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 ;)
I think while loop is alternate for the "goto_Statement". Because after 3.6 goto loop is not working anymore. I also write an example of the while loop.
str1 = "stop"
while str1 == "back":
var1 = int(input(" Enter Ist Number: "))
var2 = int(input(" Enter 2nd Number: "))
var3 = print(""" What is your next operation
For Addition Press And Enter : 'A'
For Muliplt Press And Enter : 'M'
For Division Press And Enter : 'D'
For Subtaction Press And Enter : 'S' """)
var4 = str(input("For operation press any number : "))
if(var1 == 45) and (var2 == 3):
print("555")
elif(var1 == 56) and (var2 == 9):
print("77")
elif(var1 == 56) and (var2 == 6):
print("4")
else:
if(var4 == "A" or "a"):
print(var1 + var2)
if(var4 == "M" or "m"):
print(var1 * var2)
if(var4 == "D" or "d"):
print(var1 / var2)
if(var4 == "S" or "s"):
print(var1 - var2)
print("if you want to continue then type 'stop'")
str1 = input()
print("Strt again")
For a forward Goto, you could just add:
while True:
if some condition:
break
#... extra code
break # force code to exit. Needed at end of while loop
#... continues here
This only helps for simple scenarios though (i.e. nesting these would get you into a mess)
Labels for break and continue were proposed in PEP 3136 back in 2007, but it was rejected. The Motivation section of the proposal illustrates several common (if inelegant) methods for imitating labeled break in Python.
No, Python does not support labels and goto, if that is what you're after. 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. :-)
It is technically feasible to add a 'goto' like statement to python with some work. We will use the "dis" and "new" modules, both very useful for scanning and modifying python byte code.
The main idea behind the implementation is to first mark a block of code as using "goto" and "label" statements. A special "@goto" decorator will be used for the purpose of marking "goto" functions. Afterwards we scan that code for these two statements and apply the necessary modifications to the underlying byte code. This all happens at source code compile time.
import dis, new
def goto(fn):
"""
A function decorator to add the goto command for a function.
Specify labels like so:
label .foo
Goto labels like so:
goto .foo
Note: you can write a goto statement before the correspnding label statement
"""
labels = {}
gotos = {}
globalName = None
index = 0
end = len(fn.func_code.co_code)
i = 0
# scan through the byte codes to find the labels and gotos
while i < end:
op = ord(fn.func_code.co_code[i])
i += 1
name = dis.opname[op]
if op > dis.HAVE_ARGUMENT:
b1 = ord(fn.func_code.co_code[i])
b2 = ord(fn.func_code.co_code[i+1])
num = b2 * 256 + b1
if name == 'LOAD_GLOBAL':
globalName = fn.func_code.co_names[num]
index = i - 1
i += 2
continue
if name == 'LOAD_ATTR':
if globalName == 'label':
labels[fn.func_code.co_names[num]] = index
elif globalName == 'goto':
gotos[fn.func_code.co_names[num]] = index
name = None
i += 2
# no-op the labels
ilist = list(fn.func_code.co_code)
for label,index in labels.items():
ilist[index:index+7] = [chr(dis.opmap['NOP'])]*7
# change gotos to jumps
for label,index in gotos.items():
if label not in labels:
raise Exception("Missing label: %s"%label)
target = labels[label] + 7 # skip NOPs
ilist[index] = chr(dis.opmap['JUMP_ABSOLUTE'])
ilist[index + 1] = chr(target & 255)
ilist[index + 2] = chr(target >> 8)
# create new function from existing function
c = fn.func_code
newcode = new.code(c.co_argcount,
c.co_nlocals,
c.co_stacksize,
c.co_flags,
''.join(ilist),
c.co_consts,
c.co_names,
c.co_varnames,
c.co_filename,
c.co_name,
c.co_firstlineno,
c.co_lnotab)
newfn = new.function(newcode,fn.func_globals)
return newfn
if __name__ == '__main__':
@goto
def test1():
print 'Hello'
goto .the_end
print 'world'
label .the_end
print 'the end'
test1()
Hope this answers the question.
