Python Function Call

Over the years I've come to appreciate a real lot how coherent the Python object model is. For example, the callable concept applies to every object that can be invoked, starting by functions. When you define a function, you have an object that is an instance of the types.FunctionType class, a class that has a __call__ method. In contrast, Kotlin has the invokable concept, but this is an extra (the invoke operator) that you can add to your classes, but the basic invokable element, a method/function, is not an instance of a class with an invoke operator. Functions/methods in Kotlin (at the Kotlin language level, then at a JVM or JavaScript runtime things differ) are not objects, we have to get a function reference or a method reference (with the :: syntax) to treat them as objects.

So having in mind that the callable concept applies to normal functions and methods has made me to dive deeper into what I explained in a recent post:

At the C level, all Python objects are represented by the PyObject structure, which points to a PyTypeObject (its type). The type object defines how instances of that type behave.

To make calls fast and avoid creating temporary tuple/dict arguments, CPython uses the vectorcall protocol (Py_TPFLAGS_HAVE_VECTORCALL).
When the interpreter encounters a CALL opcode, it ultimately looks at the target object's type to find its vectorcall entry point:

- Pure Python Functions (PyFunction_Type): The vectorcall pointer points to _PyFunction_Vectorcall. This function extracts the function's PyCodeObject (func->func_code), allocates a new _PyInterpreterFrame on the evaluation stack, and pushes it to _PyEval_EvalFrameDefault.

- Built-in/Native Functions (PyCFunction_Type): Native functions (like math.sqrt or print) are wrapped in a PyCFunctionObject. Their vectorcall pointer points to _PyCFunction_Vectorcall. This function extracts the underlying C function pointer (func->m_ml->ml_meth) and invokes the compiled C code directly.

But, based on the "callable protocol", when the interpreter encounters a CALL opcode, it should check if the type of the object has a __call__ method and invoke it, right? How does this fit with the above? Well, we have to separate language semantics from implementation:

It's a classic example of language semantics vs. implementation details:

At the language level (Python semantics): Everything is an object, and every object that can be invoked implements the callable protocol via __call__. Defining __call__ on function and builtin_function_or_method keeps the object model clean, predictable, and fully inspectable.

At the runtime level (C implementation): The interpreter is allowed to use any shortcut it wants as long as it doesn't break the language semantics. Since invoking fn() is the most critical operation in the language, CPython uses type-checking fast paths (ob_type) to skip the overhead of attribute lookups.

Going deeper:

Looking up __call__ via the attribute resolution mechanism on every function call would carry massive performance overhead. Here is how CPython handles this under the hood and how it fits with the previous explanation. 1. Fast Paths Bypass __call__ Entirely

For pure Python functions (types.FunctionType) and standard C builtins (types.BuiltinFunctionType), CPython completely bypasses the __call__ method lookup.

Even though type(func).__call__ exists at the Python language level (defined on function / PyFunction_Type), the bytecode interpreter does not execute a dictionary lookup for __call__ when you write func().

Instead:

The interpreter checks Py_TYPE(callable).

If it matches &PyFunction_Type or &PyCFunction_Type, it executes the direct fast paths explained previously (allocating a _PyInterpreterFrame or invoking the vectorcall C function pointer directly).

As for custom callables (instances of classes with a __call__ method), the mechanism is also optimized for performance (it's not doing an expensive MRO __call__ search for any object). The structure representing and object's type, PyTypeObject, has 2 fields, tp_call and tp_vectorcall_offset serving different generations of CPython's calling API. tp_vectorcall_offset is used among others for bound methods and functools.partial. tp_call is used for classes that define a __call__ method (and their derived classes). In that case tp_call points to the generic C wrapper that performs the lookup for "__call__" in the object's class dictionary/MRO and invokes it (and so, if the object does not have a tp_call no search is performed at all).

