The equivalent of a GOTO in python

The equivalent of a GOTO in python

Forgive me – I couldnt resist 😉

def goto(linenum):
    global line
    line = linenum

line = 1
while True:
    if line == 1:
        response = raw_input(yes or no? )
        if response == yes:
            goto(2)
        elif response == no:
            goto(3)
        else:
            goto(100)
    elif line == 2:
        print Thank you for the yes!
        goto(20)
    elif line == 3:
        print Thank you for the no!
        goto(20)
    elif line == 20:
        break
    elif line == 100:
        print Youre annoying me - answer the question!
        goto(1)

Gotos are universally reviled in computer science and programming as they lead to very unstructured code.

Python (like almost every programming language today) supports structured programming which controls flow using if/then/else, loop and subroutines.

The key to thinking in a structured way is to understand how and why you are branching on code.

For example, lets pretend Python had a goto and corresponding label statement shudder. Look at the following code. In it if a number is greater than or equal to 0 we print if it

number = input()
if number < 0: goto negative
if number % 2 == 0:
   print even
else:
   print odd
goto end
label: negative
print negative
label: end
print all done

If we want to know when a piece of code is executed, we need to carefully traceback in the program, and examine how a label was arrived at – which is something that cant really be done.

For example, we can rewrite the above as:

number = input()
goto check

label: negative
print negative
goto end

label: check
if number < 0: goto negative
if number % 2 == 0:
   print even
else:
   print odd
goto end

label: end
print all done

Here, there are two possible ways to arrive at the end, and we cant know which one was chosen. As programs get large this kind of problem gets worse and results in spaghetti code

In comparison, below is how you would write this program in Python:

number = input()
if number >= 0:
   if number % 2 == 0:
       print even
   else:
       print odd
else:
   print negative
print all done

I can look at a particular line of code, and know under what conditions it is met by tracing back the tree of if/then/else blocks it is in. For example, I know that the line print odd will be run when a ((number >= 0) == True) and ((number % 2 == 0) == False).

The equivalent of a GOTO in python

I entirely agree that goto is poor poor coding, but no one has actually answered the question. There is in fact a goto module for Python (though it was released as an April fool joke and is not recommended to be used, it does work).