Python 3.6.10 C源码导入系统：_find_and_load函数及C层机制问询

Hey there! Let's break this down clearly since you're diving deep into Python 3.6.10's C source for module imports.

First: Why You Can't Find PyId__find_and_load as a Function #

First off, PyId__find_and_load isn't a C function—it's a method name identifier used internally by Python to avoid repeated string creation. Here's the breakdown:

• It's defined via the Py_DEFINE_ID macro, which generates a global PyUnicodeObject* variable tied to the string "_find_and_load". You might find the macro invocation in files like Python/import.c or related headers (search for Py_DEFINE_ID paired with _find_and_load).
• The actual implementation behind this identifier is a Python function: importlib._bootstrap._find_and_load, which lives in Python's standard library code (not C source). The C code uses PyId__find_and_load to quickly look up this Python method without constructing a new string each time.

What That _PyObject_CallMethodIdObjArgs Line Does #

The line you're looking at is the C layer handing off the import request to Python's importlib implementation:

mod = _PyObject_CallMethodIdObjArgs(interp->importlib, &PyId__find_and_load, abs_name, interp->import_func, NULL);

Let's unpack each part:

• interp->importlib: A pointer to the importlib._bootstrap module object, loaded during interpreter initialization and acting as the core of Python's import system.
• &PyId__find_and_load: The identifier for the _find_and_load method we just discussed.
• abs_name: The absolute name of the module being imported (e.g., "a" or "package.submodule").
• interp->import_func: A C function pointer that acts as a fallback hook for imports (used for built-in modules).
• The NULL signals the end of arguments.

This call returns the loaded module object (stored in mod) or NULL if an error occurs (with an exception set).

C-Level Flow of Python's Import System (3.6.10) #

Here's the high-level sequence when you run import a:

1. Top-Level Trigger: The Python bytecode for import a calls into the C function PyImport_ImportModuleLevelObject (in Python/import.c).
2. Initial Checks: This function validates the import request, checks if the module is already loaded in sys.modules, and prepares to load it if not.
3. Hand Off to Importlib: Eventually, the code reaches the line you found, which calls importlib._bootstrap._find_and_load via the C API.
4. Python-Layer Heavy Lifting: The _find_and_load Python function handles the core import logic:
   • Calls _find_spec to locate the module (checks sys.path, built-in modules, etc.).
   • Creates a module object from the spec.
   • Loads the module's code (from a .py file, bytecode, or built-in extension).
   • Runs the module's code to initialize variables, functions, and classes.
5. Return to C Layer: The loaded module object is passed back to the C code, which adds it to sys.modules and returns it to the Python runtime.

Key point: Since Python 3.3, most of the import logic lives in Python code (importlib) rather than C. The C layer acts as a thin wrapper that bridges the interpreter's core to the flexible, Python-based import system.

内容的提问来源于stack exchange，提问作者Espada