When you use a goto statement in Python, you are basically instructing the interpreter to directly execute another line of code instead of the current one. The target line of code which you want the interpreter to execute at this moment needs to be marked in the section termed “label”.
Python does not support goto or anything equivalent. You should check out the control flow page for more information. In many other cases, you'll want to use a function to handle the logic you want to use a goto statement for. Show activity on this post. You can use infinite loop, and also explicit break if necessary.
Types of jump statements in Python: continue. break. pass.
No, Python does not support labels and goto. It's a (highly) structured programming language.
Python offers you the ability to do some of the things you could do with a goto using first class functions. For example:
void somefunc(int a)
{
if (a == 1)
goto label1;
if (a == 2)
goto label2;
label1:
...
label2:
...
}
Could be done in python like this:
def func1():
...
def func2():
...
funcmap = {1 : func1, 2 : func2}
def somefunc(a):
funcmap[a]() #Ugly! But it works.
Granted, that isn't the best way to substitute for goto. But without knowing exactly what you're trying to do with the goto, it's hard to give specific advice.
@ascobol:
Your best bet is to either enclose it in a function or use an exception. For the function:
def loopfunc():
while 1:
while 1:
if condition:
return
For the exception:
try:
while 1:
while 1:
raise BreakoutException #Not a real exception, invent your own
except BreakoutException:
pass
Using exceptions to do stuff like this may feel a bit awkward if you come from another programming language. But I would argue that if you dislike using exceptions, Python isn't the language for you. :-)
I recently wrote a function decorator that enables goto
in Python, just like that:
from goto import with_goto
@with_goto
def range(start, stop):
i = start
result = []
label .begin
if i == stop:
goto .end
result.append(i)
i += 1
goto .begin
label .end
return result
I'm not sure why one would like to do something like that though. That said, I'm not too serious about it. But I'd like to point out that this kind of meta programming is actual possible in Python, at least in CPython and PyPy, and not only by misusing the debugger API as that other guy did. You have to mess with the bytecode though.
I found this in the official python Design and History FAQ.
Why is there no goto?
You can use exceptions to provide a “structured goto” that even works across function calls. Many feel that exceptions can conveniently emulate all reasonable uses of the “go” or “goto” constructs of C, Fortran, and other languages. For example:
class label(Exception): pass # declare a label
try:
...
if condition: raise label() # goto label
...
except label: # where to goto
pass
...
This doesn’t allow you to jump into the middle of a loop, but that’s usually considered an abuse of goto anyway. Use sparingly.
It's very nice that this is even mentioned in the official FAQ, and that a nice solution sample is provided. I really like python because its community is treating even goto
like this ;)
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