A nasty little memory leak

Question

Below Python code generates memory leak.

Challenge: Fix the memory leak with minimal changes to the code as measured by Levenshtein distance to the original code. You may only change the code before t is initialised. When the del t line is removed the modified program should print I'm still alive! every second.

PS: I already know the answer (found it after several hours of debugging). This is not the programming help question! Hopefully this would be interesting to folks.

import threading, time

# this class takes callback and calls it every 1 sec
class MyTimer:
    def __init__(self, callback):
        self.callback = callback
        self.th = threading.Thread(target=self.run,daemon=True)
        self.th.start()
    def run(self):
        while(True):
            self.callback()
            time.sleep(1)

class TestMyTimer:
    def __init__(self):
        self.timer = MyTimer(self.is_alive)
    def is_alive(self):
        print("I'm still alive")

# test code for MyTimer
t = TestMyTimer()
del t

# just to be sure to collect all dead objects
import gc
gc.collect()

# if you are still seeing messages then we have memory leak!
time.sleep(60)

Here's repl.it to play with this code online.

Answer 1

Taking inspiration from the comments and from the question, just by adding 3 characters I "fixed" the memory leak.

Granted, I broke everything else at the same time

import threading, time

# this class takes callback and calls it every 1 sec
class MyTimer:
    def __init__(self, callback):
        self.callback = callback
        self.th = threading.Thread(target=self.run,daemon=True)
        self.th.start()
    def run(self):
        while(True):
            self.callback()
            time.sleep(1)

class TestMyTimer:
    def __init__(self):
        self.timer = MyTimer(self.is_alive)
    def is_alive(self):
        print("I'm still alive")

# test code for MyTimer
#t = TestMyTimer()
#del t

# just to be sure to collect all dead objects
import gc
#gc.collect()

# if you are still seeing messages then we have memory leak!
time.sleep(60)

Edit: The challenge has now been properly updated :)

Answer 2

import threading, time

# this class takes callback and calls it every 1 sec
class MyTimer:
    def __init__(self, callback):
        self.a = True
        self.callback = callback
        self.th = threading.Thread(target=self.run,daemon=True)
        self.th.start()
    def run(self):
        while(True):
            self.callback(self.a)
            time.sleep(1)


class TestMyTimer:
    def __init__(self):
        self.timer = MyTimer(self.is_alive)
    def is_alive(self, k):
        print("I'm still alive")
        if not k:
            raise SystemExit()


# test code for MyTimer
t = TestMyTimer()
t.timer.a = 0
del t.timer, t

# just to be sure to collect all dead objects
import gc
gc.collect()

# if you are still seeing messages then we have memory leak!
time.sleep(60)