"for" with "else"?

[Invalid User]

While trying to print a none empty list, I accidentaly put an "else"
statement with a "for" instead of "if". Here is what I had:

if ( len(mylist)> 0) :
for x,y in mylist:
print x,y
else:
print "Empty list"

which was supposed to be:

if ( len(mylist)> 0) :
for x,y in mylist:
print x,y
else:
print "Empty list"
Is this to be expected?
(python 2.2.2)

+++++++++++++++ +++++++++++++++ =

for x in range(5):

.... print x*x
.... else:
.... print "done"
....
0
1
4
9
16
done
+++++++++++++++ +++++++++++++++ =

[Alan Kennedy]

[Invalid User wrote]

While trying to print a none empty list, I accidentaly put an "else"
statement with a "for" instead of "if". Here is what I had:

From the Python Language Reference

"""
The for statement

for_stmt ::= "for" target_list "in" expression_list ":" suite
["else" ":" suite]

The expression list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the else clause, if present,
is executed, and the loop terminates.
"""

http://www.python.org/doc/current/ref/for.html

A quick glance through the archives confirms that this has been the
case since at least python 1.4:-

http://www.python.org/doc/1.4/ref/ref7.html#HDR2

regards,

--
alan kennedy
-----------------------------------------------------
check http headers here: http://xhaus.com/headers
email alan: http://xhaus.com/mailto/alan

[Mark Jackson]

Invalid User <us**@invalid.d omain> writes:

Is this to be expected?
(python 2.2.2)

+++++++++++++++ +++++++++++++++ =

>>> for x in range(5): .... print x*x
.... else:
.... print "done"
....
0
1
4
9
16
done
+++++++++++++++ +++++++++++++++ =

Yes. The else clause is invoked iff the for loop terminates normally.
Contrast with:

+++++++++++++++ +++++++++++++++

for x in range(5): .... print x*x
.... if x==4:
.... break
.... else:
.... print "did all"
....
0
1
4
9
16

+++++++++++++++ +++++++++++++++

(This happens to be the *reverse* of what my intuition expects - it
would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and
one must simply remember it.)

--
Mark Jackson - http://www.alumni.caltech.edu/~mjackson
There are no foolish questions and no man becomes a fool
until he has stopped asking questions.
- Charles P. Steinmetz

[Alex Martelli]

Invalid User wrote:

While trying to print a none empty list, I accidentaly put an "else"
statement with a "for" instead of "if". Here is what I had:

if ( len(mylist)> 0) :
for x,y in mylist:
print x,y
else:
print "Empty list"

which was supposed to be:

if ( len(mylist)> 0) :
for x,y in mylist:
print x,y
else:
print "Empty list"
Is this to be expected?

Sure. The else branch is executed unless the for terminates
prematurely (via break, return or exception) -- just like the
else branch of a while loop.

Typical use case:

for item in allitems:
if fraboozable(ite m):
print "first fraboozable item is", item
break
else:
print "Sorry, no item is fraboozable"
Alex

[John Roth]

"Mark Jackson" <mj******@alumn i.caltech.edu> wrote in message
news:bk******** **@news.wrc.xer ox.com...

Invalid User <us**@invalid.d omain> writes: (This happens to be the *reverse* of what my intuition expects - it
would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and
one must simply remember it.)
There's a certain amount of justification, though. You can
already put whatever amount of logic you need in the if
statement that ends with the break. You don't otherwise
have that ability for normal termination of the sequence.

Frankly, I'd like to see special case syntax for an empty
sequence, but it's not that hard to handle now.

John Roth

--
Mark Jackson - http://www.alumni.caltech.edu/~mjackson
There are no foolish questions and no man becomes a fool
until he has stopped asking questions.
- Charles P. Steinmetz

[Mark Jackson]

"John Roth" <ne********@jhr othjr.com> writes:

"Mark Jackson" <mj******@alumn i.caltech.edu> wrote in message
news:bk******** **@news.wrc.xer ox.com...

