What CPython Does With Your Objects
A Python-first developer interview rarely stops at a correct solution. The follow-up questions ask what the language does underneath: why a loop is slow, why a list uses so much memory, or what a short snippet prints. Almost every answer comes from two facts about CPython, the standard implementation. Every value is an object on the heap, and every name is a reference to one.
Every value is an object
In CPython, an integer is not a machine word. It is a heap object with a header that holds a reference count and a pointer to its type, followed by the value. sys.getsizeof(1) is 28 bytes on a 64-bit build, and a float takes 24.
A list does not hold values either. It holds pointers to objects, 8 bytes per slot. A list of a million distinct floats is therefore 8 MB of pointers plus a million 24-byte float objects, over 30 MB in total. A numpy float64 array holds the same numbers as raw doubles in 8 MB, next to each other in memory.
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