2004年04月04日 · in Ideas · 322 words
The goto for Python module implements the "goto" and "label" keywords in Python. It's intended as a joke, but their first example is something I actually want to do occasionally in Python: break out of nested loops.
for x in list1: for y in list2: if condition: goto .exit label .exit
Most other languages provide some way to exit from nested loops. Perl has labelled blocks; C has goto. The approach I'd normally use in Python (if there wasn't a nicer way to reorganise it) would be to wrap a function around it.
def function(): for x in list1: for y in list2: if condition: return function()
You could also wrap the block in a try and throw an exception to
exit, although that's more code (and less semantically sensible — you
shouldn't use exceptions for routine conditions).
Similarly, if Python had an equivalent of Scheme's call/cc, you
could use a continuation to exit the block.
There are several possible syntaxes for labelled loops in Python (assuming
break were extended to accept a label, so break outer
would exit the loop labelled outer).
A suitably general syntax would allow you to label any code block for a later
break or continue.
for x in list1 named outer: outer:: for x in list1: for outer where x in list1: for (outer) x in list1: for x in list1 :: outer: ... break outer
A simpler approach that'd work for for loops would be to let the
break statement take a loop variable.
This wouldn't allow you to break out of nested while loops, but
that may not be a problem.
for x in list1: ... break x
A final approach would be to allow break to take a numeric
argument indicating how many levels of indentation to exit; this has the
advantage of being very simple, but it seems unlikely to lead to readable code.
(Most of this came out of an IRC discussion with Giles Radford.)