3. Run It

The final step of an import operation executes the byte code of the module. All statements in the file are executed in turn, from top to bottom, and any assignments made to names during this step generate attributes of the resulting module object. This execution step therefore generates all the tools that the module’s code defines. For instance, def statements in a file are run at import time to create functions and assign attributes within the module to those functions. The functions can then be called later in the program by the file’s importers.

Because this last import step actually runs the file’s code, if any top-level code in a module file does real work, you’ll see its results at import time. For example, top-level print statements in a module show output when the file is imported. Function def statements simply define objects for later use.

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As you can see, import operations involve quite a bit of work—they search for files, possibly run a compiler, and run Python code. Because of this, any given module is imported only once per process by default. Future imports skip all three import steps and reuse the already loaded module in memory. If you need to import a file again after it has already been loaded (for example, to support end-user customization), you have to force the issue with an imp.reload call—a tool we’ll meet in the next chapter.[48]

 

[47] It’s actually syntactically illegal to include path and suffix details in a standard import. Package imports, which we’ll discuss in Chapter 23, allow import statements to include part of the directory path leading to a file as a set of period-separated names; however, package imports still rely on the normal module search path to locate the leftmost directory in a package path (i.e., they are relative to a directory in the search path). They also cannot make use of any platform-specific directory syntax in the import statements; such syntax only works on the search path. Also, note that module file search path issues are not as relevant when you run frozen executables (discussed in Chapter 2); they typically embed byte code in the binary image.

[48] As described earlier, Python keeps already imported modules in the built-in sys.modules dictionary so it can keep track of what’s been loaded. In fact, if you want to see which modules are loaded, you can import sys and print list(sys.modules.keys()). More on other uses for this internal table in Chapter 24.