If we have a function: def fn(), we can invoke it normally, via fn(), but also like this: fn.__call__(). The latter invokation form ends up creating the corresponding _PyInterpreterFrame and running fn's code_object in _PyEval_EvalFrameDefault(), but does it through quite a few extra steps. First we have the look up of the __call__ attribute in the fn object, which will find it in the Function class (types.FunctionType). Retrieving it will give us a bound method (types.MethodType), where __self__ is the fn function, and __func__ is the __call__ function. Then, we have the normal invokation of that bound method, that will end up executing the code object for the fn function.

AsyncLoop 2026

With "async loop" in this post I'm not talking about something like JavaScript's for-await or Python's async-for, where the iterator is asynchronous. What I'm talking about here is about performing an asynchronous operation in a loop (like doing a loop of http requests). Well, in the async/away world that's not something to be scared of, but in the past, when asynchronous code was based on callbacks (not even Promises) this was a bit more complicated. 15 years ago I published a couple of posts about that [1] and [2]. For whatever the reason recently my mind came to think a bit about that old stuff, and I've decided to do a sort of generic AsyncLoop class that could be used with callback based functions. It's intended for use as a sort of for-of loop (so for iterator based loops) and for normal for loops (with a specific logic for getting the next item and checking for stop).

The AsyncLoop class receives a function to obtain the next item in the iteration, a function to check if the iteration must finish and a function (actionFn) for the body of the loop. That function is the one that behaves asynchronously and expects a callback. The class provides to the action/body a callback that will take care of continuing (or stopping) with the next iteration.

class AsyncLoop {
    constructor(nextItemFn, conditionFn, actionFn, onEndFn) {
        this.nextItemFn = nextItemFn; // function that returns the next item
        this.conditionFn = conditionFn; // function that receives the current item 
        this.actionFn = actionFn; // function that receives the current item, a callback to invoke when done
        this.onEndFn = onEndFn; // function that is invoked when the loop ends
    }

    run() {
        let item = this.nextItemFn();
        if (this.conditionFn(item)) {
            this.actionFn(item, () => {
                    this.run();
                }
            );
        } 
        else {
            this.onEndFn();
        }
    }
}

We can use it like this:

// callback based asynchronous function
function getContent(item, callback) {
    setTimeout(() => {
        let content = "Content for: " + item;
        callback(content);
    }, 500);
}

function testAsyncLoop() {
    let countriesIter = ["France", "Germany", "Italy"].values();
    new AsyncLoop(
        () => countriesIter.next().value, // nextItemFn
        (item) => item !== undefined, // conditionFn
        // actionFn
        (item, nextFn) => {
            getContent(item, result => {
                console.log(result);
                nextFn();
            }); 
        },
        () => console.log("All items processed") // onEndFn
    ).run();
}

We can use it also in nested loops:

class Continent {
    constructor(name, countries) {
        this.name = name;
        this.countries = countries;
    }   
}

function testNestedAsyncLoop() {
    let continentsIter = [
        new Continent("Europe", ["France", "Germany", "Italy"]),
        new Continent("Asia", ["China", "Japan", "India"]),
        new Continent("Africa", ["Nigeria", "Egypt", "South Africa"])
    ].values();

    //nested loop
    new AsyncLoop(
        () => continentsIter.next().value, // nextItemFn
        (continent) => continent !== undefined, // conditionFn
        (continent, nextContinentFn) => {
            console.log("Processing continent: " + continent.name);
            let countriesIter = continent.countries.values();
            new AsyncLoop(
                () => countriesIter.next().value,
                (country) => country !== undefined,
                (country, nextCountryFn) => {
                    getContent(country, result => {
                        console.log(result);
                        nextCountryFn();
                    });
                },
                () => nextContinentFn() // onEndFn
            ).run();
        },
        () => console.log("All continents processed")
    ).run();
}

Async/await has always felt a bit like magic (all what the compiler does under the covers), but looking to the above code that we would have written in 2011 and comparing it to how we would write it now with async/await is like a quantum leap!

