En Sun, 11 Oct 2009 19:46:06 -0300, Philip Semanchuk
<phi...@semanchuk.com> escribió:
On Oct 11, 2009, at 5:51 PM, bartc wrote:
Mensanator wrote:
On Oct 10, 3:15�pm, kj <no.em...@please.post> wrote:
I'm coaching a group of biologists on basic Python scripting. �One
of my charges mentioned that he had come across the advice never
to use loops beginning with "while True".[...]
If you know this "exit that is always reached",
why do you pretend not to know it by writing
"while True"?
When I'm starting to code something I haven't yet fully worked out, it
often starts with an infinite loop like this, until the body is coded
and I've figured out how to escape from it.
At the end if may or may not be tidied up, depending on how much work
it is to reconcile several possible break points into a single
terminating condition to be place at one end, and whether that is
likely to break or obfuscate a currently working program.
I think you bring up a good point. I think "while True" has some
legitimate uses (like event loops), and I don't mind seeing it there.
What I don't like is goto, and to a lesser extent break, exit, and
multiple returns. I don't find too many cases where they're the clearest
way to express things. And where one sees a "while True", one can almost
always find a "break" or two lurking in the loop.
IMHO, break, goto, etc. have their place, but they're ripe for abuse
which leads to spaghetti code. Since the OP is teaching non-programmers
to write code, I think the potential for abuse is especially important
to keep in mind. I'd think that teaching them a tool like "while True"
would encourage the "code now, design later" trap that even experienced
programmers -- including myself -- sometimes fall into. Writing "while
<condition>" instead forces one to stop at the beginning of the loop and
think at least a little about exactly what it's meant to accomplish.
I don't think so; forcing ALL loops to have the condition at the start is
unnatural in some cases. Some loops are best written with the condition at
the end (do/while or repeat/until in Pascal) and some loops require a test
in the middle (in Python, while True: + break, because we don't have an
unconditional loop).
In addition, isn't it easier to figure out how this loop ends --
while (condition1) and (condition2) and (condition3):
...lots of code here...
than this one?
while True:
...lots of code here...
if not condition1:
break
...lots of code here...
if not condition2:
break
...lots of code here...
if not condition3:
break
For a loop that can be written as the first one above, sure, that's pretty
explicit.
For a loop as the second one, try to rewrite it with the condition at the
start: you'll end with duplicated tests and/or duplicated code and/or
artificial boolean variables added.
Some algorithms are *naturally* expressed as a loop with the condition in
the middle. Forcing the condition to always happen at the start requires
artificial steps that complicate unnecesarily the code.
--
Gabriel Genellina
--
http://mail.python.org/mailman/listinfo/python-list
