atexit Module Complexity¶

The atexit module registers functions to run when the interpreter terminates normally. Handlers run in last-in, first-out (LIFO) order.

Complexity Reference¶

Operation	Time	Space	Notes
`atexit.register()`	O(1)	O(1)	Append handler to stack
`atexit.unregister()`	O(n)	O(1)	n = registered handlers; removes all matches
Handler execution	O(n)	O(1)	n = registered handlers; LIFO order

When handlers run

Handlers run only on normal interpreter shutdown (e.g., program end or sys.exit()). They do not run on os._exit() or hard termination signals.

import atexit

# Register handler - O(1)
@atexit.register
def cleanup():
    print("Cleaning up")

import atexit

@atexit.register
def first():
    print("first")

@atexit.register
def second():
    print("second")

# On shutdown:
# second
# first

import atexit

def temp():
    pass

atexit.register(temp)
atexit.unregister(temp)  # O(n) scan to remove all matches