Reversing the Process in Font

Creation PDF-417 2d barcode in Font Reversing the Process

Now you ve learned about gathering up parameters in tuples and dictionaries, but it is in fact possible to do the opposite as well, with the same two operators, * and **. What might the opposite of parameter gathering be Let s say we have the following function available: def add(x, y): return x + y

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' >>> >>> Mr. >>> Mr. args = {'name': 'Mr. Gumby', 'age': 42} with_stars(**args) Gumby is 42 years old without_stars(args) 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).

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:

print 'Received redundant parameters:', others return pow(x, y) def interval(start, stop=None, step=1): 'Imitates range() for step > 0' if stop is None: # If the stop is not supplied... start, stop = 0, start # shuffle the parameters result = [] i = start # We start counting at the start index while i < stop: # Until the index reaches the stop index... result.append(i) # ...append the index to the result... i += step # ...increment the index with the step (> 0) return result Now let s try them out: >>> print story(job='king', name='Gumby') Once upon a time, there was a king called Gumby. >>> print story(name='Sir Robin', job='brave knight') Once upon a time, there was a brave knight called Sir Robin. >>> params = {'job': 'language', 'name': 'Python'} >>> print story(**params) Once upon a time, there was a language called Python. >>> del params['job'] >>> print story(job='stroke of genius', **params) Once upon a time, there was a stroke of genius called Python. >>> power(2,3) 8 >>> power(3,2) 9 >>> power(y=3,x=2) 8 >>> params = (5,) * 2 >>> power(*params) 3125 >>> power(3, 3, 'Hello, world') Received redundant parameters: ('Hello, world',) 27 >>> interval(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> interval(1,5) [1, 2, 3, 4] >>> interval(3,12,4) [3, 7, 11] >>> power(*interval(3,7)) Received redundant parameters: (5, 6) 81