具有** kwargs的功能

Question

I want to write a Python function myfun that accepts only optional parameters, a and b .

If either one of a or b is not specified when calling myfun , how can I tell myfun to use some default value for a and b ?

def myfun(**kwargs):

    a = kwargs.get('a', None)
    # if a is not specified, use default a=4.4

    b = kwargs.get('b', None)
    # if b is not specified, use default b=2.1    

    c = 2*a
    d = 3.1*b

    return c, d


c,d = myfun(a=1,b=2)
print c,d

Answer 1

**kwargs is used to collect an arbitrary number of keyword arguments. If you only want to accept two, then it is not the right tool to be using.

Instead, you should specify default values for the a and b parameters:

def myfun(a=4.4, b=2.1):

Demo:

>>> def myfun(a=4.4, b=2.1):
...     print('a={}\nb={}'.format(a, b))
...
>>> myfun()
a=4.4
b=2.1
>>> myfun(a=1)
a=1
b=2.1
>>> myfun(a=1, b=2)
a=1
b=2
>>> myfun(b=2)
a=4.4
b=2
>>>

Answer 2

set them to a default value:

def myfun(a=4.4,b=2.1):

There would be no way for a user to know they could set a and b if you did not add them as keywords in the function definition.

If you were setting a value using kwargs.get you would pass the default value to get:

a = kwargs.get('a', 4.4) # a = 4.4 if a not in kwargs

b = kwargs.get('b', 2.1)