Module pyembed::technotes

Technical Implementation Notes

When trying to understand the code, a good place to start is MainPythonInterpreter.new(), as this will initialize the CPython runtime and Python initialization is where most of the magic occurs.

A lot of initialization code revolves around mapping OxidizedPythonInterpreterConfig members to C API calls. This functionality is rather straightforward. There’s nothing really novel or complicated here. So we won’t cover it.

Python Memory Allocators

There exist several CPython APIs for memory management. CPython defines multiple memory allocator domains and it is possible to use a custom memory allocator for each using the PyMem_SetAllocator() API.

See the documentation in the pyalloc module for more on this topic.

Module Importing

The module importing mechanisms provided by this crate are one of the most complicated parts of the crate. This section aims to explain how it works. But before we go into the technical details, we need an understanding of how Python module importing works.

High Level Python Importing Overview

A meta path importer is a Python object implementing the importlib.abc.MetaPathFinder interface and is registered on sys.meta_path. Essentially, when the __import__ function / import statement is called, Python’s importing internals traverse entities in sys.meta_path and ask each finder to load a module. The first meta path importer that knows about the module is used.

By default, Python configures 3 meta path importers: an importer for built-in extension modules (BuiltinImporter), frozen modules (FrozenImporter), and filesystem-based modules (PathFinder). You can see these on a fresh Python interpreter:

   $ python3.7 -c 'import sys; print(sys.meta_path)`
   [<class '_frozen_importlib.BuiltinImporter'>, <class '_frozen_importlib.FrozenImporter'>, <class '_frozen_importlib_external.PathFinder'>]

These types are all implemented in Python code in the Python standard library, specifically in the importlib._bootstrap and importlib._bootstrap_external modules.

Built-in extension modules are compiled into the Python library. These are often extension modules required by core Python (such as the _codecs, _io, and _signal modules). But it is possible for other extensions - such as those provided by Python’s standard library or 3rd party packages - to exist as built-in extension modules as well.

For importing built-in extension modules, there’s a global PyImport_Inittab array containing members defining the extension/module name and a pointer to its C initialization function. There are undocumented functions exported to Python (such as _imp.exec_builtin() that allow Python code to call into C code which knows how to e.g. instantiate these extension modules. The BuiltinImporter calls into these C-backed functions to service imports of built-in extension modules.

Frozen modules are Python modules that have their bytecode backed by memory. There is a global PyImport_FrozenModules array that - like PyImport_Inittab - defines module names and a pointer to bytecode data. The FrozenImporter calls into undocumented C functions exported to Python to try to service import requests for frozen modules.

Path-based module loading via the PathFinder meta path importer is what most people are likely familiar with. It uses sys.path and a handful of other settings to traverse filesystem paths, looking for modules in those locations. e.g. if sys.path contains ['', '/usr/lib/python3.7', '/usr/lib/python3.7/lib-dynload', '/usr/lib/python3/dist-packages'], PathFinder will look for .py, .pyc, and compiled extension modules (.so, .pyd, etc) in each of those paths to service an import request. Path-based module loading is a complicated beast, as it deals with all kinds of complexity like caching bytecode .pyc files, differentiating between Python modules and extension modules, namespace packages, finding search locations in registry entries, etc. Altogether, there are 1500+ lines constituting path-based importing logic in importlib._bootstrap_external!

Default Initialization of Python Importing Mechanism

CPython’s internals go through a convoluted series of steps to initialize the importing mechanism. This is because there’s a bit of chicken-and-egg scenario going on. The meta path importers are implemented as Python modules using Python source code (importlib._bootstrap and importlib._bootstrap_external). But in order to execute Python code you need an initialized Python interpreter. And in order to execute a Python module you need to import it. And how do you do any of this if the importing functionality is implemented as Python source code and as a module?!

A few tricks are employed.

At Python build time, the source code for importlib._bootstrap and importlib._bootstrap_external are compiled into bytecode. This bytecode is made available to the global PyImport_FrozenModules array as the _frozen_importlib and _frozen_importlib_external module names, respectively. This means the bytecode is available for Python to load from memory and the original .py files are not needed.

During interpreter initialization, Python initializes some special built-in extension modules using its internal import mechanism APIs. These bypass the Python-based APIs like __import__. This limited set of modules includes _imp and sys, which are both completely implemented in C.

During initialization, the interpreter also knows to explicitly look for and load the _frozen_importlib module from its frozen bytecode. It creates a new module object by hand without going through the normal import mechanism. It then calls the _install() function in the loaded module. This function executes Python code on the partially bootstrapped Python interpreter which culminates with BuiltinImporter and FrozenImporter being registered on sys.meta_path. At this point, the interpreter can import compiled built-in extension modules and frozen modules. Subsequent interpreter initialization henceforth uses the initialized importing mechanism to import modules via normal import means.

Later during interpreter initialization, the _frozen_importlib_external frozen module is loaded from bytecode and its _install() is also called. This self-installation adds PathFinder to sys.meta_path. At this point, modules can be imported from the filesystem. This includes .py based modules from the Python standard library as well as any 3rd party modules.

Interpreter initialization continues on to do other things, such as initialize signal handlers, initialize the filesystem encoding, set up the sys.std* streams, etc. This involves importing various .py backed modules (from the filesystem). Eventually interpreter initialization is complete and the interpreter is ready to execute the user’s Python code!

Our Importing Mechanism

We use the multi-phase initialization mechanism provided by CPython 3.8+ (PEP-587) to import oxidized_importer and inject its OxidizedFinder onto sys.meta_path during interpreter initialization.

Essentially:

1. Add oxidized_importer to PyImport_Inittab so it can be serviced by BuiltinImporter.
2. Enable multi-phase initialization by setting PyConfig._init_main = 0.
3. Call Py_InitializeFromConfig() to initialize Python up to the point where .py based modules need to be loaded.
4. Construct an OxidizedFinder and install it on sys.meta_path. This entails loading resources data, indexing built-ins and frozen modules, and clearing out sys.met_path of the default meta path importers.
5. Call _Py_InitializeMain() to finish Python initialization. OxidizedFinder is able to service Python standard library imports.
6. Clear out sys.meta_path and sys.path_hooks from unwanted changes made as part of initializing “external” importers.

By injecting OxidizedFinder onto sys.meta_path[0], we effectively make it the highest priority importer. And if it has indexed everything needed as part of Python interpreter initialization, it essentially preempts the other standard library importers from doing anything.