Cpython notes

Cpython is the official implementation of Python. It is written in C (hence the name).

So a Python intrepreter is basically a large complex C program. Let’s try to make sense of it.

Download the source

The source is hosted at github.com/python/cpython.

Every C program starts running from a main function.

The main function for Cpython is located inside Programs/python.c.

#include "Python.h"

int main(int argc, char **argv) {
    return Py_BytesMain(argc, argv);
}

// C extensions should only #include <Python.h> and not
// directly include the other Python header files included by <Python.h>

This leads to the Modules/main.c file.

int Py_BytesMain(int argc, char **argv) {
    _PyArgv args = {
        // something here
        // The argc and argv are packed into this structure
        // and passed into pymain_main
    }
    return pymain_main(&args);
}

This leads to another function in the same file.

static int pymain_main(_PyArgv *args) {
    // TODO: Look into this function
    PyStatus status = pymain_init(args);

    // some grunt work here

    return Py_RunMain();
}

Deeper down the rabbit hole, in the same file.

int Py_RunMain(void) {
    int exitcode = 0;

    pymain_run_python(&exitcode);

    pymain_free();

    // handle keyboard interrupt

    return exitcode;
}

Finally, we arrive at a function that actually does something interesting.

static void pymain_run_python(int *exitcode) {
    PyInterpreterState *interp = _PyInterpreterState_GET(); // Interpreter state
    PyConfig *config = _PyInterpreterState_GetConfig(interp);// PyConfig

    if (config->run_command) {
        *exitcode = pymain_run_command(config->run_command);
    }
    else if (config->run_module) {
        *exitcode = pymain_run_module(config->run_module, 1);
    }
    else if (main_importer_path != NULL) {
        *exitcode = pymain_run_module(L"__main__", 0);
    }
    else if (config->run_filename != NULL) {
        *exitcode = pymain_run_file(config);
    }
    else {
        *exitcode = pymain_run_stdin(config);
    }
    pymain_repl(config, exitcode);


}

Let’s take a look at the pymain_run_command since that seems to be the most barebones way of running a line of Python code.

static int pymain_run_command(wchar_t *command) {
    PyObject *unicode, *bytes;
    int ret;

    unicode = PyUnicode_FromWideChar

    bytes = PyUnicode_AsUTF8String(unicode);

    ret = PyRun_SimpleStringFlags(PyBytes_AsString(bytes), &cf);
}

Questions here:

  • What is a wchar_t? It looks like it means WideChar.
  • What is Unicode and UTF8 string?
  • It seems like we always do Py_DECREF() after we’ve finished using it.

This function takes us to the Python/pythonrun.c file.

int PyRun_SimpleStringFlags(const char *command, PyCompiler *flags) {
    PyObject *m, *d, *v;
    m = PyImport_AddModule("__main__");

    d = PyModule_GetDict(m);

    v = PyRun_StringStringFlags(command, Py_file_input, d, d, flags);

    return 0 else return -1 if error;
}

This takes us to another function that uses an interesting structure.

PyObject *PyRun_StringFlags(
    const char *str,
    int start,
    PyObject *globals,
    PyObject *locals,
    PyCompilerFlags *flags
) {
    PyObject *ret = NULL;
    PyArena *arena = _PyArena_New();

    mod = _PyParser_ASTFromString(start, flags, arena); // shortened code

    ret = run_mod(mod, globals, locals, flags, arena); // shortened code
}

This leads to a couple of interesting functions. Let’s take a look at the run_mod first.

static PyObject* run_mod(...) {
    PyThreadState
    PyCodeObject *co = _PyAST_Compile(...);

    PyObject *v = run_eval_code_obj(tstate, co, globals, locals);

    return v;
}

run_eval_code_obj leads to PyEval_EvalCode

This inturn leads to PyEval_EvalFrame. This quickly leads to _PyEval_EvalFrameDefault

This is the core functionality of the interpreter.

What is _Py_HOT_FUNCTION This function uses COMPUTED_GOTOS, which is faster than a switch statement, I presume.

This …

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