جامع البيانات المهملة والمراجع الدورية

لماذا لا يكفي عدّ المراجع؟

# Reference cycle: a → b → a
a = {}
b = {"other": a}
a["other"] = b
del a, b
# Both objects are unreachable but refcount > 0
# Only the cyclic GC can collect them

الوحدة gc

import gc

gc.collect()   # trigger immediately
print(gc.garbage)   # objects with __del__ in cycles (cannot auto-collect)

الجمع حسب الأجيال

import gc

print(gc.get_count())     # (gen0, gen1, gen2) allocations since last collection
print(gc.get_threshold()) # (700, 10, 10) — thresholds for each generation
gc.set_threshold(1000, 15, 10)  # tune

اكتشاف الدورات باستخدام gc.get_referents

import gc

a = []
b = [a]
a.append(b)

for ref in gc.get_referents(a):
    print(ref)   # shows b

gc.get_objects()

import gc

before = len(gc.get_objects())
create_lots_of_objects()
after = len(gc.get_objects())
print(f"Leaked: {after - before} objects")

تجنب الدورات

import weakref

class Node:
    def __init__(self, parent):
        # weakref avoids cycle:
        self.parent = weakref.ref(parent)

class Tree:
    def __init__(self):
        self.child = Node(self)   # no hard cycle

__del__ والدورات

import gc

class Leaky:
    def __del__(self): pass

a = Leaky()
b = Leaky()
a.other = b
b.other = a
del a, b

gc.collect()
print(gc.garbage)   # [Leaky, Leaky] — cannot collect

gc.freeze() — Python 3.7 فما بعد

import gc

# After all globals and imports are in place:
gc.freeze()   # no GC scanning for these objects ever again

# Useful for: gunicorn pre-fork, background servers

تعطيل GC وإعادة تمكينها

import gc

gc.disable()
try:
    result = tight_loop_no_cycles()
finally:
    gc.enable()
    gc.collect()

isenabled وisfinalized

import gc

print(gc.isenabled())      # True by default
obj = object()
print(gc.is_finalized(obj))  # False (still alive)

أنماط تسرب الذاكرة

# Typical leaks:
global_cache = {}   # grows forever — use lru_cache with maxsize

class EventBus:
    listeners = []
    # Listeners hold references to objects:
    # @classmethod
    # def on(cls, fn): cls.listeners.append(fn)
    # Must call off() to detach

تحقق سريع

مراجعة