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L6·Expert

🧠Memory, References & gc

id, is vs ==, refcounting, and the cycle collector.

gcidisrefcount

§ 1is vs ==

== compares values. is compares identity (same object in memory). id(x) returns the identity.

§ 2Refcounting

CPython deallocates when the refcount hits zero. sys.getrefcount(x) inspects it.

§ 3Cycles

Reference cycles (a→b→a) can't be freed by refcounting alone. The gc module collects them.

Example

1import sys, gc​
2a = [1, 2]​
3b = a​
4print(a is b, a == b)     # True True​
5c = [1, 2]​
6print(a is c, a == c)     # False True​
7print(sys.getrefcount(a)) # includes the temp arg​
8gc.collect()​

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