读 Python behind the scenes #1: how the CPython VM works

def g(x):
    return x + 3

# opcode & argument 各占一个 byte
$ python -m dis example1.py
...                                         #  opcode, argument  
2           0 LOAD_FAST            0 (x)    # (     124,        0)
            2 LOAD_CONST           1 (3)    # (     100,        1)
            4 BINARY_ADD                    # (      23,        0)
            6 RETURN_VALUE                  # (      83,        0)

def f(x):
    return x + 1

print(f(1))

$ python -m dis example2.py

1           0 LOAD_CONST               0 (<code object f at 0x10bffd1e0, file "example.py", line 1>)
            2 LOAD_CONST               1 ('f')
            4 MAKE_FUNCTION            0
            6 STORE_NAME               0 (f)

4           8 LOAD_NAME                1 (print)
           10 LOAD_NAME                0 (f)
           12 LOAD_CONST               2 (1)
           14 CALL_FUNCTION            1
           16 CALL_FUNCTION            1
           18 POP_TOP
           20 LOAD_CONST               3 (None)
           22 RETURN_VALUE
...

/* Function objects and code objects should not be confused with each other:
 *
 * Function objects are created by the execution of the 'def' statement.
 * They reference a code object in their __code__ attribute, which is a
 * purely syntactic object, i.e. nothing more than a compiled version of some
 * source code lines.  There is one code object per source code "fragment",
 * but each code object can be referenced by zero or many function objects
 * depending only on how many times the 'def' statement in the source was
 * executed so far.
 */

def make_add_x(x):
    def add_x(y):
        return x + y
    return add_x

add_4 = make_add_x(4)
add_5 = make_add_x(5)

struct PyCodeObject {
    PyObject_HEAD
    int co_argcount;            /* #arguments, except *args */
    int co_posonlyargcount;     /* #positional only arguments */
    int co_kwonlyargcount;      /* #keyword only arguments */
    int co_nlocals;             /* #local variables */
    int co_stacksize;           /* #entries needed for evaluation stack */
    int co_flags;               /* CO_..., see below */
    int co_firstlineno;         /* first source line number */
    PyObject *co_code;          /* instruction opcodes */
    PyObject *co_consts;        /* list (constants used) */
    PyObject *co_names;         /* list of strings (names used) */
    PyObject *co_varnames;      /* tuple of strings (local variable names) */
    PyObject *co_freevars;      /* tuple of strings (free variable names) */
    PyObject *co_cellvars;      /* tuple of strings (cell variable names) */

    Py_ssize_t *co_cell2arg;    /* Maps cell vars which are arguments. */
    PyObject *co_filename;      /* unicode (where it was loaded from) */
    PyObject *co_name;          /* unicode (name, for reference) */
        /* ... more members ... */
};

typedef struct {
    PyObject_HEAD
    PyObject *func_code;        /* A code object, the __code__ attribute */
    PyObject *func_globals;     /* A dictionary (other mappings won't do) */
    PyObject *func_defaults;    /* NULL or a tuple */
    PyObject *func_kwdefaults;  /* NULL or a dict */
    PyObject *func_closure;     /* NULL or a tuple of cell objects */
    PyObject *func_doc;         /* The __doc__ attribute, can be anything */
    PyObject *func_name;        /* The __name__ attribute, a string object */
    PyObject *func_dict;        /* The __dict__ attribute, a dict or NULL */
    PyObject *func_weakreflist; /* List of weak references */
    PyObject *func_module;      /* The __module__ attribute, can be anything */
    PyObject *func_annotations; /* Annotations, a dict or NULL */
    PyObject *func_qualname;    /* The qualified name */
    vectorcallfunc vectorcall;
} PyFunctionObject;

struct _frame {
    PyObject_VAR_HEAD
    struct _frame *f_back;      /* previous frame, or NULL */
    PyCodeObject *f_code;       /* code segment */
    PyObject *f_builtins;       /* builtin symbol table (PyDictObject) */
    PyObject *f_globals;        /* global symbol table (PyDictObject) */
    PyObject *f_locals;         /* local symbol table (any mapping) */
    PyObject **f_valuestack;    /* points after the last local */

    PyObject **f_stacktop;          /* Next free slot in f_valuestack.  ... */
    PyObject *f_trace;          /* Trace function */
    char f_trace_lines;         /* Emit per-line trace events? */
    char f_trace_opcodes;       /* Emit per-opcode trace events? */

    /* Borrowed reference to a generator, or NULL */
    PyObject *f_gen;

    int f_lasti;                /* Last instruction if called */
    /* ... */
    int f_lineno;               /* Current line number */
    int f_iblock;               /* index in f_blockstack */
    char f_executing;           /* whether the frame is still executing */
    PyTryBlock f_blockstack[CO_MAXBLOCKS]; /* for try and loop blocks */
    PyObject *f_localsplus[1];  /* locals+stack, dynamically sized */
};

from threading import Thread

def f():
    """Perform an I/O-bound task"""
    pass

t = Thread(target=f)
t.start()
t.join()

# generated with gpt-5.5
process
└─ runtime state
   ├─ interpreter state A  <-->  VM A
   │  ├─ modules / builtins / __main__
   │  └─ thread state(s)
   │     └─ call stack: frame -> frame -> frame
   │                     │
   │                     ├─ value stack, 
   │                     └─ code object (bytecode, consts, ..)
   │
   └─ interpreter state B  <-->  VM B
      └─ ...

# 一段 code object 可以被多个 function object 引用
function object
└─ __code__ ─────────────> code object
   + defaults / globals / closure / name / ...