barcode generator in c# windows application codeproject you have a small number of mandatory parameters and many modifying parameters with default values. in Font

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you have a small number of mandatory parameters and many modifying parameters with default values.
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For example, our hello function might require a name, but allow us to (optionally) specify the greeting and the punctuation: def hello_4(name, greeting='Hello', punctuation='!'): print '%s, %s%s' % (greeting, name, punctuation) This function can be called in many ways. Here are some of them:
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>>> hello_4('Mars') Hello, Mars! >>> hello_4('Mars', 'Howdy') Howdy, Mars! >>> hello_4('Mars', 'Howdy', '...') Howdy, Mars... >>> hello_4('Mars', punctuation='.') Hello, Mars. >>> hello_4('Mars', greeting='Top of the morning to ya') Top of the morning to ya, Mars! >>> hello_4() Traceback (most recent call last): File "<pyshell#64>", line 1, in hello_4() TypeError: hello_4() takes at least 1 argument (0 given)
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Note If I had given name a default value as well, the last example wouldn t have raised an exception.
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That s pretty flexible, isn t it And we didn t really have to do much to achieve it either. In the next section we get even more flexible.
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Collecting Parameters
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Sometimes it can be useful to allow the user to supply any number of parameters. For example, in the name-storing program (described earlier in this chapter), you can store only one name at a time. It would be nice to be able to store more names, like this: >>> store(data, name1, name2, name3) For this to be useful, you should be allowed to supply as many names as you wanted. Actually, that s quite possible. Try the following function definition: def print_params(*params): print params Here, I seemingly specify only one parameter, but it has an odd little star (or asterisk) in front of it. What does that mean Let s call the function with a single parameter and see what happens: >>> print_params('Testing') ('Testing',) You can see that what is printed out is a tuple because it has a comma in it. (Those tuples of length one are a bit odd, aren t they ) So using a star in front of a parameter puts it in a tuple The plural in params ought to give a clue about what s going on:
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>>> print_params(1, 2, 3) (1, 2, 3) The star in front of the parameter puts all the values into the same tuple. It gathers them up, so to speak. I wonder if we can combine this with ordinary parameters . . . Let s write another function and see: def print_params_2(title, *params): print title print params Let s try it: >>> print_params_2('Params:', 1, 2, 3) Params: (1, 2, 3) It works! So the star means Gather up the rest of the positional parameters. I bet if I don t give any parameters to gather, params will be an empty tuple: >>> print_params_2('Nothing:') Nothing: () Indeed. How useful. Does it handle keyword arguments (the same as parameters), too >>> print_params_2('Hmm...', something=42) Traceback (most recent call last): File "<pyshell#60>", line 1, in print_params_2('Hmm...', something=42) TypeError: print_params_2() got an unexpected keyword argument 'something' Doesn t look like it. So we probably need another gathering operator for keyword arguments. What do you think that might be Perhaps ** def print_params_3(**params): print params At least the interpreter doesn t complain about the function. Let s try to call it: >>> print_params_3(x=1, y=2, z=3) {'z': 3, 'x': 1, 'y': 2} Yep. We get a dictionary rather than a tuple. Let s put them all together: def print_params_4(x, y, z=3, *pospar, **keypar): print x, y, z print pospar print keypar This works just like expected: >>> print_params_4(1, 2, 3, 5, 6, 7, foo=1, bar=2) 1 2 3
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(5, 6, 7) {'foo': 1, 'bar': 2} >>> print_params_4(1, 2) 1 2 3 () {} By combining all these techniques, you can do quite a lot. If you wonder how some combination might work (or whether it s allowed), just try it! (In the next section, you see how * and ** can be used at the point of call as well, regardless of whether they were used in the function definition.) Now, back to the original problem: how you can use this in the name-storing example. The solution is shown here: def store(data, *full_names): for full_name in full_names: names = full_name.split() if len(names) == 2: names.insert(1, '') labels = 'first', 'middle', 'last' for label, name in zip(labels, names): people = lookup(data, label, name) if people: people.append(full_name) else: data[label][name] = [full_name] Using this function is just as easy as using the previous version, which only accepted one name: >>> d = {} >>> init(d) >>> store(d, 'Han Solo') But now you can also do this: >>> store(d, 'Luke Skywalker', 'Anakin Skywalker') >>> lookup(d, 'last', 'Skywalker') ['Luke Skywalker', 'Anakin Skywalker']
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