Python Programming/Modules and how to use them

Modules are libraries that can be called from other scripts. For example, a popular module is the time module. You can call it using:

import time

Then, create a new python file, you can name it anything (except time.py, since it'd mess up python's module importing, you'll see why later):

import time
 
def main():
    # define the variable 'current_time' as a tuple of time.localtime()
    current_time = time.localtime() 
    print(current_time)              # print the tuple
 
    # if the year is 2009 (first value in the current_time tuple)
    if current_time[0] == 2009: 
        print('The year is 2009')    # print the year
 
if __name__ == '__main__':     # if the function is the main function ...
    main() # ...call it

Modules can be called in a various number of ways. For example, we could alias the time module with the reserved word as:

import time as t    # import the time module and call it 't'
 
def main():
    current_time = t.localtime() 
    print(current_time)
    if current_time[0] == 2009: 
        print('The year is 2009')
 
if __name__ == '__main__': 
    main()

It is not necessary to import the whole module. If you only need a certain function or class, use from-import. Note that a from-import would import the name directly into the global namespace. In other words, when invoking the imported function, don't include the module's name:

from time import localtime         #1
 
def main():
    current_time = localtime()     #2
    print(current_time)
    if current_time[0] == 2009: 
        print 'The year is 2009'
 
if __name__ == '__main__': 
    main()

The from-import form can also be aliased:

from time import localtime as lt 
 
def main():
    current_time = lt()
    print(current_time)
    if current_time[0] == 2009: 
        print('The year is 2009')
 
if __name__ == '__main__': 
    main()

People who do test-first programming or perform regression testing write a main() function in every Python module, and a main() method in every Java class, to run automated tests. When a person executes the file directly, the main() function executes and runs the automated tests for that file. When a person executes some other Python file that in turn imports many other Python files, only one main() function is executed -- the main() function of the directly-executed file.

Summary

Every module can be easily searched or imported. A module is just an ordinary file in the form <filename>.py. A module called mymodule.py can be imported into main.py by inserting import mymodule.py at the top of main.py. After importing mymodule.py, you can modify and use the module's functions and variables in main.py. To access them, use the form mymodule.myfunction(). To access any documentation, use the form pydoc mymodule.

See also

• Python Programming/Modules
• "What does ' if __name__ == "__main__":' do?"