Learning Python - Mark Lutz [301]
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 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. Module Usage Clients can use the simple module file we just wrote by running an import or from statement. Both statements find, compile, and run a module file’s code, if it hasn’t yet been loaded. The chief difference is that import fetches the module as a whole, so you must qualify to fetch its names; in contrast, from fetches (or copies) specific names out of the module. Let’s see what this means in terms of code. All of the following examples wind up calling the printer function defined in the prior section’s module1.py module file, but in different ways. The import Statement In the first example, the name module1 serves two different purposes—it identifies an external file to be loaded, and it becomes a variable in the script, which references the module object after the file is loaded: >>> import module1 # Get module as a whole >>> module1.printer('Hello world!') # Qualify to get names Hello world! Because import gives a name that refers to the whole module object, we must go through the module name to fetch its attributes (e.g., module1.printer). The from Statement By contrast, because from also copies names from one file over to another scope, it allows us to use the copied names directly in the script without going through the module (e.g., printer): >>> from module1 import printer # Copy out one variable >>> printer('Hello world!') # No need to qualify name Hello world! This has the same effect as the prior example, but because the imported name is copied into the scope where the from statement appears, using that name in the script requires less typing: we can use it directly instead of naming the enclosing module. As you’ll see in more detail later, the from statement is really just a minor extension to the import statement—it imports the module file as usual, but adds an extra step that copies one or more names out