Lesson 15 Notes: Modules and Shelves

• The Problems
  • Code is getting very long.
    • With hundreds of lines, it's easy to get lost
    • When working with partners, it is hard to work on a long body of code.
    • Be nice to break code into smaller, independent files
  • Cannot save data structures
    • If you create a dictionary, we know how to save it as a text file, but we can't read it in as a dictionary.
    • It would be nice to save lists as lists, dictionaries as dictionaries, and so forth and get them back again easily.
• Breaking Up Code
  • Code can be broken into separate files called "modules".
  • You have already used a lot of modules that come with Python (string, random, time, etc.).
  • To create a module, just type your Python code and save it as usual.
  • Your saved module can be imported and used like any other Python module.
• Creating a Module
  • Write reusable code.
  • Code should be primarily functions or classes
  • Avoid having variables or commands outside function definitions
  • Functions can require values from the calling program
  • Save code as a regular .py file.
  • In main program, import module and use it
• Adding New Directory to Path
  • Python only looks inside certain directories for modules.
  • Most built-in modules are saved in c:\python\lib
  • If you save your module in a different place, you will need to add the new directory to sys.path.
  • To edit sys.path, in the interactive interpreter type:
    > > >import sys
> > >sys.path #prints current directories in path list
> > >sys.path.append("k:\\mydirectory\\subdir")

  • Be sure to use double back-slashes
  • Change name of directory to the directory you want to add.
• Making Modules User Friendly
  • Comments in triple-quotes are used to add documentation to your code.
  • Example:
    def my_function(x,y):
    """my_function (first name, last name) """

  • Be certain to use three quotes at beginning and end
  • Do NOT make this a "print" line.
  • Text in triple quotes should explain what the function, class, or module does.
• Using a Module
  • In main program, include command to import the module.
  • When calling a module function, use its fully qualified name:
    my_module.my_function(name1, name2)
my_module.another_function()
• Module Tricks
  • A module can be run as an independent program if you include the following line at the end:
    if __name__ == '__main__':
    run_function()

  • Substitute "run_function()" for the name of the main function in the module.
• Preserving Python Data
  • Two main methods for preserving data:
  • pickle module:
    Stores python data structures in a binary file. Limited to only one data structure per file.
  • shelve module:
    Stores Python data structures, but allows more than one data structure and can be indexed by a key.
• How to Shelve
  • Import shelve module
  • Open a shelve file
  • Assign an item, by key, to the shelve file
  • To retrieve, open the shelve file and access item by key
  • shelve works like a dictionary, you add, access and remove items by using their key.
  • shelve works like a dictionary, you add, access and remove items by using their key.
• Shelve Example
  import shelve
colors = ["green", "red", "blue"]
states = ["TN", "KY", "MS"]
shelf = shelve.open('filename')
# To shelve an item
shelf['colors'] = colors
shelf['ST'] = states
# Now, to get it back
newitem = shelf['colors']
# And close it up
shelf.close()
• Other shelve functions
  • To get a list of all the keys available in a shelve file, use the keys() function:
    list = shelf.keys()
  • To delete an item, use the del function
    del shelf('ST')
  • To see if a key exists, use has_key():
    if shelf.has_key('ST'): print "yep"
• Review
  • How do you create a module?
  • If a module is saved in my folder, but cannot be imported correctly, what might be the problem?
  • What advantage is there in using the shelve module over saving data to a text file?
  • What advantage is there in using the shelve module over saving data to a text file?
  
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