function getContentPromisified(item) {
    return new Promise((res, rej) => getContent(item, res));
}

async function testUsingAsyncAwait(){
    let continents = [
        new Continent("Europe", ["France", "Germany", "Italy"]),
        new Continent("Asia", ["China", "Japan", "India"]),
        new Continent("Africa", ["Nigeria", "Egypt", "South Africa"])
    ];
    for (let continent of continents) {
        console.log("Processing continent: " + continent.name);
        for (let country of continent.countries) {  
            console.log(await getContentPromisified(country));  
        }
    }
    console.log("All continents processed");
}

Python And Native Modules

When we think about running Python code, particularly in the CPython interpreter, we normally think about interpreted code (a bytecode interpreter), but we also know that we can write native modules (extension modules) normally in C or C++. but also using cython, nuitka or even rust. These extension modules are not used just by some third party libraries, but the python runtime/environment/standard library also makes good use of them. In this sense, as explained in this article we have 2 types of these modules:

• A built-in extension module is a module built and shipped with the Python interpreter. A built-in module is statically linked into the interpreter, thereby lacking a __file__ attribute.
• A shared (or dynamic) extension module is built as a shared library (.so or .dll file) and is dynamically linked into the interpreter. In particular, the module’s __file__ attribute contains the path to the .so or .dll file.

Normally the Python interpreter is contained in the python3.XX binary (.exe in Windows), that is like 30 MBs in size, but in some builds (those done with the --enable-shared flag), most of the code is put in a libpython3.14.so (.dll in Windows) dynamic libray, and the python binary just bootstraps it.

built-in extension modules (like sys, builtins, _thread, gc...) are obviously part of the Python distribution (they are inside the interpreter as we've seen), while for shared extension modules, we have those that are part of the python distribution (like math, array, _ssl, _socket) and those that are developed by third parties.

You can check all the built-in extension modules like this:

>>> sys.builtin_module_names
('_abc', '_ast', '_codecs', '_collections', '_contextvars', '_datetime', '_functools', '_imp', '_io', '_locale', '_opcode', '_operator', '_signal', '_sre', '_stat', '_string', '_suggestions', '_symtable', '_sysconfig', '_thread', '_tokenize', '_tracemalloc', '_types', '_typing', '_warnings', '_weakref', 'atexit', 'builtins', 'errno', 'faulthandler', 'gc', 'itertools', 'marshal', 'posix', 'pwd', 'sys', 'time')

While for the shared extension modules that are part of your distribution, just check the lib-dynload folder in your installation, e.g.:

ls -la /usr/local/lib/python3.14/lib-dynload/
total 27032
drwxr-xr-x  2 root root    4096 mar 14 13:26 .
drwxr-xr-x 43 root root    4096 mar 14 13:26 ..
-rwxr-xr-x  1 root root  298704 mar 14 13:26 array.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root  399192 mar 14 13:26 _asyncio.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root  201048 mar 14 13:26 binascii.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root   97240 mar 14 13:26 _bisect.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root 1705384 mar 14 13:26 _blake2.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root  105048 mar 14 13:26 _bz2.cpython-314-x86_64-linux-gnu.so
-rwxr-xr-x  1 root root  166800 mar 14 13:26 cmath.cpython-314-x86_64-linux-gnu.so
...

So when running a Python application you have normal Python functions (that as first step of the execution have been compiled to Python bytecodes) that have to be interpreted, and other functions, living in those extension modules that are already native code and have to be executed as such, without interpretation. How does Python manage that?

The essential part in most interpreters is the interpreter loop. This is the code that loops through the bytecode instructions to be interpreted (or traverses the tree of nodes in Tree-parsing interpreters). I say "most" rather than "all" because in tree parsing interpreters we can have things like Truffle, where each node execute() method takes care of moving to the next node (and also manages its own specialization). In CPython, this interpreter loop lives in the _PyEval_EvalFrameDefault function. It's nicely explained here

When a Python function is invoked we have a Function object and a CALL bytecode instruction. If the function being called is not a native one (so a normal function that was written in pure Python and compiled to bytecodes to be interpreted) the interpreter handles this CALL by creating a frame object (the optimized _PyInterpreterFrame that I discussed here), that contains all the information needed for the function execution: local variables (including arguments and closure cells), the code object... and invokes _PyEval_EvalFrameDefault() with that frame. Indeed, there's a very interesting performance optimization added in Python3.11, _PyEval_EvalFrameDefault no longer recursively calls itself when it finds a new CALL, so we keep a flat C stack. From a GPT:

Ordinary Python→Python calls are frameless on the C stack. A chain of plain function calls runs inside a single _PyEval_EvalFrameDefault C invocation. The CALL opcode pushes a lightweight _PyInterpreterFrame onto a per thread data stack (chunked heap) and dispatches within the same C frame. Plain recursion is bounded by sys.getrecursionlimit() (a logical / data-stack counter), not the C stack.

