Python Memory Model Deep Dive: Id, Pointers, and Mutability Traps

Understanding the CPython memory model is essential for passing senior Python engineering interviews. Unlike C where variables are memory addresses storing values, Python variables are name tags pointing to heap objects. Practice tracing memory references live at /practice/.

1. Integer Interning and the Small Integer Cache

a = 256
b = 256
print(a is b)  # True

x = 257
y = 257
print(x is y)  # False (in interactive REPL)

CPython pre-allocates an array of integer objects for numbers between -5 and 256. Any variable assigned an integer in this range points to the exact same pre-allocated memory object!

2. Mutable Objects Inside Immutable Containers

t = ([1, 2], 3)
t[0].append(99)
print(t)

A tuple is immutable because its reference pointers cannot be re-bound to different objects. However, if a tuple contains a pointer to a mutable list, that list itself CAN be mutated in-place! Output: ([1, 2, 99], 3).