如何使用** kwargs自动填写格式参数

Question

I want to automate/simplyfy this:

def test(stream, tag):
    subprocess.call('git submodule checkout {stream}-{tag}'.format(stream=stream, tag=tag))

ie I want to get rid of stream=stream and tag=tag and somehow make use of something like **kwargs. Is this possible?

Answer 1

My 2 cents: don't abuse **kwargs , it should be used only if the number of parameters is not known a priori .

Here are some approaches not involving **kwargs :

Easy

If your concern is length of line, you can save space by using implicit order:

def test(stream, tag):
    subprocess.call('git submodule checkout {}-{}'.format(stream, tag))

This comes at the price of format string readability, but for a one-liner it might just do it.

Object Style

Wrap the parameters in a Checkout object:

class Checkout:
    def __init__(self, stream, tag):
        self.stream = stream
        self.tag = tag

#...

def test(checkout):
    subprocess.call('git submodule checkout {0.stream}-{0.tag}'.format(checkout))

or even:

class Checkout:
    def __init__(self, stream, tag):
        self.stream = stream
        self.tag = tag

    def test(self):
        subprocess.call('git submodule checkout {0.stream}-{0.tag}'.format(self))

This is verbose, but the Checkout object is more than a simple wrapper, it might be reused somewhere else or serialized.

Answer 2

this should work:

def test(**kwargs):
    subprocess.call("git submodule checkout {stream}-{tag}".format(**kwargs))

Now, you could add some default values, or raise clearer error messages.

def test(**kwargs):
    #set a default value for "stream"
    if "stream" not in kwargs:
        kwargs["stream"] = "mystream"
    if "tag" not in kwargs:
        raise ValueError("Please add some tags")
    subprocess.call("git submodule checkout {stream}-{tag}".format(**kwargs))

Now, when the tag argument is not set, the message will tell you so. Without this code, the only information you get is a KeyError with the name of the missing key.

Answer 3

You could use locals() to pass the dictionary of local variables:

def test(stream, tag):
    subprocess.call('git submodule checkout {stream}-{tag}'.format(**locals()))