Re: for what are for/while else clauses
From: Fredrik Lundh (fredrik_at_pythonware.com)
Date: 11/21/03
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Date: Fri, 21 Nov 2003 12:45:25 +0100 To: python-list@python.org
Mel Wilson wrote:
> >for a while-statement, the controlling condition is the test at
> >the top.
> >
> >for a for loop, the condition is "is there another item" (to quote the
> >language reference: "When the items are exhausted (which is imme-
> >diately when the sequence is empty) ... the loop terminates.".
> >
> >for a try-except clause, the condition is "did the suite raise an
> >exception".
>
> Interesting way to think about it. Thanks.
>
> So in some sample code:
>
> while some_condition:
> some_action ()
> else:
> last_action ()
> following_code ()
>
> If the loop results in some_action being done, say, 17
> times; then that means that some_condition was found true 17
> times. The 18th time, some_condition is found to be false,
> the else takes effect and last_action gets done one time.
imagine a dialect of Python that supports C-style goto's and labels.
in this dialect,
while some_condition:
some_action ()
else:
last_action ()
can be rewritten as
this_statement:
if some_condition:
some_action ()
goto this_statement
else:
last_action ()
next_statement:
(which, of course, is exactly what Python's current compiler does, but
on the bytecode level).
"break" and "continue" can now be rewritten as "goto next_statement"
and "goto this_statement".
for "for-in" and "try-except", the code calculating the "some_condition"
value is a bit different, but the rest works in exactly the same way.
here's the for-in version:
<set up the iterator>
this_statement:
<fetch next value from iterator>
if <value found>:
variable = <value>
some_action ()
goto this_statement
else:
last_action ()
next_statement:
and here's the try-except version (somewhat simplified):
this_statement:
<enable error handling>
some_action ()
error_handler:
<disable error handling>
if <matching error occurred>:
some_action ()
else:
other_action ()
> The only wrinkle then is that the while loop construct
> passes control to the following code after that one
> last_action. But we expect that from a while loop.
most Python statements pass control to the next statement
when they're done.
> The effect of a break in the suite controlled by the
> while is to blow away execution of the whole while
> construct, including the else.
>
> As an explanation, I like it.
me too.
</F>
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