在以下python代码场景中使用kwargs是否正确？

Question

Is it correct to use kargs in the following scenario?

I have a function that needs five parameters in the call with two of them optional. So, I can delete it as:

def f(p1, p2, p3, p4, p5, o1=None, o2=None)

or

def f(**kwargs)

Because I'm doing things like this with the params:

orig_path = "{0}/{1}/{3}/{4}".format(p1, p2, p3, p4)

or

orig_path = "{p1}/{p2}/{p3}/{p4}".format(kwargs)

I don't know which style is the correct way in Python to use in the declarations. Is there any guide to look for the correct use of kwargs?. I've seen in the Amazon boto3 api usually uses the kwargs construction but...

Answer 1

As a rule of thumb, you should always write out all parameters in a function definition, if you can, and ideally give them descriptive names.

The Zen of Python says

Explicit is better than implicit.

and also

Readability counts.

Making the parameters explicit helps you prevent many mistakes, like calling the function with too many or too few parameters or with keyword parameters that don't exist.

It also helps readability a lot, as you can easily spot from the definition how many and which parameters the function expects and in which order and what the defaults for possible keyword parameters are. Sometimes, you can even guess what the function does via good parameter names without having to look at the documentation.

Some IDEs can even look up the definition automatically when you write a call and alert you to possible problems while you're typing.

Do this:

def twiddle_knobs(tuning, volume, panning=None, bass=6.0, midrange=-2.0, treble=1.5):
    """Modify the most important radio controls."""

    my_radio.modify_control(radio.TUNEFREQ, value=max(min(tuning, 120.0), 85.0))
    my_radio.modify_control(radio.VOL_MASTER, value=(volume / 100.0))

    if panning is not None:
        my_radio.modify_control(radio.VOL_LEFT, value=min(1.0 - panning / 100.0, 1.0))
        my_radio.modify_control(radio.VOL_RIGHT, value=min(1.0 + panning / 100.0, 1.0))

    for control, value in ((radio.EQU_BASS, bass),
                           (radio.EQU_MID,  midrange),
                           (radio.EQU_TREB, treble)):

        my_radio.modify_control(control, value=value, unit=radio.DB)

Do not do this:

 def twiddle_knobs(*args, **kwargs): # WARNING: never do this! my_radio.modify_control(radio.TUNEFREQ, max(min(args[0], 120.0), 85.0)) my_radio.modify_control(radio.VOL_MASTER, args[1] / 100.0) if kwargs.get("panning") is not None: my_radio.modify_control(radio.VOL_LEFT, min(1.0 - kwargs["panning"] / 100.0, 1.0)) my_radio.modify_control(radio.VOL_RIGHT, min(1.0 + kwargs["panning"] / 100.0, 1.0)) for control, param, default in ((radio.EQU_BASS, "bass", 6.0), (radio.EQU_MID, "midrange", -2.0), (radio.EQU_TREB, "treble", 1.5)): my_radio.modify_control(control, kwargs.get(param, default), radio.DB)

So when is it OK to use *args and *kwargs ?

• One use for *args is when you need to receive a list of things that are alike and serve the same function . However, you could also just use a normal parameter and have the caller explicitly pass in a list. Sometimes, calling the function with func([thing]) can seem awkward, though, particularly when most of the time you pass only one thing or no thing at all. Those are the times when *args would improve the situation.

• Also, when you need to pass some of the positional and/or keyword arguments your function received on to another function you don't have control over and you don't want to dictate which parameters your caller is allowed to have handed through, then *args and *kwargs - used after your explicit parameters - are the way to achieve that.

• If you have a set of keyword parameters that serve a strongly related function and you cannot exactly predefine upfront what the keywords will be , because they might depend upon something that's dynamically changing, that would also be a good reason for using *kwargs .

This list is not exhaustive, but it should give you a general idea.