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4.5: Optional Parameters

  • Page ID
    15436
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    We have seen built-in functions and methods that take a variable number of arguments. It is possible to write user-defined functions with optional arguments, too. For example, here is a function that prints the most common words in a histogram

    def print_most_common(hist, num=10):
        t = most_common(hist)
        print 'The most common words are:'
        for freq, word in t[:num]:
            print word, '\t', freq
    

    The first parameter is required; the second is optional. The default value of num is 10.

    If you only provide one argument:

    print_most_common(hist)
    

    num gets the default value. If you provide two arguments:

    print_most_common(hist, 20)
    

    num gets the value of the argument instead. In other words, the optional argument overrides the default value.

    If a function has both required and optional parameters, all the required parameters have to come first, followed by the optional ones.


    This page titled 4.5: Optional Parameters is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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