5 ms·

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

by Psyonic 12y 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.)

thatswrong0 12y 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 12y ago
  If you're using break to signal success, that is how else acts.
  - thatswrong0 12y ago
    I've been using break to signal failure. I'll keep that in mind.
  - maxerickson 12y ago
    Just to illustrate: >>> for item in (1,2): print(item) else: print('Done!') 1 2 Done! >>> for item in (1,2): print(item) break else: print('Done!') 1 >>> edit: also: >>> for item in (): print(item) else: print('Done') Done
- deleted 12y ago
  [deleted]
simonw 12y 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.
MBlume 12y 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 12y 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.
  - maxerickson 12y ago
    It's analogous to the natural termination of a while-else, where the else block is executed after the condition evaluates to false: >>> a=1 >>> while a>0: print(4) a=a-1 else: print('done') 4 done clarity cribbed from here: https://mail.python.org/pipermail/python-list/1999-July/004457.html https://mail.python.org/pipermail/python-list/1999-July/0044...
    - MBlume 12y ago
      Oh, that helps too -- it's an else clause for the conditional of the while loop :D
  - MBlume 12y ago
    finally seems much more confusing to me -- the point of finally is that it will definitely be executed...
- pyre 12y 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 12y ago
Raymond Hettinger has suggested that instead of `else:` the keyword should have been called `nobreak:` which conveys intentions better.
- avn2109 12y ago
  What about using "finally," which if IIRC is already a thing in Python and encapsulates this idea pretty neatly?
  - dbarlett 12y ago
    finally blocks are always executed; this kind of else block is only executed if the for block exited normally (i.e. did not throw an exception).
    - aperture 12y ago
      How about a keyword such as "after"? I feel like that could have extensible potential in other areas of the language, and the name makes a little sense logically speaking.
      - gingerrr 12y ago
        or something like "then"?
- BerislavLopac 12y ago
  Or "unbroken", even... ;-)
enneff 12y 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. :-/
blahedo 12y 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 12y 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… ☺
  - blahedo 12y ago
    Remember that I was talking about novice programmers, here. I'll concede that "idiomatic" was a bit strong (and the True/False thing was a brain fart), but I think my main point stands: encouraging the use of an "else:" that can attach to loops, with an unobvious and somewhat nuanced semantics, is not kind to newcomers. And the fact remains that the difference between a correct implementation and one that is wrong in exactly the way that a lot of newcomers get it wrong is nothing but indentation.

by Psyonic 12y ago

thatswrong0 12y 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 12y ago
If you're using break to signal success, that is how else acts.
- thatswrong0 12y ago
  I've been using break to signal failure. I'll keep that in mind.
- maxerickson 12y ago
  Just to illustrate: >>> for item in (1,2): print(item) else: print('Done!') 1 2 Done! >>> for item in (1,2): print(item) break else: print('Done!') 1 >>> edit: also: >>> for item in (): print(item) else: print('Done') Done
deleted 12y ago
[deleted]

simonw 12y 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.

MBlume 12y 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 12y 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.
- maxerickson 12y ago
  It's analogous to the natural termination of a while-else, where the else block is executed after the condition evaluates to false: >>> a=1 >>> while a>0: print(4) a=a-1 else: print('done') 4 done clarity cribbed from here: https://mail.python.org/pipermail/python-list/1999-July/004457.html https://mail.python.org/pipermail/python-list/1999-July/0044...
  - MBlume 12y ago
    Oh, that helps too -- it's an else clause for the conditional of the while loop :D
- MBlume 12y ago
  finally seems much more confusing to me -- the point of finally is that it will definitely be executed...
pyre 12y 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 12y ago

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

avn2109 12y ago
What about using "finally," which if IIRC is already a thing in Python and encapsulates this idea pretty neatly?
- dbarlett 12y ago
  finally blocks are always executed; this kind of else block is only executed if the for block exited normally (i.e. did not throw an exception).
  - aperture 12y ago
    How about a keyword such as "after"? I feel like that could have extensible potential in other areas of the language, and the name makes a little sense logically speaking.
    - gingerrr 12y ago
      or something like "then"?
BerislavLopac 12y ago
Or "unbroken", even... ;-)

enneff 12y 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. :-/

blahedo 12y 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 12y 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… ☺
- blahedo 12y ago
  Remember that I was talking about novice programmers, here. I'll concede that "idiomatic" was a bit strong (and the True/False thing was a brain fart), but I think my main point stands: encouraging the use of an "else:" that can attach to loops, with an unobvious and somewhat nuanced semantics, is not kind to newcomers. And the fact remains that the difference between a correct implementation and one that is wrong in exactly the way that a lot of newcomers get it wrong is nothing but indentation.