[Python-Dev] Python 2.4 | 7.3 The for statement

[Nick Coghlan]

Juan Carlos Rodrigo wrote:
>>Interesting idea, but not really needed given the existence of the break statement:
> 
> 
> Goto = break
> I'm not interested.

All non-sequential control structures are merely constrained ways of using goto 
(the underlying machine code will devolve into conditional and unconditional 
branches and jumps - i.e. goto's). 'break' is a highly constrained form of goto 
and a fundamental part of structured programming (as is 'continue') - each is 
limited to a local effect on the loop that contains them. This is a far cry from 
the ability to jump to an arbitrary label that gives goto its bad reputation.

I used to share your sentiment regarding break and continue - experience 
(especially Python experience) has convinced me otherwise. Python embraces the 
concept of breaking out of a loop to the point that it even has an 'else' clause 
on loop structures that is executed only if the loop is exited naturally rather 
than via a break statement.

Regardless of whether you personally choose to use break and continue, the 
existence of those statements is the main reason that the addition of a flag 
condition to for loops is highly unlikely. If you want to terminate a for loop 
before the iterable is exhausted, the recommended solution is to use a break 
statement.

To illustrate the point about all control structures being gotos, even a simple 
do-nothing for loop results in a JUMP_ABSOLUTE (goto!) in the generated bytecode:

Py> def f():
...   for item in range(10):
...     pass
...
Py> import dis
Py> dis.dis(f)
   2           0 SETUP_LOOP              20 (to 23)
               3 LOAD_GLOBAL              0 (range)
               6 LOAD_CONST               1 (10)
               9 CALL_FUNCTION            1
              12 GET_ITER
         >>   13 FOR_ITER                 6 (to 22)
              16 STORE_FAST               0 (item)

   3          19 JUMP_ABSOLUTE           13
         >>   22 POP_BLOCK
         >>   23 LOAD_CONST               0 (None)
              26 RETURN_VALUE

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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