The optional “else” in Python loops

http://shahriar.svbtle.com/pythons-else-clause-in-loops

by s16h • 12 years ago

67 48 12 years ago

shahriar.svbtle.com

Psyonic• 12 years ago

While in theory this is useful, it's one of the least intuitive parts of python for me. I avoid them whenever possible because I feel "else" conveys the intent very poorly. (And I've been writing python for years.)

thatswrong02• 12 years ago

I agree. I use it, but intuitively, I think of else acting the opposite way - if the loop finishes successfully, great! else, do something.

eurleif• 12 years ago

If you're using break to signal success, that is how else acts.

simonw• 12 years ago

I agree. Intuitively, I would expect for item in list: .... else: ... to execute the "else" clause only if the list was empty and hence the for loop didn't iterate at all.

MBlume3• 12 years ago

What finally made it intuitive for me: assuming the loop contains an if condition: break, you can consider the else-clause to be the else of the if statement within the loop.

BWStearns• 12 years ago

That logic definitely helps me think about it. But it does seem like a switch from the other wording that python uses. Feels like finally would be more intuitive, whereas without thinking about the search loop case else feels like something that would be triggered by a break or return. I wonder if using the feature would be more common if it was more intuitively worded.

pyre3• 12 years ago

I think the real issue is that the word 'else' is ambiguous in the context. Everyone comes in with a different idea of what the 'else' is a fallback clause to. Guido obviously picked one particular case, but there are other valid meanings.

For example, I think that it's more common to want to do something like:

  # mnemonic:
  #   for thing in list_of_things DO_SOMETHING else DO_SOMETHING_ELSE

  if list_of_things:
    for thing in list_of_things:
      print thing
  else:
    print "No things!"

Than:

  for thing in list_of_things:
    if thing.is_awesome:
      break
  else:
    print "No awesome things!"

And what about conditional code that you want to execute when a loop does break early?

  error_found = False

  for thing in list_of_things:
    if thing.error_condition:
      error_found = True
      break

  if error_found:
    pass

wting• 12 years ago

Raymond Hettinger has suggested that instead of `else:` the keyword should have been called `nobreak:` which conveys intentions better.

avn2109• 12 years ago

What about using "finally," which if IIRC is already a thing in Python and encapsulates this idea pretty neatly?

BerislavLopac• 12 years ago

Or "unbroken", even... ;-)

enneff• 12 years ago

I totally agree. I wrote Python code professionally for five years and am shocked to find I was wrong about this all this time. I must have introduced quite a few bugs. :-/

blahedo2• 12 years ago

What's more, it dances very close to something that is a super-common error for novice programmers: the early return from a loop. I grade AP exams every year, and one of the hands-down most common conceptual mistakes I've seen (on problems where this is relevant) goes something like this:

    boolean lookForSomething(int parameter) {
      for (Item item: list) {
        if (item.matches(parameter))
          return true;
        else
          return false;
      }
    }

where a correct answer would omit the "else" and put the "return false;" outside the bracketed loop (or, keep a boolean variable updated and then return that after the loop is done. Let's translate that to python:

    def lookForSomething(parameter):
      for item in list:
        if matches(item, parameter):
          return true
        else:
          return false

As a conceptual matter, for a beginner who is still trying to nail down the whole notion of "can stop early when found, but have to scan the whole list if not found", it is just plain nasty that the following code is not only correct but idiomatic:

    def lookForSomething(parameter):
      for item in list:
        if matches(item, parameter):
          return true
      else:
        return false

chrismorgan• 12 years ago

That last one is not idiomatic (I’ll ignore the lowercase first letter of True and False).

The `else` there is superfluous; that cuts it down to this:

    def look_for_something(parameter):
        for item in list:
            if matches(item, parameter):
                return True
        return False

And then after that one should just replace the entire loop with an `any` call:

    def look_for_something(parameter):
        return any(matches(item, parameter) for item in list)

Also, if we assume a `matches()` that is simple equality, then it would just be

    def look_for_something(parameter):
        return parameter in list

… and even then, you shouldn’t have named a variable `list`.

