Python exit commands: quit(), exit(), sys.exit() and os._exit()

The functions quit(), exit(), sys.exit() and os._exit() have almost same functionality as they raise the SystemExit exception by which the Python interpreter exits and no stack traceback is printed.
We can catch the exception to intercept early exits and perform cleanup activities; if uncaught, the interpreter exits as usual.

When we run a program in Python, we simply execute all the code in file, from top to bottom. Scripts normally exit when the interpreter reaches the end of the file, but we may also call for the program to exit explicitly with the built-in exit functions.

  1. quit()

    It works only if the site module is imported so it should not be used in production code. Production code means the code is being used by the intended audience in a real-world situation. This function should only be used in the interpreter.

    It raises the SystemExit exception behind the scenes. If you print it, it will give a message:

    Example:



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    # Python program to demonstrate
    # quit()
      
    for i in range(10):
          
        # If the value of i becomes 
        # 5 then the program is forced
        # to quit
        if i == 5:
              
            # prints the quit message
            print(quit)
            quit()
        print(i)

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    Output:

    0
    1
    2
    3
    4
    Use quit() or Ctrl-D (i.e. EOF) to exit
    
  2. exit()

    exit() is defined in site.py and it works only if the site module is imported so it should be used in the interpreter only. It is like a synonym of quit() to make the Python more user-friendly. It too gives a message when printed:

    Example:

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    # Python program to demonstrate
    # exit()
      
    for i in range(10):
          
        # If the value of i becomes 
        # 5 then the program is forced
        # to exit
        if i == 5:
              
            # prints the exit message
            print(exit)
            exit()
        print(i)

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    Output:

    0
    1
    2
    3
    4
    Use exit() or Ctrl-D (i.e. EOF) to exit
    
  3. sys.exit([arg])

    Unlike quit() and exit(), sys.exit() is considered good to be used in production code for the sys module is always available. The optional argument arg can be an integer giving the exit or another type of object. If it is an integer, zero is considered “successful termination”.

    Note: A string can also be passed to the sys.exit() method.

    Example – A program which stops execution if age is less than 18.

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    # Python program to demonstrate
    # sys.exit()
      
      
    import sys
      
      
    age = 17
      
      
    if age < 18:
          
        # exits the program
        sys.exit("Age less than 18")    
    else:
        print("Age is not less than 18")

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    Output:

    An exception has occurred, use %tb to see the full traceback.
    
    SystemExit: Age less than 18
    
  4. os._exit(n)

    os._exit() method in Python is used to exit the process with specified status without calling cleanup handlers, flushing stdio buffers, etc.

    Note: This method is normally used in child process after os.fork() system call. The standard way to exit the process is sys.exit(n) method.

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    # Python program to explain os._exit() method  
        
    # importing os module   
    import os 
        
    # Create a child process 
    # using os.fork() method  
    pid = os.fork() 
        
        
    # pid greater than 0 
    # indicates the parent process  
    if pid > 0
        
        print("\nIn parent process"
        # Wait for the completion  
        # of child process and     
        # get its pid and  
        # exit status indication using 
        # os.wait() method 
        info = os.waitpid(pid, 0
        
            
        # os.waitpid() method returns a tuple 
        # first attribute represents child's pid 
        # while second one represents 
        # exit status indication 
        
        # Get the Exit code  
        # used by the child process 
        # in os._exit() method 
            
        # firstly check if 
        # os.WIFEXITED() is True or not 
        if os.WIFEXITED(info[1]) : 
            code = os.WEXITSTATUS(info[1]) 
            print("Child's exit code:", code) 
        
    else
        print("In child process"
        print("Process ID:", os.getpid()) 
        print("Hello ! Geeks"
        print("Child exiting.."
            
        # Exit with status os.EX_OK 
        # using os._exit() method 
        # The value of os.EX_OK is 0         
        os._exit(os.EX_OK)

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    Output:

    In child process
    Process ID: 25491
    Hello ! Geeks
    Child exiting..
    
    In parent process
    Child's exit code: 0
    

Among above four exit functions, sys.exit() is preferred mostly, because the exit() and quit() functions cannot be used in production code while os._exit() is for special cases only when immediate exit is required.




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