad code, but I wanted to know how it would be possible. I think your answer is valuable for many future visitors, but is not the answer to my question. I wouldn't delete it though (as the first answerer did, who suggested the same thing and got downvoted to the ninth layer of Baator – Jetport

Oops sorry, should've read properly. See edit in original comment. – Psychopathology

Answer 7

Unfortunately, there's no way to do what you want. Even widely adopted python libraries/frameworks use your first approach. It's an extra line of code, but it is quite readable.

Do not use the alpha("FOO", myp2 or "bar") approach, because, as you mention yourself, it creates a terrible kind of coupling, since it requires the caller to know details about the function.

Regarding work-arounds: you could make a decorator for you function (using the inspect module), which checks the arguments passed to it. If one of them is None, it replaces the value with its own default value.

Januaryjanuisz

@deceze: really? is there any language feature that allows you to do this? Obviously there can be various work-arounds, but this is 3 times longer and harder to read than what the OP wants to avoid. – Januaryjanuisz

If you just have a single if..else, sure, that is actually shorter. But for functions with more than two or three arguments this longer workaround quickly pays off. And yes, I'd regard ** as a language feature to solve this issue. – Premiere

@deceze: although ** is definitely a language feature, it's surely not created for solving this particular problem. Your suggestion works, so does mine. Which one works better depends on the rest of the OP's code. However, there is still no way to write f(x or dont_pass_it_at_all) – Januaryjanuisz

@blue_note That's a cool idea! I think I came pretty close to that with this. (I don't intend to actually work out all the kinks though) – Jetport

@lucidbrot: regardless, if you have 2-3 arguments, I think you should stick with your original if p2 is not None. If you have 10 arguments, 5 of which are often None, your should probably re-think your design. – Januaryjanuisz

@blue_note I'm fairly certain that the ** feature was created for exactly this use case: dynamically building up an argument set. What in the world else can you use dict unpacking for? Nothing else accepts a set of unpacked key-value pairs like methods (including initializers) do. – Athalie

@jpmc26: of course it was created for variable arguments. I'm talking about this particular case. In the OP's case, it's better to go with if x is not None. – Januaryjanuisz

@jpmc26: turns out, the question has been edited since I wrote the answer. The ** fits in this case. – Januaryjanuisz

It fit the original version, too. – Athalie

Answer 8

Not a direct answer, but I think this is worth considering:

See if you can break your function into several functions, neither of which has any default arguments. Factor any shared functionality out to a function you designate as internal.

def alpha():
    _omega('foo', 'bar')

def beta(p1):
    _omega(p1, 'bar')

def _omega(p1, p2):
     print('{0},{1}'.format(p1, p2))

This works well when the extra arguments trigger "extra" functionality, as it may allow you to give the functions more descriptive names.

Functions with boolean arguments with True and/or False defaults frequently benefit from this type of approach.

Detail

Answer 9

Another possibility is to simply do alpha("FOO", myp2 or "bar"), but that requires us to know the default value. Usually, I'd probably go with this approach, but I might later change the default values for alpha and this call would then need to be updated manually in order to still call it with the (new) default value.

Just create a constant:

P2_DEFAULT = "bar"

def alpha(p1="foo", p2=P2_DEFAULT):
     print('{0},{1}'.format(p1, p2))

and call the function:

alpha("FOO", myp2 or P2_DEFAULT)

If default values for alpha will be changed, we have to change only one constant.

Be careful with logical or for some cases, see https://mcmap.net/q/73661/-is-there-a-python-equivalent-of-the-c-null-coalescing-operator

One more (better) use case

For example, we have some config (dictionary). But some values are not present:

config = {'name': 'Johnny', 'age': '33'}
work_type = config.get('work_type', P2_DEFAULT)

alpha("FOO", work_type)

So we use method get(key, default_value) of dict, which will return default_value if our config (dict) does not contain such key.

Monatomic

Answer 10

As I cannot comment on answers yet, I'd like to add that the first solution (unpacking the kwargs) would fit nicely in a decorator as follows:

def remove_none_from_kwargs(func):
    @wraps(func)
    def wrapper(self, *args, **kwargs):
        func(self,*args, **{k: v for k, v in kwargs.items() if v is not None})
    return wrapper

Henbit