16.1 Module Creation

To define a module, use your text editor to type Python code into a text file. Names assigned at the top level of the module become its attributes (names associated with the module object), and are exported for clients to use. For instance, if we type the def below into a file called module1.py and import it, we create a module object with one attribute—the name printer, which happens to be a reference to a function object:

def printer(x):           # Module attribute
    print x

A word on module filenames: you can call modules just about anything you like, but module filenames should end in a .py suffix if you plan to import them. The .py is technically optional for top-level files that will be run, but not imported; but adding it in all cases makes the file's type more obvious.

Since module names become variables inside a Python program without the .py, they should also follow the normal variable name rules we learned in Chapter 8. For instance, you can create a module file named if.py, but cannot import it, because if is a reserved word—when you try to run import if, you'll get a syntax error. In fact, both the names of module files and directories used in package imports must conform to the rules for variable names presented in Chapter 8. This becomes a larger concern for package directories; their names cannot contain platform-specific syntax such as spaces.

When modules are imported, Python maps the internal module name to an external filename, by adding directory paths in the module search path to the front, and a .py or other extension at the end. For instance, a module name M ultimately maps to some external file <directory>\M.<extension> that contains our module's code.