Call function without optional arguments if they are None

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.

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:])])

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.

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.

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

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.

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

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://stackoverflow.com/a/4978745/3605259

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.