And from this very in depth article

In CPython 3.10 and earlier, the CALL instruction used to create a new interpreter stackframe for the function being called and then it used to recursively reenter the interpreter by calling its entry point _PyEval_EvalFrameDefault.

This was bad for performance from many angles at the hardware level. The recursive call into the interpreter required saving the registers for the current function, and pushing a new C stackframe. It would lead to increased memory usage because each recursive interpreter call would allocate its own local variables on the stack, and other heap allocations. Apart from that it would also lead to poor instruction cache locality due to the constant jumps in and out of the bytecode evaluation loop.

In the 3.11 release this was fixed by eliminating the recursive call to the interpreter. Now the CALL instruction simply creates the stackframe for the called function, after that it immediately starts evaluating the new function’s bytecode without ever leaving the loop.

I've explained so far how the interpreter manages "normal" functions, but what about native functions? The Function object for a native function does not have a code object (as obviously they don't have associated bytecodes), but a function pointer to the native code. From a GPT:

Native functions: Stored as PyCFunctionObject with a function pointer
No __code__ attribute
Instead, has a function pointer (ml_meth) stored in PyMethodDef
CPython calls the C function pointer directly, bypassing the interpreter loop
The C function executes natively and returns a PyObject*

How does the interpreter check if this 'CALL function_object' should be treated one way or another (_PyInterpreterFrame + _PyEval_EvalFrameDefault() vs native call)? Basically at a low level different objects are used for a "normal" function and a native function. But the whole story goes like this (GPT explanation)

At the C level, all Python objects are represented by the PyObject structure, which points to a PyTypeObject (its type). The type object defines how instances of that type behave.

To make calls fast and avoid creating temporary tuple/dict arguments, CPython uses the vectorcall protocol (Py_TPFLAGS_HAVE_VECTORCALL).
When the interpreter encounters a CALL opcode, it ultimately looks at the target object's type to find its vectorcall entry point:

- Pure Python Functions (PyFunction_Type): The vectorcall pointer points to _PyFunction_Vectorcall. This function extracts the function's PyCodeObject (func->func_code), allocates a new _PyInterpreterFrame on the evaluation stack, and pushes it to _PyEval_EvalFrameDefault.

- Built-in/Native Functions (PyCFunction_Type): Native functions (like math.sqrt or print) are wrapped in a PyCFunctionObject. Their vectorcall pointer points to _PyCFunction_Vectorcall. This function extracts the underlying C function pointer (func->m_ml->ml_meth) and invokes the compiled C code directly.

But things are even more interesting. I already talked about how in Python3.11 the Python interpreter turned into a Python Adaptive Specializing Interpreter. Thanks to that, Function Calls are optimized like this (as explained by a GPT)

If the interpreter had to look up the type and vectorcall pointer from scratch on every single loop iteration, it would be slow. To solve this, CPython uses Opcodes Specialization (PEP 659 Adaptive Interpreter). When a generic CALL opcode executes, it starts in an "adaptive" state. It looks at the object being called and patches itself in memory to a specialized version based on what it sees:

- Python-to-Python Calls
If the target is a pure Python function, the CALL opcode specializes itself into CALL_PY_EXACT_ARGS (or CALL_PY_BOUND_METHOD).

The Check: It performs a strict pointer comparison on the target's type to ensure it is exactly PyFunction_Type.
The Action: It skips the generic lookup, directly grabs the code object, creates the _PyInterpreterFrame, and increments the frame depth.