Invalid User <us**@invalid.d omain> writes:

(This happens to be the *reverse* of what my intuition expects - it
would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and
one must simply remember it.)

There's a certain amount of justification, though. You can
already put whatever amount of logic you need in the if
statement that ends with the break. You don't otherwise
have that ability for normal termination of the sequence.

Sure - I wasn't arguing the other meaning was needed, clearly it is
not. It's the use of the keyword "else" which creates the [purely
mental] problem for me.

--
Mark Jackson - http://www.alumni.caltech.edu/~mjackson
There are no foolish questions and no man becomes a fool
until he has stopped asking questions.
- Charles P. Steinmetz

[John Roth]

"Mark Jackson" <mj******@alumn i.caltech.edu> wrote in message
news:bk******** **@news.wrc.xer ox.com...

"John Roth" <ne********@jhr othjr.com> writes:

"Mark Jackson" <mj******@alumn i.caltech.edu> wrote in message
news:bk******** **@news.wrc.xer ox.com...

Invalid User <us**@invalid.d omain> writes:

(This happens to be the *reverse* of what my intuition expects - it
would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and
one must simply remember it.)

There's a certain amount of justification, though. You can
already put whatever amount of logic you need in the if
statement that ends with the break. You don't otherwise
have that ability for normal termination of the sequence.

Sure - I wasn't arguing the other meaning was needed, clearly it is
not. It's the use of the keyword "else" which creates the [purely
mental] problem for me.

Good point.

John Roth
--
Mark Jackson - http://www.alumni.caltech.edu/~mjackson
There are no foolish questions and no man becomes a fool
until he has stopped asking questions.
- Charles P. Steinmetz

[Michael Geary]

Mark Jackson wrote:

(This happens to be the *reverse* of what my intuition expects - it
would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and
one must simply remember it.)

I'm with you there--the use of "else" here has seemed a bit confusing to me.

The use case that Alex just posted gave me another way to think about it:

for item in allitems:
if fraboozable(ite m):
print "first fraboozable item is", item
break
else:
print "Sorry, no item is fraboozable"

If I think of the else statement as being related to the if statement inside
the loop, then it makes some sense--the else statement is executed when the
if statement never comes up true. (It's twisted logic, I know, but at least
it helps me keep track of what "else" does here.)

-Mike

[Aahz]

In article <vn************ @corp.supernews .com>,
Michael Geary <Mi**@DeleteThi s.Geary.com> wrote:

The use case that Alex just posted gave me another way to think about it:

for item in allitems:
if fraboozable(ite m):
print "first fraboozable item is", item
break
else:
print "Sorry, no item is fraboozable"

If I think of the else statement as being related to the if statement
inside the loop, then it makes some sense--the else statement is
executed when the if statement never comes up true. (It's twisted
logic, I know, but at least it helps me keep track of what "else" does
here.)

Maybe it's twisted logic, but it's the only time I've ever used for/else
in real code. It helps if instead of thinking of the else as connected
to the if, you think of the entire for loop as being an extended if
statement.
--
Aahz (aa**@pythoncra ft.com) <*> http://www.pythoncraft.com/

"It is easier to optimize correct code than to correct optimized code."
--Bill Harlan

[Terry Reedy]

"Michael Geary" <Mi**@DeleteThi s.Geary.com> wrote in message
news:vn******** ****@corp.super news.com...

Mark Jackson wrote:

(This happens to be the *reverse* of what my intuition expects - it would seem more natural if the syntax meant "do this for loop to
completion, else do this extra stuff," but this is the way it is and one must simply remember it.)

I'm with you there--the use of "else" here has seemed a bit

confusing to me.

How I plan to remember now: if the abnormal exit is a return or
exception, then all code end of for block, including any else part, to
the end of the function or try block will be skipped, so else part can
only apply to normal exit.

Terry J. Reedy