Cut down to its essence like this, the function probably shouldn’t have even existed… ☺

stbarnes• 12 years ago

The 'else' after the 'if' is unnecessary, but so what? We're not running out of letters. IMO including an "else" makes it clearer.

agrover2• 12 years ago

Raymond Hettinger, one of the main Python devs, advises that the "else" in this situation should mentally be thought of as "notfound".

eddyg• 12 years ago

Glad you posted this. I came to the comments to post the same thing[1] because thinking of it as "notfound" makes remembering (and comprehending it) easier. :)

[1] https://www.youtube.com/watch?v=OSGv2VnC0go#t=17m12s

agrover2• 12 years ago

Ah, I misremembered, "nobreak", not "notfound".

a3n• 12 years ago

I think "else" should be pronounced "then" for the for and while loops. And the meaning is "then and only then."

kazinator• 12 years ago

This badly named else is reminiscent of the "result forms" in Lisp loops like `dolist`, `do` or `loop`.

    (loop for x below 10
                finally (princ "iterated over everything, ") (princ "indeed"))

But, of course, these Lisp loops have a result value, and the result forms are an "implicit progn" which produces that value.

gejjaxxita• 12 years ago

I always strongly discourage the use of things like this construct in any language.

A language is meant to be read not just written. A non-expert python programmer who encounters this will be confused.

solox3• 12 years ago

There will always be non-expert programmers of many languages. Avoiding language-specific constructs will not solve the issue.

As far as portability is concerned (if you port software by hand a lot, for example), this is indeed an issue; but if you work with people who write python code, you might as well use everything in your toolbox.

pyre3• 12 years ago

There will always be non-expert programmers of many languages. Avoiding language-specific constructs will not solve the issue.

This is a language-specific issue that occurs rarely in the wild. Yes, it's part of the language, but it's used rarely enough that it can trip up even experienced Python programmers.

For example, Perl (until more recently) allowed one to change arrays to be 1-indexed, rather than 0-indexed at runtime. Just because it's part of the language doesn't mean that it's a good thing to use it.

collyw3• 12 years ago

As someone who did years of Perl (and that's not the most intuitive of languages), I was really confused when I first saw this in Python.

est• 12 years ago

who encounters this will be confused

And get it after 30 seconds doc reading?

andrewchambers• 12 years ago

In my opinion it's just poorly named.

collyw3• 12 years ago

After 2 hours of searching to find the appropriate piece of documentation.

I encountered this situation. I assumed the else belonged to an if, but the indentation rules were somehow being broken. What would you expect me to look up in the docs?

jw2013• 12 years ago

This is all in Jeff Knupp's Writing Idiomatic Python:

http://www.jeffknupp.com/blog/2012/10/04/writing-idiomatic-p...

Many so-called Python Idioms are really non-intuitive and I don't really appreciate.

abuddy• 12 years ago

They are intuitive and very easy to understand and read the code.

If you use Python you should code in Python, not trying to translate C to Python.

mdaniel• 12 years ago

For me, the optional else on for loops isn't nearly as mentally destabilizing as the optional else on try-except blocks. An ex-employee used to love doing that:

    try:
        None.doit()
    except Exception as e:
        logger.exception('Nope')
    else:
        # now what?
        pass

edavis• 12 years ago

try/except/else used to throw me, but I've grown to like the idiom.

All it means is the try clause didn't raise any exceptions. And a "finally" clause is ran whether an exception was raised or not.

gomesnayagam• 12 years ago

it clear to me that this notion is not maintainable at all after looking the comments though it is nice feature to have.

enesunal• 12 years ago

I would choose `finally` keyword over the `else`.

bashwork• 12 years ago

Created an account to point out this gem by Guido himself that makes use of this and the fact that python variables are not scoped inside for loops:

https://github.com/aosabook/500lines/blob/master/crawler/cra...

ludwik• 12 years ago

Oh my God! I would be so confused reading this code!

ddandd• 12 years ago

Not only it is not very intuitive, I also find it to hurt the linearity of code. The code which will be executed after the loop ends depends on how the loop ended. break statement is usually a glorified but mostly harmless go to, but I think Python's go to statement isn't so harmless as it adds complexity but doesn't have big benefits.

But, I do think that the else clause on try\except blocks is less harmful. It is less harmful because the control flow complexity of exceptions handling is already there on the catch clause, so while exceptions does add big control flow changes, the addition of the else clause causes a relative minor change on the linearity of code (as it is already broken by catch: clause.)

mhd• 12 years ago

Also known as "Dijkstra's loop" to give credit where credit is due. Only other language I know that has it though is Oberon (and that just introduced it in a pretty recent update)

parham• 12 years ago

Might be worth while looking at the video below for more Pythonic gems like this one.

https://www.youtube.com/watch?v=OSGv2VnC0go

The optional “else” in Python loops

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