- Python-to-Native Calls

If the target is a native C function, the CALL opcode specializes into CALL_BUILTIN_FAST or CALL_BUILTIN_CLASS.

The Check: It verifies that the object's type is PyCFunction_Type and often checks if the specific function handler matches what was cached.
The Action: It bypasses the interpreter frame creation entirely, sets up the C arguments, and jumps straight into the native C function.

That's fascinating!

If out of curiosity we want to know if a function is pure Python or native code we can check its type. For normal functions its type is FunctionType, for native functions it's BuiltinFunctionType

# Normal functions:
import types
import math

def f1(): pass
l1 = lambda x: x

isinstance(f1, types.FunctionType)
True
isinstance(l1, types.FunctionType)
True

isinstance(len, types.FunctionType)
False
isinstance(len, types.BuiltinFunctionType)
True
isinstance(math.sqrt, types.BuiltinFunctionType)

Additionally, as I've mentioned, native functions lack a __code__ attribute.

f1.__code__
code object f1 at 0x76be17bfcac0, file "", line 1

len.__code__
Traceback (most recent call last):
  File "", line 1, in 
    len.__code__
AttributeError: 'builtin_function_or_method' object has no attribute '__code__'. 

Python unpacking and multiple assignment (again)

When I talked about Python destructuring assignment I also mentioned the * and ** syntax, and referred to it as "operators" which indeed is not right (though it's common to name them like that). They should be better referred as unpacking/packing syntax or star expressions. For the different use cases of this syntax:

• function invokation: f1(*args, **kwargs) or function definition: def f2(*args, **kwargs) we should talk about arguments unpacking or packing
• When used in collection literals we should talk about iterator unpacking: [*items1, *items2] and dictionary unpacking: {**dict1, **dict2}, with the expressions being called "starred expressions"
• When used on the left side of an assignment: first, *reminder = my_list, we talk of a "starred target".

As we know the unpacking (destructuring) happens automatically when performing a multiple assignment, with no need of using * at all

x, y = [1, 2]
print(f"x: {x}, y: {y}")
# x: 1, y: 2

There are some advanced uses that I tend to forget. We can use multiple assignment with object attributes or dictionary keys:

@dataclass
class Person:
    name: str
    age: int
    country: str

# multiple assignment to attributes of an object
p1 = Person("Antoine", 47, "France")
p1.name, p1.age = ["Francois", 48]
print(f"p1.name: {p1.name}, p1.age: {p1.age}")

# multiple assignment to dictionary keys
d1 = {}
d1["name"], d1["age"] = ["Francois", 48]
print(f"d1['name']: {d1['name']}, d1['age']: {d1['age']}")

As I explained in this post we can unpack nested structures:

x, [a, b], y = [1, [2, 3], 4]
print(f"x: {x}, a: {a}, b: {b}, y: {y}")
# x: 1, a: 2, b: 3, y: 4

But notice that nested unpacking works for assignment, but not for function parameters. Surprisingly this is something that worked in Python2 but was lost in Python3.

>>> def f(a, (b, c)):
...     return c

SyntaxError: Function parameters cannot be parenthesized


>>> def f(a, [b, c]):
...     return c
...     
            
SyntaxError: invalid syntax

Python is missing the object destructuring assignment feature present JavaScript, I mean:

const user = {
  id: 42,
  isVerified: true,
};

const { id, isVerified } = user;

So we have to use this more verbose approach (notice that itemgetter and attrgetter are a nice option for dynamic scenarios)

p1 = Person("Antoine", 47, "France")

name, country = p1.name, p1.country
name, country = attrgetter("name", "country")(p1)

name, age = d1["name"], d1["age"]
name, age = itemgetter("name", "age")(d1)

Python Class Body to the Limit

In my 2 previous posts [1] and [2] we've seen how the class body of a Python class statement is just executable code that is put in a code object around which a synthetic function is created. This code is executed during the class creation (receiving a namespace object, a dictionary, created by the __prepare__ method of the metaclass). This is pretty powerful, as you can put complex initialization code there, not just a normal assignment). We can apply a decorator conditionally, create multiple function alias, define functions conditionally... Let's see some examples.

