Updating a variable by its id within python

Question

I know how to get the value of a variable by its id in Python like:

a = "hello world!"
ctypes.cast(id(a), ctypes.py_object).value

I wonder if it possible to overwrite the variables value by id?

The simplest way, this one:

ctypes.cast(id(a), ctypes.py_object).value = "new value"

does not work.

Answer 1

Why it did not work

The object ctypes.cast(id(a), ctypes.py_object) only provides a view on an object in memory. So when updating the value attribute you do not update the object itself, all you do is create a new object and make value point to it.

import ctypes

a = "Hello World!"
py_obj = ctypes.cast(id(a), ctypes.py_object)

id(py_obj.value) # 1868526529136

py_obj.value = 'Bye Bye World!'

# Here we can see that `value` now points to a new object
id(py_obj.value) # 1868528280112

How to mutate any object

It is possible, with ctypes, to update memory directly and thus to mutate any object. That is even true for strings which are said to be immutable.

The following is interesting as an exercice, but should never be used in other circumstances. Among other things it can corrupt object reference count, leading to memory management errors.

import ctypes
import sys

def mutate(obj, new_obj):
    if sys.getsizeof(obj) != sys.getsizeof(new_obj):
        raise ValueError('objects must have same size')

    mem = (ctypes.c_byte * sys.getsizeof(obj)).from_address(id(obj))
    new_mem = (ctypes.c_byte * sys.getsizeof(new_obj)).from_address(id(new_obj))

    for i in range(len(mem)):
        mem[i] = new_mem[i]

Here are examples. Among these you will find reasons why you must not use the above code unless you really know what you are doing or as an exercice.

s = 'Hello World!'
mutate(s, 'Bye World!!!')
print(s) # prints: 'Bye World!!!'

# The following happens because of Python interning
mutate('a', 'b')
print('a') # prints: 'b'

mutate(1, 2)
print(1) # prints: 2

In particular, the above example makes Python exit with an unknown error code or crash, depending on the version and environment.

Answer 2

a is a string, and strings are immutable in Python.

Example from documentation:

>>> s = "Hello, World"
>>> c_s = c_wchar_p(s)
>>> print(c_s)
c_wchar_p(139966785747344)
>>> print(c_s.value)
Hello World
>>> c_s.value = "Hi, there"
>>> print(c_s)              # the memory location has changed
c_wchar_p(139966783348904)
>>> print(c_s.value)
Hi, there
>>> print(s)                # first object is unchanged
Hello, World
>>>