Python Meta Path Finders

Python allows providing custom Python types to handle the low-level machinery behind the import statement. The way this works is a meta path finder instance (as defined by the interface) is registered on sys.meta_path. When an import is serviced, Python effectively iterates the objects on sys.meta_path and asks each one can you service this request until one does.

These meta path finder not only service basic Python module loading, but they can also facilitate loading resource files and package metadata. There are a handful of optional methods available on implementations.

This documentation will often refer to a meta path finder as an importer, because it is primarily used for importing Python modules.

Normally when you start a Python process, the Python interpreter itself will install 3 meta path finders on sys.meta_path before your code even has a chance of running:

Handles importing of built-in extension modules, which are compiled into the Python interpreter. These include modules like sys.
Handles importing of frozen bytecode modules, which are compiled into the Python interpreter. This finder is typically only used to initialize Python’s importing mechanism.
Handles filesystem-based loading of resources. This is what is used to import .py and .pyc files. It also handles .zip files. This is the meta path finder that most imports are traditionally serviced by. It queries the filesystem at import time to find and load resources.