def log_deco(fn):
    def log_wrapper(*args, **kwargs):
        print(f"invoking {fn.__name__}")
        return fn(*args, **kwargs)
    return log_wrapper

def do_nothing_deco(fn):
    return fn

log_mode = True
profile_mode = Trumult

class Person:
    def __init__(self, name: str): 
        self.name = name

    # conditional decorator
    @(log_deco if log_mode else do_nothing_deco)
    def say_hi(self, to_x: str):
        print(f"hi {to_x} I'm {self.name}")

    # declare a function conditionally
    if profile_mode:
        def get_memory_consumption(self):
            print("my memory consumtpion is ...")

    def do_something(self):
        print(f"{self.name} is doing_something")

    # function aliases
    work = do_something
    cook = do_something
    sleep = do_something

p1 = Person("Francois")
p1.say_hi("Iyan")
p1.get_memory_consumption()
p1.sleep()


# invoking say_hi
# hi Iyan I'm Francois
# my memory consumtpion is ...
# Francois is doing_something

That's pretty nice, right? As the class body ends up being executed as a function you can put any code in it, and the compiler will compile any declarations that you put in it as attributes in the namespace object (that then is passed to the __new__ and __init__ of the metaclass to create the class). But there's one limitation. What if I want to create several alias for a same function using a loop? in principle we can't.

class MyGeoManager:
    _not_implemented = lambda self, item:  print(f"Method is not implemented yet")
    for name in ["get_city", "get_country"]:
        # obviously this does not do what we would like
        name = _not_implemented 

Obviously the above is not doing what we want. It's just creating an attribute named "name" and assigning it in a loop. How could we add an attribute get_city and an attribute get_countr?

Well, we can leverage something we've seen in previous posts, our friend frame.f_locals. I mentioned it in my last post and talked about it in depthhere. f_locals gives us a write-through proxy to the locals of a function. When using an optimized scope, adding variables to that proxy has no particular useful effect, as the function has been compiled to acces by index to the variables that were found at compile time, but in the kind of scope used in a class body, that is a real dictionary, not a fast-array, adding new variables via f_locals adds them to the namespace dictionary that then is passed to __new__ and __init__. So we can do this:

class MyGeoManager: 
    local_namespace = inspect.currentframe().f_locals
    not_implemented = lambda self, item:  print(f"Method is not implemented yet")
    for name in ["get_city", "get_country"]:
        local_namespace[name] = not_implemented

print(f"get_country: {MyGeoManager.get_country}")
print(f"get_city: {MyGeoManager.get_city}")

It works like a charm!

There's another way to do this, derived from how class scope differs from optimized scopes. We are used to the builtin functions exec, compile and eval working with a snapshot of the locals namespace (because we are used to work in optimized scopes), but in a class scope, these functions receive the real namespace object. So we can leverage exec() like this:

class MyGeoManager:
    not_implemented = lambda self, item: print("Method is not implemented yet")
    for name in ["get_city", "get_country"]:
        exec(f"{name} = not_implemented")

Python Class Body and Lexical Scope

In my previous post about class creation in Python I mentioned how a code object is created for the code in the class body, and then that code object is executed as a function receiving a namespace object (created by __prepare__) as its locals. The class body will add attributes to that namespace, and can use whatever is already present in that namespace (if __prepare__ has put something there). This has made me wonder if apart from that, the class body can have access to its enclosing scope. Just remember that in this post we saw that methods in a class have access to its enclosing scope (they close over variables defined outside the class).