No, Python does not support labels and goto, if that is what you're after. It's a (highly) structured programming language.
A working version has been made: http://entrian.com/goto/.
Note: It was offered as an April Fool's joke. (working though)
# Example 1: Breaking out from a deeply nested loop:
from goto import goto, label
for i in range(1, 10):
for j in range(1, 20):
for k in range(1, 30):
print i, j, k
if k == 3:
goto .end
label .end
print "Finished\n"
Needless to say. Yes its funny, but DONT use it.
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 think while loop is alternate for the "goto_Statement". Because after 3.6 goto loop is not working anymore. I also write an example of the while loop.
str1 = "stop"
while str1 == "back":
var1 = int(input(" Enter Ist Number: "))
var2 = int(input(" Enter 2nd Number: "))
var3 = print(""" What is your next operation
For Addition Press And Enter : 'A'
For Muliplt Press And Enter : 'M'
For Division Press And Enter : 'D'
For Subtaction Press And Enter : 'S' """)
var4 = str(input("For operation press any number : "))
if(var1 == 45) and (var2 == 3):
print("555")
elif(var1 == 56) and (var2 == 9):
print("77")
elif(var1 == 56) and (var2 == 6):
print("4")
else:
if(var4 == "A" or "a"):
print(var1 + var2)
if(var4 == "M" or "m"):
print(var1 * var2)
if(var4 == "D" or "d"):
print(var1 / var2)
if(var4 == "S" or "s"):
print(var1 - var2)
print("if you want to continue then type 'stop'")
str1 = input()
print("Strt again")
To answer the @ascobol's question using @bobince's suggestion from the comments:
for i in range(5000):
for j in range(3000):
if should_terminate_the_loop:
break
else:
continue # no break encountered
break
The indent for the else block is correct. The code uses obscure else after a loop Python syntax. See Why does python use 'else' after for and while loops?
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 ;)
No, Python does not support labels and goto, if that is what you're after. It's a (highly) structured programming language.
Labels for break and continue were proposed in PEP 3136 back in 2007, but it was rejected. The Motivation section of the proposal illustrates several common (if inelegant) methods for imitating labeled break in Python.
you can use User-defined Exceptions to emulate goto
example:
class goto1(Exception):
pass
class goto2(Exception):
pass
class goto3(Exception):
pass
def loop():
print 'start'
num = input()
try:
if num<=0:
raise goto1
elif num<=2:
raise goto2
elif num<=4:
raise goto3
elif num<=6:
raise goto1
else:
print 'end'
return 0
except goto1 as e:
print 'goto1'
loop()
except goto2 as e:
print 'goto2'
loop()
except goto3 as e:
print 'goto3'
loop()
I have my own way of doing gotos. I use separate python scripts.
If I want to loop:
file1.py
print("test test")
execfile("file2.py")
a = a + 1
file2.py
print(a)
if a == 10:
execfile("file3.py")
else:
execfile("file1.py")
file3.py
print(a + " equals 10")
(NOTE: This technique only works on Python 2.x versions)
I wanted the same answer and I didnt want to use goto. So I used the following example (from learnpythonthehardway)
def sample():
print "This room is full of gold how much do you want?"
choice = raw_input("> ")
how_much = int(choice)
if "0" in choice or "1" in choice:
check(how_much)
else:
print "Enter a number with 0 or 1"
sample()
def check(n):
if n < 150:
print "You are not greedy, you win"
exit(0)
else:
print "You are nuts!"
exit(0)
No, Python does not support labels and goto, if that is what you're after. It's a (highly) structured programming language.
Though there isn't any code equivalent to goto/label in Python, you could still get such functionality of goto/label using loops.
Lets take a code sample shown below where goto/label can be used in a arbitrary language other than python.
String str1 = 'BACK'
label1:
print('Hello, this program contains goto code\n')
print('Now type BACK if you want the program to go back to the above line of code. Or press the ENTER key if you want the program to continue with further lines of code')
str1 = input()
if str1 == 'BACK'
{
GoTo label1
}
print('Program will continue\nBla bla bla...\nBla bla bla...\nBla bla bla...')
