The equivalent of a GOTO in python [duplicate]

问题:

This question already has an answer here:

• Is there a label/goto in Python? 15 answers

I am self teaching myself python 2.7. I have some experience in using BATCH, which has a GOTO statement. How do I do that in python? For example, suppose I want to jump from line 5 to line 18.

I realize there have been previous questions regarding this topic, but I have not found them sufficiently informative or, are too high level in python for my current understanding.

回答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 can't 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 can't 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).

回答2:

Forgive me - I couldn't 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 "You're annoying me - answer the question!"
        goto(1)

回答3:

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).

回答4:

There's no goto instruction in the Python programming language. You'll have to write your code in a structured way... But really, why do you want to use a goto? that's been considered harmful for decades, and any program you can think of can be written without using goto.

Of course, there are some cases where an unconditional jump might be useful, but it's never mandatory, there will always exist a semantically equivalent, structured solution that doesn't need goto.

回答5:

Disclaimer: I have been exposed to a significant amount of F77

The modern equivalent of goto (arguable, only my opinion, etc) is explicit exception handling:

Edited to highlight the code reuse better.

Pretend pseudocode in a fake python-like language with goto:

def myfunc1(x)
    if x == 0:
        goto LABEL1
    return 1/x

def myfunc2(z)
    if z == 0:
        goto LABEL1
    return 1/z

myfunc1(0) 
myfunc2(0)

:LABEL1
print 'Cannot divide by zero'.

Compared to python:

def myfunc1(x):
    return 1/x

def myfunc2(y):
    return 1/y


try:
    myfunc1(0)
    myfunc2(0)
except ZeroDivisionError:
    print 'Cannot divide by zero'

Explicit named exceptions are a significantly better way to deal with non-linear conditional branching.

回答6:

answer = None
while True:
    answer = raw_input("Do you like pie?")
    if answer in ("yes", "no"): break
    print "That is not a yes or a no"

Would give you what you want with no goto statement.