So the answer is YES, and it's just the closures mechanism in action. Let's see an example:

def create_class(id: str):
    class MyClass:
        # the class initialiation (the class body) has access to "external" variables in the enclosing scope, such as "id"
        # the code in the class body is placed in a codeobject that will run in a function having trapped (closed over) the "id" free variable
        class_id = id
        
        print(f"Free variables: {inspect.currentframe().f_code.co_freevars}")
        # Free variables: ('id',)

        def __init__(self, value):
            self.value = value

        def display(self):
            print(f"MyClass value: {self.value}, Class ID: {self.class_id}")

    return MyClass

cl = create_class("123")
instance = cl("aa")
instance.display()

# Free variables: ('id',)
# locals: {'__module__': '__main__', '__qualname__': 'create_class..MyClass', '__firstlineno__': 7, 'class_id': '123'}
# MyClass value: aa, Class ID: 123

As you can see, that class body (my understanding is that Python will execute the code object corresponding to that class body by putting it in a "synthetic function") has access to the id variable in the outer scope and can assign it to one of its attributes. We can see that 'id' in the list of freevars for the code object of the class body (that we get accessing the current frame from the class body itself). However, I came across something that confused me. If I print locals() from the class body, I can't see 'id' there. That's very strange, if I do the same from a normal function, locals shows both "normal" variables and those that the function has trapped in its closure, but, as I've said, for the class body, 'id' is missing in locals:

def create_class(id: str):
    class MyClass:
        # the class initialiation (the class body) has access to "external" variables in the enclosing scope, such as "id"
        # the code in the class body is placed in a codeobject that will run in a function having trapped (closed over) the "id" free variable
        class_id = id
        
        print(f"Free variables: {inspect.currentframe().f_code.co_freevars}")
        # Free variables: ('id',)

        # notice that locals() does not show the id free var
        # that's because we are running in "class scope" and locals() just shows its namespace, not the closure cells. The closure is a separate object that holds references to the free variables, and it is not part of the local namespace of the class body. However, the class body can still access the free variable "id" through the closure.
        print(f"locals: {locals()}")
        # {'__module__': '__main__', '__qualname__': 'create_class..MyClass', '__firstlineno__': 4, 'class_id': '123'}

While for a normal function, it's well there:

def outer(id):
    def inner(value):
        # locals() shows "id" free variable because we are in a function scope (optimized scope)
        print(f"locals: {locals()}")
        # {'value': 'bb', 'id': '456'}
        print(f"Inner function value: {value}, Outer ID: {id}")
    return inner

inner_func = outer("456")
inner_func("bb")

So, why is that? At the time of the Python3.13 release I wrote a post about locals(), f_locals and the "local namespace" (this is related to PEP-667). Well, indeed what I mention on that post (based on other articles) about the "local namespace" as a sort of dictionary is not correct for normal functions in recent Python versions. In "normal" functions we are working in an Optimized Scope. In this optimized scope local variables are not placed in a dictionary and accessed by key, but in a fastarray, and accessed by index (you can see this using dis to check the bytecodes of a function). This locals fastarray, part of the _PyInterpreterFrame for each running function, contains both local variables (including function arguments and those local variables that are cells, cellvars, because they are trapped by inner functions in its closure) and variables trapped by the function itself in its closure:

In CPython's frame object, the `fastlocals` array is laid out as:

[regular locals] [cellvars] [freevars]

At the beginning of a function that has variables in its __closure__ the `COPY_FREE_VARS` bytecode instruction copies cell references from `__closure__` into the frame's fastlocals array for quick access!*
After `COPY_FREE_VARS` executes, all variables (normal locals, cellvars and freevars) are accessed from the fastlocals array during function execution.

By the way, regarding the aforementioned _PyInterpreterFrame, I'll leverage to copy here some GPT wisdom about frames in recent (Python 3.11 and above) Python versions

The _PyInterpreterFrame is an internal C struct introduced in Python 3.11 that represents a stack frame for execution, aiming to improve performance by reducing the overhead of allocating full Python PyFrameObject objects.

Purpose: It holds the execution state for code objects, including local variables, globals, builtins, and the instruction pointer (f_lasti).
Performance: Unlike older Python versions where every frame was a full heap-allocated PyFrameObject, _PyInterpreterFrame is designed to be lightweight and often lives on the C stack, reducing garbage collection pressure.

The traditional PyFrameObject still exists, but it has been relegated to a "shadow" role. It is now treated purely as a compatibility API wrapper.