Now the same functionality of the above code sample can be achieved in python by using a while loop as shown below.
str1 = 'BACK'
while str1 == 'BACK':
print('Hello, this is a python program containing python equivalent code for goto code\n')
print('Now type BACK if you want the program to go back to the above line of code. Or press the ENTER key if you want the program to continue with further lines of code')
str1 = input()
print('Program will continue\nBla bla bla...\nBla bla bla...\nBla bla bla...')
I was looking for some thing similar to
for a in xrange(1,10):
A_LOOP
for b in xrange(1,5):
for c in xrange(1,5):
for d in xrange(1,5):
# do some stuff
if(condition(e)):
goto B_LOOP;
So my approach was to use a boolean to help breaking out from the nested for loops:
for a in xrange(1,10):
get_out = False
for b in xrange(1,5):
if(get_out): break
for c in xrange(1,5):
if(get_out): break
for d in xrange(1,5):
# do some stuff
if(condition(e)):
get_out = True
break
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 wanted the same answer and I didnt want to use goto. So I used the following example (from learnpythonthehardway)
def sample():
print "This room is full of gold how much do you want?"
choice = raw_input("> ")
how_much = int(choice)
if "0" in choice or "1" in choice:
check(how_much)
else:
print "Enter a number with 0 or 1"
sample()
def check(n):
if n < 150:
print "You are not greedy, you win"
exit(0)
else:
print "You are nuts!"
exit(0)
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 have my own way of doing gotos. I use separate python scripts.
If I want to loop:
file1.py
print("test test")
execfile("file2.py")
a = a + 1
file2.py
print(a)
if a == 10:
execfile("file3.py")
else:
execfile("file1.py")
file3.py
print(a + " equals 10")
(NOTE: This technique only works on Python 2.x versions)
In lieu of a python goto equivalent I use the break statement in the following fashion for quick tests of my code. This assumes you have structured code base. The test variable is initialized at the start of your function and I just move the "If test: break" block to the end of the nested if-then block or loop I want to test, modifying the return variable at the end of the code to reflect the block or loop variable I'm testing.
def x:
test = True
If y:
# some code
If test:
break
return something
I was looking for some thing similar to
for a in xrange(1,10):
A_LOOP
for b in xrange(1,5):
for c in xrange(1,5):
for d in xrange(1,5):
# do some stuff
if(condition(e)):
goto B_LOOP;
So my approach was to use a boolean to help breaking out from the nested for loops:
for a in xrange(1,10):
get_out = False
for b in xrange(1,5):
if(get_out): break
for c in xrange(1,5):
if(get_out): break
for d in xrange(1,5):
# do some stuff
if(condition(e)):
get_out = True
break
There is now. goto
I think this might be useful for what you are looking for.
To answer the @ascobol's question using @bobince's suggestion from the comments:
for i in range(5000):
for j in range(3000):
if should_terminate_the_loop:
break
else:
continue # no break encountered
break
The indent for the else block is correct. The code uses obscure else after a loop Python syntax. See Why does python use 'else' after for and while loops?
In lieu of a python goto equivalent I use the break statement in the following fashion for quick tests of my code. This assumes you have structured code base. The test variable is initialized at the start of your function and I just move the "If test: break" block to the end of the nested if-then block or loop I want to test, modifying the return variable at the end of the code to reflect the block or loop variable I'm testing.
def x:
test = True
If y:
# some code
If test:
break
return something
pip3 install goto-statement
Tested on Python 2.6 through 3.6 and PyPy.
