コンテンツにスキップ

未翻訳のページ

このページはまだ日本語に翻訳されていないため、英語の原文を表示しています。 翻訳に協力する

exit() and quit() Functions

The exit() and quit() objects terminate the Python interpreter. They are _sitebuiltins.Quitter instances installed into builtins by the site module, intended for interactive use. In scripts, use sys.exit() instead.

Both do exactly two things when called: close sys.stdin, then raise SystemExit(code). Everything about their cost follows from that.

Complexity Reference

Operation Time Space Notes
exit / quit (bare name) O(1) O(1) Returns a fixed hint string from __repr__
exit() / quit() (the call) O(1) O(1) Closes sys.stdin, raises SystemExit
Resulting stack unwinding O(d) O(1) d = frames with finally or __exit__ to run
Resulting atexit handlers O(h) + handlers O(1) h = registered handlers
exit(code) with an int O(1) O(1) Stored on the exception, becomes exit status
exit(message) with a string O(n) O(n) n = message length, written to stderr; status 1

The call itself is constant time. What follows it is not: raising SystemExit unwinds the stack and runs the interpreter's normal shutdown, so the real cost is dominated by whatever cleanup your program has registered.

What exit and quit Are

# O(1) - type lookups
print(type(exit))    # O(1) - <class '_sitebuiltins.Quitter'>
print(type(quit))    # O(1) - <class '_sitebuiltins.Quitter'>

# The bare name prints a hint rather than exiting - O(1)
print(repr(exit))    # Use exit() or Ctrl-D (i.e. EOF) to exit
print(repr(quit))    # Use quit() or Ctrl-D (i.e. EOF) to exit

The two objects are identical apart from the name they print. Neither is a function; both are callable instances.

Availability

They are installed by the site module, so they do not exist under -S:

$ python -c "exit(7)"; echo $?
7

$ python -S -c "exit(7)"
NameError: name 'exit' is not defined

This is the main reason they do not belong in a script.

Calling Them

Exit Status

exit() passes its argument straight to SystemExit, so it accepts the same values as any exit status.

# All O(1) - the value is stored on the exception
exit()      # status 0
exit(0)     # status 0 - success
exit(1)     # status 1 - general error
exit(2)     # status 2 - misuse

$ python -c "exit(7)"; echo $?
7

Passing a String

A non-integer argument is printed to stderr and the status becomes 1.

# O(n) - n = length of the message written to stderr; status is 1
exit("error: config.json not found")

The stdin Side Effect

Before raising, Quitter closes sys.stdin. sys.exit() does not do this, and it can surprise code that still expects to read input.

import sys

try:
    exit()                     # O(1) - closes sys.stdin, then raises
except SystemExit:
    print(sys.stdin.closed)    # O(1) - True

What Calling Them Triggers

SystemExit propagates like any other exception, so cleanup still runs before the process ends.

import atexit
import sys

atexit.register(lambda: print("3. atexit handler"))   # O(1) to register

class Resource:
    def __enter__(self):
        return self

    def __exit__(self, *exc):
        print("2. __exit__")                          # O(1)
        return False

try:
    with Resource():
        try:
            exit(3)                                   # O(1) to raise
        finally:
            print("1. finally")                       # O(1)
except SystemExit as e:
    print("caught, code =", e.code)                   # O(1) - 3

# Output order: 1. finally, 2. __exit__, caught, then 3. atexit handler

Total cost of the exit is therefore O(d + h) plus whatever those handlers do. exit() is only "immediate" in the sense that the call returns nothing.

Interactive Use

# In the REPL, calling either one ends the session - O(1) to raise
exit()
quit()

# Ctrl-D (Unix/macOS) or Ctrl-Z then Enter (Windows) does the same

Typing the bare name is the common mistake: it prints the hint instead of exiting, which is exactly what __repr__ is there for.

Best Practices

✅ Do:

  • Treat exit() and quit() as REPL conveniences
  • Pass an integer for the exit status, or a string to report an error
  • Remember they raise SystemExit, so finally and atexit still run

❌ Avoid:

  • Using them in scripts - they are absent under -S, and they close sys.stdin. Use sys.exit()
  • Calling either from a library - raise an exception and let the caller decide
  • Assuming the exit is instantaneous - unwinding and handlers run first