Python variable value for default argument

Question

In Python, what value can a variable take, so that when a function is invoked with the variable as an argument, the function uses its default value for the parameter instead?

Consider the following code:

def foo(a=100):
    print(a)

b = None #blank value
foo(b)

Desired output:
100

Actual output:
None

I hypothesized that None would work, but clearly it doesn't. What value can I choose for b, so that foo(b) is equivalent to foo()? Or is this simply not possible? I'm in a situation where the value for b can either be defined, or I would like to use the default value of the parameter.

I would like to be able to change the default value only one place and for it to still work — Eivind, Jul 21 '22 at 11:38
But you're not using the default, you're actively telling it to use a different value instead of the default. — Sayse, Jul 21 '22 at 11:39
You could do something like `def foo(a=None)` and then in the first line of the function: `if a is None: a=100` — slothrop, Jul 21 '22 at 11:43
to use the default value, call the function without passing the arg, e.g. `foo()` — Anentropic, Jul 21 '22 at 11:43
@khelwood Assuming the function *has* a `__defaults__` attribute. (A function defined in C may not have one.) — chepner, Jul 21 '22 at 11:49
`int` is kind of a good example; it's not "obvious" that `int()` and `int(0)` should be the same. (OK, it's *kind of* obvious, but that's more a property of knowing what `int` does rather than of how it is defined.) — chepner, Jul 21 '22 at 11:52

chepner · Accepted Answer · 2022-07-21T11:50:20.413

(This answer assumes that you cannot modify foo, and that you cannot use reflection or introspection to determine what the default argument value is.)

It's the absence of an argument, not any particular value used as an argument, that triggers the use of the default value. The only way you can produce nothing out of something is to unpack an empty mapping

foo(**{})

or an empty sequence

foo(*())

Both * and ** are part of the function-call syntax, though, not part of the argument value, so with a variable, it still looks like

b = {}
foo(**b)

b = ()
foo(*b)

score 2 · Answer 2 · answered Jul 21 '22 at 11:45

2

If you want None to revert to a default value, the easiest way is to do the logic in the function itself.

def foo(a=None):
    if a is None:
        a = 100
    print(a)

answered Jul 21 '22 at 11:45

Peter

3,186
3
26
59

Better to keep a=100 in function params to keep showing the intention of the function (and also keep your additional code). – Roy Levy Jul 21 '22 at 11:58

score 0 · Answer 3 · answered Jul 21 '22 at 12:02

Function default parameter uses when there don't pass any parameter for the argument. And None is not a blank value. None is an object of NoneType Datatype in python similar to Other Datatype Object.

Instead, you can use

def foo(a=100):
    a=100 if a is None else a //Ternary operator 
    print(a)

b = None #blank value
foo(b)

Output: 100

score 0 · Answer 4 · answered Jul 21 '22 at 12:12

0

In my case, I ended up using the inspect module to create a helper function which extracts the default values of the function as described here:

import inspect

def get_defaults(func):
    signature = inspect.signature(func)
    return { k: v.default for k, v
             in signature.parameters.items()
             if v.default is not inspect.Parameter.empty }


def foo(a=100):
    print(a)

b = get_defaults(foo)['a']
foo(b)

Output: 100

answered Jul 21 '22 at 12:12

Eivind

75
4

May I ask *why* you did this? That's an awful lot of work to do something equivalent to simply not passing the value at all. It works, sure, but so does making a `dict` and only adding the mapping for `"a"` if you have a non-default, then unconditionally doing `foo(**mydict)`. I suspect [an XY problem](https://meta.stackexchange.com/q/66377/322040). – ShadowRanger Jul 21 '22 at 14:57
@ShadowRanger Making a `dict` with all the parameters and unpacking it is probably a more elegant solution, but it does turn a single line function call into multiple lines of creating an argument `dict`, checking for which arguments should be added (we're talking upwards of 20), and finally the function call itself, with unpacking arguably being a less familiar operation to novice python developers. – Eivind Jul 22 '22 at 07:34
I'll grant you it's a multiline thing, but then, any reasonable case for "do I want to use the default or not?" is also going to be multiline. I won't grant the unpacking argument; it's a key skill to learn, more important than `inspect`-based reflection code, and novice Python developers that want to become even journeymen need to learn it at some point. – ShadowRanger Jul 22 '22 at 11:32

Python variable value for default argument

4 Answers4