"import module" - Two-Step Process

Python Tutorials - Herong's Tutorial Examples

∟"import module" - Two-Step Process

This section describes the two-step process of the 'import module' statement execution.

In the last tutorial, we learned how to create a "module" object using the "import module" statement. Now let's take a closer look at the behavior of the "import module" statement.

Keep the module file in the current directory and run the following code:

herong$ ls -l
-rw-r--r--@ 1 herong  staff  423 Apr 1 07:35 module_test.py

herong$ python

# import the module
>>> import module_test
Hi there! - from 'module_test' module
Welcome on board! - from 'first' class

# import the module again
>>> import module_test

As you can see from the output, module sub-statements are executed only when the module is imported for the first time.

This is because the "import" statement is designed as a two-step process with a cache mechanism:

Step 1: Loading - In this step, the Python system will do nothing, if the requested module already exists in the cache. Otherwise, it will:

Search for the module file, module_name.py, in locations given in sys.path.
Load the module file in memory.
Execute sub-statements stored in the module file. This may define variables (attributes), functions and classes in the module.
Create a "module" object to store module's attributes, functions and classes defined during the execution.
Save the module object in the cache.

This is why when you run the "import module_test" statement for the second time, you don't see those "print" statements get executed. The requested module is already in the cache. It will not be loaded again.