Does Python Cache Imported Files?

Consider the following: a.py foo = 1 b.py bar = 2 c.py import a kik = 3 d.py import a import c def main(): import b main() main() How many times is a.py loaded? How many

Solution 1:

Both a and b are loaded once. When you import a module, its content is cached so when you load the same module again, you're not calling upon the original script for the import, done using a "finder":

• https://www.python.org/dev/peps/pep-0451/#finder
• https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder

This works across modules so if you had a d.py of which import b, it will bind to the same cache as an import within c.py.

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Some interesting builtin modules can help understand what happens during an import:

• https://docs.python.org/3/reference/import.html#importsystem

  When a module is first imported, Python searches for the module and if found, it creates a module object 1, initializing it.

  Notably here the first import, all imports after follow the __import__. Internal caches of finders are stored at sys.meta_path.

• https://docs.python.org/3/library/functions.html#import

You can leverage the import system to invalidate those caches for example:

https://docs.python.org/3/library/importlib.html#importlib.import_module

If you are dynamically importing a module that was created since the interpreter began execution (e.g., created a Python source file), you may need to call invalidate_caches() in order for the new module to be noticed by the import system.

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The imp (and importlib py3.4+) allows the recompilation of a module after import:

import imp
import a
imp.reload(a)

• https://docs.python.org/3/library/importlib.html#importlib.reload

  Python module’s code is recompiled and the module-level code re-executed, defining a new set of objects which are bound to names in the module’s dictionary by reusing the loader which originally loaded the module.

• https://docs.python.org/3/library/imp.html