return in loop for ?

[yomgui]

hello,

is it legal to return inside a for loop
or am I obliged to break out of the loop before returning ?

thanks

yomgui

[Mike Meyer]

yomgui <no*@valid.or g> writes:

is it legal to return inside a for loop
or am I obliged to break out of the loop before returning ?

Try it and see:

def f(): .... for i in range(20):
.... if i > 10: return i
.... print f() 11

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

[yomgui]

Mike Meyer wrote:

yomgui <no*@valid.or g> writes:

is it legal to return inside a for loop
or am I obliged to break out of the loop before returning ?

Try it and see:

it is not because it does work on an implementation of python
that it will work on any other platform or in the futur.

yomgui

[Mike Meyer]

yomgui <no*@valid.or g> writes:

Mike Meyer wrote:

yomgui <no*@valid.or g> writes:

> is it legal to return inside a for loop
> or am I obliged to break out of the loop before returning ?

Try it and see:

it is not because it does work on an implementation of python
that it will work on any other platform or in the futur.

That's true no matter how you arrive at your answer.

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.

[Steve Holden]

Mike Meyer wrote:

yomgui <no*@valid.or g> writes:

Mike Meyer wrote:

yomgui <no*@valid.or g> writes:

is it legal to return inside a for loop
or am I obliged to break out of the loop before returning ?

Try it and see:

it is not because it does work on an implementation of python
that it will work on any other platform or in the futur.

That's true no matter how you arrive at your answer.

But, to answer the specific question, there's nothing in the language
definition that suggests the programmer should refrain from using a
return inside a loop, so for any implementation to impose such a
restriction would be a violation of the language definition.

Yomgui: I am not a language lawyer, but I think you can feel safe
returning from inside a loop. Just as a matter of interest, how else
would you propose to implement the functionality Mike showed:

def f():

... for i in range(20):
... if i > 10: return i
...

Python is supposed to cleanly express the programmer's intent. I can;t
think of a cleaner way that Mike's - can you?

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[bonono]

Steve Holden wrote:

Yomgui: I am not a language lawyer, but I think you can feel safe
returning from inside a loop. Just as a matter of interest, how else
would you propose to implement the functionality Mike showed:

>>def f():

... for i in range(20):
... if i > 10: return i
...

Python is supposed to cleanly express the programmer's intent. I can;t
think of a cleaner way that Mike's - can you?

Interestingly, I just saw a thread over at TurboGears(or is it this
group, I forgot) about this multiple return issue and there are people
who religiously believe that a function can have only one exit point.

def f():
r = None
for i in range(20):
if i > 10:
r = 10
break
if r is None: something
else: return r

[Steve Holden]

bo****@gmail.co m wrote:

Steve Holden wrote:

Yomgui: I am not a language lawyer, but I think you can feel safe
returning from inside a loop. Just as a matter of interest, how else
would you propose to implement the functionality Mike showed:

>>def f():

... for i in range(20):
... if i > 10: return i
...

Python is supposed to cleanly express the programmer's intent. I can;t
think of a cleaner way that Mike's - can you?

Interestingly, I just saw a thread over at TurboGears(or is it this
group, I forgot) about this multiple return issue and there are people
who religiously believe that a function can have only one exit point.

def f():
r = None
for i in range(20):
if i > 10:
r = 10
break
if r is None: something
else: return r

Well, I'm happy in this instance that practicality beats purity, since
the above is not only ugly in the extreme it's also far harder to read.

What are the claimed advantages for a single exit point? I'd have
thought it was pretty obvious the eight-line version you gave is far
more likely to contain errors than Mike's three-line version, wouldn't
you agree?

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[bonono]

Steve Holden wrote:

Well, I'm happy in this instance that practicality beats purity, since
the above is not only ugly in the extreme it's also far harder to read.

What are the claimed advantages for a single exit point? I'd have
thought it was pretty obvious the eight-line version you gave is far
more likely to contain errors than Mike's three-line version, wouldn't
you agree?

I thought it is about easier to spot error(that the return value is
done in single place) but didn't give too much attention as I don't buy
it. I call them "gotophobia "

[Duncan Booth]

Steve Holden wrote:

Interestingly, I just saw a thread over at TurboGears(or is it this
group, I forgot) about this multiple return issue and there are people
who religiously believe that a function can have only one exit point.

def f():
r = None
for i in range(20):
if i > 10:
r = 10
break
if r is None: something
else: return r

To bonono, not Steve:

So if a function should religiously have only one exit point why does the
example you give have two exit points? i.e. the 'return r', and the implied
'return None' if you execute the 'something' branch.
Well, I'm happy in this instance that practicality beats purity, since
the above is not only ugly in the extreme it's also far harder to read.

What are the claimed advantages for a single exit point? I'd have
thought it was pretty obvious the eight-line version you gave is far
more likely to contain errors than Mike's three-line version, wouldn't
you agree?

Arguments include: any cleanup code you need when returning from a function
is all in one place (mostly applicable to languages where you have to do
your own memory management---see the recent AST discussion on the python
dev list); or it stops you accidentally forgetting to return a value (as
demonstrated above :^) )

In practice it is impossible to write code in Python (or most
languages) with only one return point from a function: any line could throw
an exception which is effectively another return point, so the cleanup has
to be done properly anyway.

[bonono]

Duncan Booth wrote:

To bonono, not Steve:

So if a function should religiously have only one exit point why does the
example you give have two exit points? i.e. the 'return r', and the implied
'return None' if you execute the 'something' branch. I may have remember it wrong, but don't ask me. I am not the one
advocate of it. Oh, you mean the something. It is that I forgot what it
was, the original one did have one return.
Arguments include: any cleanup code you need when returning from a function
is all in one place (mostly applicable to languages where you have to do
your own memory management---see the recent AST discussion on the python
dev list); or it stops you accidentally forgetting to return a value (as
demonstrated above :^) )

The faintly remember it was about the last reason, or why it has been
brought up.