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How Imports Work
The prior section talked about importing modules without really explaining what happens when you do so. Because imports are at the heart of program structure in Python, this section goes into more detail on the import operation to make this process less abstract.
Some C programmers like to compare the Python module import operation to a C #include, but they really shouldn’t—in Python, imports are not just textual insertions of one file into another. They are really runtime operations that perform three distinct steps the first time a program imports a given file:
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- Find the module’s file.
- Compile it to byte code (if needed).
- Run the module’s code to build the objects it defines.
To better understand module imports, we’ll explore these steps in turn. Bear in mind that all three of these steps are carried out only the first time a module is imported during a program’s execution; later imports of the same module bypass all of these steps and simply fetch the already loaded module object in memory. Technically, Python does this by storing loaded modules in a table named sys.modules and checking there at the start of an import operation. If the module is not present, a three-step process begins.
