Any file that contains Python code can be imported as a module. For example, suppose you have a file named
wc.py with the following code:
def linecount(filename): count = 0 for line in open(filename): count += 1 return count print linecount('wc.py')
If you run this program, it reads itself and prints the number of lines in the file, which is 7. You can also import it like this:
>>> import wc 7
Now you have a module object
>>> print wc <module 'wc' from 'wc.py'>
That provides a function called
>>> wc.linecount('wc.py') 7
So that’s how you write modules in Python.
The only problem with this example is that when you import the module it executes the test code at the bottom. Normally when you import a module, it defines new functions but it doesn’t execute them.
Programs that will be imported as modules often use the following idiom:
if __name__ == '__main__': print linecount('wc.py')
__name__ is a built-in variable that is set when the program starts. If the program is running as a script,
__name__ has the value
__main__; in that case, the test code is executed. Otherwise, if the module is being imported, the test code is skipped.
Type this example into a file named
wc.py and run it as a script. Then run the Python interpreter and
import wc. What is the value of
__name__ when the module is being imported?
Warning: If you import a module that has already been imported, Python does nothing. It does not re-read the file, even if it has changed.
If you want to reload a module, you can use the built-in function
reload, but it can be tricky, so the safest thing to do is restart the interpreter and then import the module again.