Comptage des références dans CPython

Qu’est-ce que le comptage des références ?

import sys

x = [1, 2, 3]
print(sys.getrefcount(x))   # 2 (x + getrefcount arg)
y = x
print(sys.getrefcount(x))   # 3

sys.getrefcount()

import sys

a = "hello"
print(sys.getrefcount(a))   # baseline (may be high for interned strings)

b = a
print(sys.getrefcount(a))   # one more
del b
print(sys.getrefcount(a))   # back to baseline

Incrémentation et décrémentation

x = []          # refcount = 1
y = x           # refcount = 2
z = [x]         # refcount = 3  (list holds a reference)
del y           # refcount = 2
z.clear()       # refcount = 1
del x           # refcount = 0 → deallocated

Internement des objets

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

c = 257
d = 257
print(c is d)   # may be False (not interned)

Références faibles

import weakref

class BigObject:
    pass

obj = BigObject()
ref = weakref.ref(obj)
print(ref())   # <BigObject instance>
del obj
print(ref())   # None (object was collected)

Cycles de références

a = []
b = [a]
a.append(b)   # cycle: a → b → a
del a, b
# Both are unreachable but refcount > 0
# The cyclic GC collects them

Le module gc

import gc

gc.collect()   # force a collection cycle
print(gc.get_count())    # (gen0, gen1, gen2) allocation counts
print(gc.get_threshold())  # when each generation is collected

Réservoirs de mémoire (pymalloc)

# You do not call pymalloc directly — CPython uses it internally
# for all Python-level object allocations

# The pool pre-allocates large memory arenas
# and carves them into pools of fixed-size blocks

Finaliseurs __del__

class Resource:
    def __del__(self):
        print(f"Freeing {self}")

# Better: use context manager
class SafeResource:
    def __enter__(self): return self
    def __exit__(self, *a): self.cleanup()

Objets compacts avec __slots__

import sys

class Reg:
    def __init__(self, x, y): self.x, self.y = x, y

class Slotted:
    __slots__ = ("x","y")
    def __init__(self, x, y): self.x, self.y = x, y

print(sys.getsizeof(Reg(1,2)))      # ~48 + dict overhead
print(sys.getsizeof(Slotted(1,2)))  # ~48 (no dict)

Désactiver le ramasse-miettes cyclique

import gc

gc.disable()   # no cyclic GC — objects are only freed by refcounting

# Use only if you KNOW no cycles are created
# (pure functional/immutable data)

gc.enable()    # re-enable

Vérification rapide

Récapitulatif