Learning Python - Mark Lutz [49]
>>> dir(threenames)
['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'a', 'b', 'c']
I ran this on Python 3.0 and 2.6; older Pythons may return fewer names. When the dir function is called with the name of an imported module passed in parentheses like this, it returns all the attributes inside that module. Some of the names it returns are names you get “for free”: names with leading and trailing double underscores are built-in names that are always predefined by Python and that have special meaning to the interpreter. The variables our code defined by assignment—a, b, and c—show up last in the dir result.
Modules and namespaces
Module imports are a way to run files of code, but, as we’ll discuss later in the book, modules are also the largest program structure in Python programs.
In general, Python programs are composed of multiple module files, linked together by import statements. Each module file is a self-contained package of variables—that is, a namespace. One module file cannot see the names defined in another file unless it explicitly imports that other file, so modules serve to minimize name collisions in your code—because each file is a self-contained namespace, the names in one file cannot clash with those in another, even if they are spelled the same way.
In fact, as you’ll see, modules are one of a handful of ways that Python goes to great lengths to package your variables into compartments to avoid name clashes. We’ll discuss modules and other namespace constructs (including classes and function scopes) further later in the book. For now, modules will come in handy as a way to run your code many times without having to retype it.
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Note
import versus from: I should point out that the from statement in a sense defeats the namespace partitioning purpose of modules—because the from copies variables from one file to another, it can cause same-named variables in the importing file to be overwritten (and won’t warn you if it does). This essentially collapses namespaces together, at least in terms of the copied variables.
Because of this, some recommend using import instead of from. I won’t go that far, though; not only does from involve less typing, but its purported problem is rarely an issue in practice. Besides, this is something you control by listing the variables you want in the from; as long as you understand that they’ll be assigned values, this is no more dangerous than coding assignment statements—another feature you’ll probably want to use!
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import and reload Usage Notes
For some reason, once people find out about running files using import and reload, many tend to focus on this alone and forget about other launch options that always run the current version of the code (e.g., icon clicks, IDLE menu options, and system command lines). This approach can quickly lead to confusion, though—you need to remember when you’ve imported to know if you can reload, you need to remember to use parentheses when you call reload (only), and you need to remember to use reload in the first place to get the current version of your code to run. Moreover, reloads aren’t transitive—reloading a module reloads that module only, not any modules it may import—so you sometimes have to reload multiple files.
Because of these complications (and others we’ll explore later, including the reload/from issue mentioned in a prior note in this chapter), it’s generally a good idea to avoid the temptation to launch by imports and reloads for now. The IDLE Run→Run Module menu option described in the next section, for example, provides a simpler and less error-prone way to run your files, and always runs the current version of your code. System shell command lines offer similar benefits. You don’t need to use reload if you use these techniques.
In addition, you may run into trouble if you use modules in unusual ways at this point