Python-覆盖函数参数

Question

Writing a cli function that should do the following:

In case, the function parameters are not set (I get them from docopt), I would like to look them up from environment. The following does not work due to fast loading in functions:

def my_function(a=None, b=None, c=None):
    for v in ("a", "b", "c"):
        if vars()[v] is None:
            locals()[v] = getenv("env_{}".format(v).upper())
    do_something_with(a, b, c)

What would be the pythonic way to achieve this?

Answer 1

You can do this easily with keyword arguments:

def my_function(**kwargs):
    for var in ('a', 'b', 'c'):
        if kwargs.get(var) is None:
            kwargs[var] = getenv("env_{}".format(v).upper())
    do_something_with(**kwargs)

If you want to keep the signature, you can create a decorator ( functools.wraps takes care of aligning the signatures):

def defaults_from_env(function):
    @functools.wraps(function)
    def wrapper(**kwargs):
        for var in kwargs:
            if kwargs.get(var) is None:
                kwargs[var] = getenv("env_{}".format(var).upper())
        return function(**kwargs)
    return wrapper


@defaults_from_env
def my_function(a=None, b=None, c=None):
    print(a, b, c)

However, this forces you to name all parameters when calling the decorated my_function (ie a=.., b=.., c=.. ). To avoid this, you could use inspect.signature to bind the parameters in the wrapper; this would allow you to get the name and value of all parameters, both *arg and **kwarg .