o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

124 CHAPTER 6 ■ ABSTRACTION
Also, let’s say you have a tuple with two numbers that you want to add:
params = (1, 2)
This is more or less the opposite of what we did previously. Instead of gathering the
parameters, we want to distribute them. This is simply done by using the asterisk operator in
the “other end,” that is, when calling the function rather than when defining it:
>>> add(*params)
3
This works with parts of a parameter list, too, as long as the expanded part is last. You can
use the same technique with dictionaries, using the double asterisk operator. Assuming that
you have defined hello_3 as before, you can do the following:
>>> params = {'name': 'Sir Robin', 'greeting': 'Well met'}
>>> hello_3(**params)
Well met, Sir Robin!
Using the asterisk (or double asterisk) both when you define and call the function will
simply pass the tuple or dictionary right through, so you might as well not have bothered:
>>> def with_stars(**kwds):
print kwds['name'], 'is', kwds['age'], 'years old'
>>> def without_stars(kwds):
print kwds['name'], 'is', kwds['age'], 'years old'
>>> args = {'name': 'Mr. Gumby', 'age': 42}
>>> with_stars(**args)
Mr. Gumby is 42 years old
>>> without_stars(args)
Mr. Gumby is 42 years old
As you can see, in with_stars, I use stars both when defining and calling the function. In
without_stars, I don’t use the stars in either place but achieve exactly the same effect. So the
stars are only really useful if you use them either when defining a function (to allow a varying
number of arguments) or when calling a function (to “splice in” a dictionary or a sequence).
Example
With so many ways of supplying and receiving parameters, it’s easy to get confused. So let me tie it all together with
an example. First, let me define some functions:
def story(**kwds):
return 'Once upon a time, there was a ' \
'%(job)s called %(name)s.' % kwds
def power(x, y, *others):
if others: