Module Creation

To define a module, simply use your text editor to type some Python code into a text file, and save it with a “.py” extension; any such file is automatically considered a Python module. All the 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 you type the following def into a file called module1.py and import it, you create a module object with one attribute—the name printer, which happens to be a reference to a function object:

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def printer(x):                   # Module attribute
    print(x)

Before we go on, I should say a few more words about 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 your files’ types more obvious and allows you to import any of your files in the future.

Because module names become variable names inside a Python program (without the .py), they should also follow the normal variable name rules outlined in Chapter 11. For instance, you can create a module file named if.py, but you 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 the names of directories used in package imports (discussed in the next chapter) must conform to the rules for variable names presented in Chapter 11; they may, for instance, contain only letters, digits, and underscores. Package directories also cannot contain platform-specific syntax such as spaces in their names.

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

As mentioned in the preceding chapter, it is also possible to create a Python module by writing code in an external language such as C or C++ (or Java, in the Jython implementation of the language). Such modules are called extension modules, and they are generally used to wrap up external libraries for use in Python scripts. When imported by Python code, extension modules look and feel the same as modules coded as Python source code files—they are accessed with import statements, and they provide functions and objects as module attributes. Extension modules are beyond the scope of this book; see Python’s standard manuals or advanced texts such as Programming Python for more details.