Link: goto-statement
foo.py
from goto import with_goto
@with_goto
def bar():
label .bar_begin
...
goto .bar_begin
To answer the @ascobol's question using @bobince's suggestion from the comments:
for i in range(5000):
for j in range(3000):
if should_terminate_the_loop:
break
else:
continue # no break encountered
break
The indent for the else block is correct. The code uses obscure else after a loop Python syntax. See Why does python use 'else' after for and while loops?
you can use User-defined Exceptions to emulate goto
example:
class goto1(Exception):
pass
class goto2(Exception):
pass
class goto3(Exception):
pass
def loop():
print 'start'
num = input()
try:
if num<=0:
raise goto1
elif num<=2:
raise goto2
elif num<=4:
raise goto3
elif num<=6:
raise goto1
else:
print 'end'
return 0
except goto1 as e:
print 'goto1'
loop()
except goto2 as e:
print 'goto2'
loop()
except goto3 as e:
print 'goto3'
loop()
no there is an alternative way to implement goto statement
class id:
def data1(self):
name=[]
age=[]
n=1
while n>0:
print("1. for enter data")
print("2. update list")
print("3. show data")
print("choose what you want to do ?")
ch=int(input("enter your choice"))
if ch==1:
n=int(input("how many elemet you want to enter="))
for i in range(n):
name.append(input("NAME "))
age.append(int(input("age ")))
elif ch==2:
name.append(input("NAME "))
age.append(int(input("age ")))
elif ch==3:
try:
if name==None:
print("empty list")
else:
print("name \t age")
for i in range(n):
print(name[i]," \t ",age[i])
break
except:
print("list is empty")
print("do want to continue y or n")
ch1=input()
if ch1=="y":
n=n+1
else:
print("name \t age")
for i in range(n):
print(name[i]," \t ",age[i])
n=-1
p1=id()
p1.data1()
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. :-)
There is now. goto
I think this might be useful for what you are looking for.
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. :-)
pip3 install goto-statement
Tested on Python 2.6 through 3.6 and PyPy.
Link: goto-statement
foo.py
from goto import with_goto
@with_goto
def bar():
label .bar_begin
...
goto .bar_begin
A working version has been made: http://entrian.com/goto/.
Note: It was offered as an April Fool's joke. (working though)
# Example 1: Breaking out from a deeply nested loop:
from goto import goto, label
for i in range(1, 10):
for j in range(1, 20):
for k in range(1, 30):
print i, j, k
if k == 3:
goto .end
label .end
print "Finished\n"
Needless to say. Yes its funny, but DONT use it.
Though there isn't any code equivalent to goto/label in Python, you could still get such functionality of goto/label using loops.
Lets take a code sample shown below where goto/label can be used in a arbitrary language other than python.
String str1 = 'BACK'
label1:
print('Hello, this program contains goto code\n')
print('Now type BACK if you want the program to go back to the above line of code. Or press the ENTER key if you want the program to continue with further lines of code')
str1 = input()
if str1 == 'BACK'
{
GoTo label1
}
print('Program will continue\nBla bla bla...\nBla bla bla...\nBla bla bla...')
Now the same functionality of the above code sample can be achieved in python by using a while loop as shown below.
str1 = 'BACK'
while str1 == 'BACK':
print('Hello, this is a python program containing python equivalent code for goto code\n')
print('Now type BACK if you want the program to go back to the above line of code. Or press the ENTER key if you want the program to continue with further lines of code')
str1 = input()
print('Program will continue\nBla bla bla...\nBla bla bla...\nBla bla bla...')
no there is an alternative way to implement goto statement
class id:
def data1(self):
name=[]
age=[]
n=1
while n>0:
print("1. for enter data")
print("2. update list")
print("3. show data")
print("choose what you want to do ?")
ch=int(input("enter your choice"))
if ch==1:
n=int(input("how many elemet you want to enter="))
for i in range(n):
name.append(input("NAME "))
age.append(int(input("age ")))
elif ch==2:
name.append(input("NAME "))
age.append(int(input("age ")))
elif ch==3:
try:
if name==None:
print("empty list")
else:
print("name \t age")
for i in range(n):
print(name[i]," \t ",age[i])
break
except:
print("list is empty")
print("do want to continue y or n")
ch1=input()
if ch1=="y":
n=n+1
else:
print("name \t age")
for i in range(n):
print(name[i]," \t ",age[i])
n=-1
p1=id()
p1.data1()
For a forward Goto, you could just add:
while True:
if some condition:
break
#... extra code
break # force code to exit. Needed at end of while loop
#... continues here
This only helps for simple scenarios though (i.e. nesting these would get you into a mess)
Source: Stackoverflow.com