Python only creates a PyFrameObject on demand when a tool or a piece of code explicitly asks to inspect the call stack. This process is often referred to as materializing a frame.

Class Statement vs Dynamic Class Creation

We know that along with the standard class statement, Python also allows us to create classes dynamically by calling type() (or another metaclass if our class has a metaclass other than type)

I already dedicated a rather thick post to type. Basically we use it for creating a new class like this: type(classname, superclasses, namespace). (the namespace is just a dictionary with the attributes).

So I was wondering if the compiler translates a class statement into a call to type(), and yes, more or less we can say so, but there are some extras. I've had a really insightful conversation with a GPT about this, and additionally I've found an excellent article that explains it in full detail

The steps that Python follows when it comes across with a class statement (class Foo(Base, metaclass=Meta): x = 1) are these (I'm taking it from a GPT discussion, it's basically the same that is explained in the linked article)

• Step 1 — Determine the metaclass. Python calls __build_class__ (a builtin), which inspects the bases and the explicit metaclass= kwarg to resolve which metaclass to use (with MRO-based metaclass conflict resolution).
• Step 2 — Prepare the namespace. The metaclass's __prepare__ classmethod is called: namespace = Meta.__prepare__('Foo', (Base,), **kwargs). This returns the dict (or dict-like object) that will serve as the class namespace. For type, this is just a plain dict. For enum.EnumMeta, for example, it returns a special _EnumDict.
• Step 3 — Execute the class body. The compiled code object for the class body is executed as a function, with the namespace from step 2 as its locals(). This is the key insight: it's essentially exec(body_code, globals(), namespace). After this, namespace contains {'x': 1, '__module__': ..., '__qualname__': ...}.
• Step 4 — Call the metaclass. Meta('Foo', (Base,), namespace) is called — which, for the default type, invokes type.__call__ → type.__new__ → type.__init__. This is where the actual class object is constructed.

How do the above steps look at the bytecode level? When the Python compiler (yes, in Python, where compilation is like a hidden step that happens the first time our code is run (or has changed), it's sometimes confusing to establish the difference between compilation time and execution time), comes across a class statement, it creates a code object for the code that we've placed inside that statement (the body of the class statement), along with code objects for each function (method) defined in that code, and creates a sequence of bytecode instructions that at runtime will make use of that code object (and many more things) to create a class object (yes, remember that classes are objects).

That sequence of bytecode instructions can vary slightly with Python versions (what I'll show below, that corresponds to python 3.14 is slightly different from what is shown in the aforementioned article), but the intent is the same.

class Person:
	pass

# translates into:

 0           RESUME                   0

  1           LOAD_BUILD_CLASS
              PUSH_NULL
              LOAD_CONST               0 (code object Person at 0x78d07576e730, file "class_creation.py", line 1)
              MAKE_FUNCTION
              LOAD_CONST               1 ('Person')
              CALL                     2
              STORE_NAME               0 (Person)
              LOAD_CONST               2 (None)
              RETURN_VALUE

So those seem like very few instructions for the complex 4 steps that I've just described!. Well, that's because all the magic happens in a builtin function __build_class, that is loaded by the LOAD_BUILD_CLASS bytecode instruction. The article makes a great job explaining these opcodes.

When we create a class dynamically using type() (or any other metaclass), we are directly at step 4, we are skipping the first 3 steps. Obviously it's us who choose the metaclass to use, and there's not class body to execute. And as we create ourselves the namespace object to pass to the metaclass the __prepare__ method that helps prepare that namespace is not executed. That's maybe the main difference then, that in the dynamic class creation the metaclass __prepare__ method does not intervene. That's interesting, cause indeed I was not familiar with that __prepare__ method (also referred as hook).

When talking about metaclasses I always think about __new__ and __init__ (and __call__ that intervenes when instances of a class created by the metaclass are created), I've talked about them in different posts, one of the most interesting being this, but was unfamiliar with __prepare__. We've seen that it allows us to prepare the namespace, OK, but when can we need that? Well, very rarely (this is particularly dark metaclass stuff). That can be material for another post, for now I'll just say that enum.EnumMeta makes use of it.