How to leak memory in Python

leak.py

# A simple test showing possible memory leaks in Python

# import gc
import time
import resource

counter = 0
def get_new_id():
    global counter
    counter += 1
    return counter - 1

class MyClass():

    def __init__(self):
        self.my_id  = get_new_id()
        self.friend = None
        self.blob   = b'1' * 1024 * 1024  # 1 MiB data
        print('__init__', self.my_id)

    def set_friend(self, friend):
        self.friend = friend

    def __del__(self):
        if hasattr(self, 'my_id'):
            print('__del__', self.my_id)


def my_fun():
    print('my_fun: creating objects')
    x = MyClass()
    y = MyClass()
    z = MyClass()
    print('my_fun: setting friends')
    x.set_friend(y)
    y.set_friend(z)
    z.set_friend(x)
    print('my_fun: return')

def main():
    for _ in range(100):
        print('Memory KiB:', resource.getrusage(resource.RUSAGE_SELF).ru_maxrss)
        my_fun()
        # gc.collect()
        print('Memory KiB:', resource.getrusage(resource.RUSAGE_SELF).ru_maxrss)
        time.sleep(2)

if __name__ == '__main__':
    main()

# You will notice that the memory keeps increasing on each iteration,
# and the past iteration objects' destructors are not being called,
# despite the fact that the objects are inaccessible, suggesting a memory leak.
# The culprit is the reference cycle together with the fact that the __del__ method is defined.
# CPython's garbage collector cannot decide which to (safely) call first!
# If you comment out the __del__ definition or break the cycle, the leak will be gone,
# as the GC will collect past objects on each iteration.
#
# Note that the topic is more nuanced than this, and with Python >= 3.4
# the GC will *eventually* destroy the objects, keeping memory usage bounded,
# while with older versions the process will eventually be killed by running OOM.
# more info here:
# https://docs.python.org/2.7/library/gc.html#gc.garbage
# https://docs.python.org/3.5/library/gc.html#gc.garbage
# https://peps.python.org/pep-0442/#disposal-of-cyclic-isolates

# Fun things to try:
# - Run with python2.7
# - Comment out memory printing and sleep(), increase the iterations
#   and run with different interpreter versions, while watching memory usage.
# - Comment out "z.set_friend(x)"
# - Uncomment the gc lines