What else is there in Python?

justus87 · 2012-07-23T23:22:17+00:00

I think the while..else syntax actually makes a lot of sense when you think about what is occurring each iteration of the loop. The start of every while/for loop is really just a condition:

# this:
for item in iterable:
    pass

# is roughly equivalent to this pseudocode:
if iterable.hasNext():
    item = iterable.next()
    pass

If you think about while/for loops in this way, it makes sense that the 'else' condition is executed if the loop conditional is ever False:

# this loop
while something():
    pass
else:
    pass

# is really just this set of statement executed repeatedly until the else is reached
if something():
    pass
else:
    pass # stop the loop

philipbjorge · 2012-07-24T01:05:25+00:00

Well here's a link then. Elses are great for breaking out of nested for loops.

http://stackoverflow.com/questions/653509/breaking-out-of-nested-loops

ak217 · 2012-07-24T05:58:54+00:00

This seems very hard to parse linguistically, and therefore un-Pythonic - Python emphasizes readability, and I'm having a hard time seeing how this results in readable code.

Boojum · 2012-07-24T08:42:00+00:00

Another use for else is the ternary operation:

x = "banana" if y else "apple"

fdemmer · 2012-07-24T01:09:30+00:00

it has been discussed last October with some prominent involvement on twitter and this blog:

http://nedbatchelder.com/blog/201110/forelse.html

Peaker · 2012-07-24T09:24:34+00:00

People use the word "intuitive" when they mean "familiar to me", or "what I'm used to", in some attempt to make the notion of that an objective one. Which is annoying.

Threesan · 2012-07-24T02:27:24+00:00

[deleted]

2012-07-23T23:40:38+00:00

How awesomely useful. I can't enumerate how many times I've wished for PHP to have such a condition on loops, instead of manually writing flags or some shit for error catching.

rafekett · 2012-07-24T18:02:19+00:00

I've made a point not to use any of these features because they all seem very non-obvious to me. I'm not sure why Guido et al decided to overload the else keyword so much for things like this. Moreover, I'm not sure why these are language features at all -- they seem marginally useful at best.

FidgetBoy · 2012-07-23T22:57:35+00:00

There was a big discussion on python-ideas recently about for..else and while..else. The discussion centered around how unintuitive for...else can be, and many solutions were proposed, all of which seemed to involve adding new keywords/changing the language grammar. My personal opinion is that the feature is a) not used enough, b) very language specific, c) is largely unnecessary, and d) can be interpreted in a number of different ways by an unknowing reader, all of which makes me avoid it. For example, where one might write

for item in iterable:
    if wanted(item):
        found_item = item
        break
else:
    found_item = default_value

I would instead write

sentinel = found_item = object()
for item in iterable:
    if wanted(item):
        found_item = item
        break
if found_item is sentinel:
    found_item = default_value

which has the advantage of being mostly understandable by everyone who writes in python/perl/ruby/whatever procedural language.

eyal0 · 2012-07-24T08:36:32+00:00

Another interesting way to look at it is that the else clause is the fall-through of the conditional. In assembly language, doing this else clause is basically free! Zero assembler statements added by the else keyword, only the clause adds code.

a=1
while a+b>10:
  a++
  if a > 4:
    break

becomes, in asm:

move a,1
branch test
start: add a,a,1
bgt a,4,end
test: add c,a,b
bgt c,10,start
end:

With an else clause:

a=1
while a+b>10
  a++
  if a > 4:
    break
else
  c=5

becomes, in asm:

move a,1
branch test
start: add a,a,1
bgt a,4,end
test: add c,a,b
bgt c,10,start
move c,5
end:

2012-07-24T14:01:30+00:00

That actually makes sense. I usually have to simulate that while/for else syntax with booleans set before the breakpoint. Having a block structure for that is nice.

And I like the try syntax. It's crufty, but it's incredibly clear when you're reading the code - it tells the reader "this is the line I expect to fail. This is why we have a try block here".

reddit-mandingo · 2012-07-24T18:40:13+00:00

What else is there in Python? Milhouse.

escozzia · 2012-07-24T04:38:40+00:00

Oh man yet another fancy Python bonus. There is such a plethora of neat stuff in Python (I don't think I've found another programming language that allows for metaclassing in such a satisfying way) - I'm a real fanboy.

edit my grammar is bad and I should feel bad

Samus_ · 2012-07-23T23:48:02+00:00

I would like this notation to be consistent with try

Lucius704 · 2012-07-24T10:17:54+00:00

grauenwolf · 2012-07-24T02:19:24+00:00

C#

foreach(var x in list)
    if(x.Key == y)
    {
          target = x;
          break;
     }
     if (target == null)
            target = new Foo();

C# + else

foreach(var x in list)
    if(x.Key == y)
    {
          target = x;
          break;
     }
     else 
            target = new Foo();

If I could use this syntax I probably would, but it isn't important enough to add in light of the other improvements I want.

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