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Abstraction and Structure
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Abstraction can be useful as a labor saver, but it is actually more important than that. It is the key to making computer programs understandable to humans (which is essential, whether you re writing them or reading them). The computers themselves are perfectly happy with very concrete and specific instructions, but humans generally aren t. If you ask me for directions to the cinema, for example, you wouldn t want me to answer, Walk 10 steps forward, turn 90 degrees to your left, walk another 5 steps, turn 45 degrees to your right, walk 123 steps. You would soon lose track, wouldn t you Now, if I instead told you to Walk down this street until you get to a bridge, cross the bridge, and the cinema is to your left, you would certainly understand me. The point is that you already know how to walk down the street and how to cross a bridge. You don t need explicit instructions on how to do either. You structure computer programs in a similar fashion. Your programs should be quite abstract, as in Download page, compute frequencies, and print the frequency of each word. This is easily understandable. In fact, let s translate this high-level description to a Python program right now: page = download_page() freqs = compute_frequencies(page) for word, freq in freqs: print word, freq From reading this, you can understand what the program does. However, you haven t explicitly said anything about how it should do it. You just tell the computer to download the page and compute the frequencies. The specifics of these operations will need to be written somewhere else in separate function definitions.
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CHAPTER 6 ABSTRACTION
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Creating Your Own Functions
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A function is something you can call (possibly with some parameters the things you put in the parentheses), which performs an action and returns a value.1 In general, you can tell whether something is callable or not with the built-in function callable: >>> import math >>> x = 1 >>> y = math.sqrt >>> callable(x) False >>> callable(y) True
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Note The function callable no longer exists in Python 3.0. With that version, you will need to use the
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expression hasattr(func, __call__). For more information about hasattr, see 7.
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As you know from the previous section, creating functions is central to structured programming. So how do you define a function You do this with the def (or function definition ) statement: def hello(name): return 'Hello, ' + name + '!' After running this, you have a new function available, called hello, which returns a string with a greeting for the name given as the only parameter. You can use this function just like you use the built-in ones: >>> print hello('world') Hello, world! >>> print hello('Gumby') Hello, Gumby! Pretty neat, huh Consider how you would write a function that returned a list of Fibonacci numbers. Easy! You just use the code from before, and instead of reading in a number from the user, you receive it as a parameter: def fibs(num): result = [0, 1] for i in range(num-2): result.append(result[-2] + result[-1]) return result
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1. Actually, functions in Python don t always return values. More on this later in the chapter.
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CHAPTER 6 ABSTRACTION
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After running this statement, you ve basically told the interpreter how to calculate Fibonacci numbers. Now you don t have to worry about the details anymore. You simply use the function fibs: >>> [0, >>> [0, fibs(10) 1, 1, 2, 3, 5, 8, 13, 21, 34] fibs(15) 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377]
The names num and result are quite arbitrary in this example, but return is important. The return statement is used to return something from the function (which is also how we used it in the preceding hello function).
Documenting Functions
If you want to document your functions so that you re certain that others will understand them later on, you can add comments (beginning with the hash sign, #). Another way of writing comments is simply to write strings by themselves. Such strings can be particularly useful in some places, such as immediately after a def statement (and at the beginning of a module or a class you learn more about classes in 7 and modules in 10). If you put a string at the beginning of a function, it is stored as part of the function and is called a docstring. The following code demonstrates how to add a docstring to a function: def square(x): 'Calculates the square of the number x.' return x*x The docstring may be accessed like this: >>> square.__doc__ 'Calculates the square of the number x.'
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