Learning Python - Mark Lutz [309]
To be even more dynamic, though, such systems can automatically reload the Python customization code periodically at runtime. That way, users’ changes are picked up while the system is running; there is no need to stop and restart each time the Python code is modified. Not all systems require such a dynamic approach, but for those that do, module reloads provide an easy-to-use dynamic customization tool.
* * *
Chapter Summary
This chapter delved into the basics of module coding tools—the import and from statements, and the reload call. We learned how the from statement simply adds an extra step that copies names out of a file after it has been imported, and how reload forces a file to be imported again without stopping and restarting Python. We also surveyed namespace concepts, saw what happens when imports are nested, explored the way files become module namespaces, and learned about some potential pitfalls of the from statement.
Although we’ve already seen enough to handle module files in our programs, the next chapter extends our coverage of the import model by presenting package imports—a way for our import statements to specify part of the directory path leading to the desired module. As we’ll see, package imports give us a hierarchy that is useful in larger systems and allow us to break conflicts between same-named modules. Before we move on, though, here’s a quick quiz on the concepts presented here.
Test Your Knowledge: Quiz
How do you make a module?
How is the from statement related to the import statement?
How is the reload function related to imports?
When must you use import instead of from?
Name three potential pitfalls of the from statement.
What...is the airspeed velocity of an unladen swallow?
Test Your Knowledge: Answers
To create a module, you just write a text file containing Python statements; every source code file is automatically a module, and there is no syntax for declaring one. Import operations load module files into module objects in memory. You can also make a module by writing code in an external language like C or Java, but such extension modules are beyond the scope of this book.
The from statement imports an entire module, like the import statement, but as an extra step it also copies one or more variables from the imported module into the scope where the from appears. This enables you to use the imported names directly (name) instead of having to go through the module (module.name).
By default, a module is imported only once per process. The reload function forces a module to be imported again. It is mostly used to pick up new versions of a module’s source code during development, and in dynamic customization scenarios.
You must use import instead of from only when you need to access the same name in two different modules; because you’ll have to specify the names of the enclosing modules, the two names will be unique.
The from statement can obscure the meaning of a variable (which module it is defined in), can have problems with the reload call (names may reference prior versions of objects), and can corrupt namespaces (it might silently overwrite names you are using in your scope). The from * form is worse in most regards—it can seriously corrupt namespaces and obscure the meaning of variables, so it is probably best used sparingly.
What do you mean? An African or European swallow?
Chapter 23. Module Packages
So far, when we’ve imported modules, we’ve been loading files. This represents typical module usage, and it’s probably the technique you’ll use for most imports you’ll code early on in your Python career. However, the module import story is a bit richer than I have thus far implied.
In addition to a module name, an import can name a directory path. A directory of Python code is said to be a package, so such imports are known as package imports. In effect, a package import turns a directory