Why does python use else after for and while loops

Question

I understand how this construct works   for i in range 10       print i       if i    9          print  Too big - I m giving up            break  else      print  Completed successfully     But I don t understand why else is used as the keyword here  since it suggests the code in question only runs if the for block does not complete  which is the opposite of what it does  No matter how I think about it  my brain can t progress seamlessly from the for statement to the else block  To me  continue or continuewith would make more sense  and I m trying to train myself to read it as such    I m wondering how Python coders read this construct in their head  or aloud  if you like   Perhaps I m missing something that would make such code blocks more easily decipherable

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Since the technical part has been pretty much answered, my comment is just in relation with the confusion that produce this recycled keyword.

Being Python a very eloquent programming language, the misuse of a keyword is more notorious. The else keyword perfectly describes part of the flow of a decision tree, "if you can't do this, (else) do that". It's implied in our own language.

Instead, using this keyword with while and for statements creates confusion. The reason, our career as programmers has taught us that the else statement resides within a decision tree; its logical scope, a wrapper that conditionally return a path to follow. Meanwhile, loop statements have a figurative explicit goal to reach something. The goal is met after continuous iterations of a process.

if / else indicate a path to follow. Loops follow a path until the "goal" is completed.

The issue is that else is a word that clearly define the last option in a condition. The semantics of the word are both shared by Python and Human Language. But the else word in Human Language is never used to indicate the actions someone or something will take after something is completed. It will be used if, in the process of completing it, an issue rises (more like a break statement).

At the end, the keyword will remain in Python. It's clear it was mistake, clearer when every programmer tries to come up with a story to understand its usage like some mnemonic device. I'd have loved if they have chosen instead the keyword then. I believe that this keyword fits perfectly in that iterative flow, the payoff after the loop.

It resembles that situation that some child has after following every step in assembling a toy: And THEN what Dad?

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I read it like  When the iterable is exhausted completely  and the execution is about to proceed to the next statement after finishing the for  the else clause will be executed   Thus  when the iteration is broken by break  this will not be executed

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There s an excellent presentation by Raymond Hettinger  titled Transforming Code into Beautiful  Idiomatic Python  in which he briefly addresses the history of the for     else construct  The relevant section is  Distinguishing multiple exit points in loops  starting at 15 50 and continuing for about three minutes  Here are the high points    The for     else construct was devised by Donald Knuth as a replacement for certain GOTO use cases  Reusing the else keyword made sense because  it s what Knuth used  and people knew  at that time  all  for statements  had embedded an if and GOTO underneath  and they expected the else   In hindsight  it should have been called  no break   or possibly  nobreak    and then it wouldn t be confusing     So  if the question is   Why don t they change this keyword   then Cat Plus Plus probably gave the most accurate answer     at this point  it would be too destructive to existing code to be practical  But if the question you re really asking is why else was reused in the first place  well  apparently it seemed like a good idea at the time   Personally  I like the compromise of commenting   no break in-line wherever the else could be mistaken  at a glance  as belonging inside the loop  It s reasonably clear and concise  This option gets a brief mention in the summary that Bjorn linked at the end of his answer      For completeness  I should mention that with a slight change in   syntax  programmers who want this syntax can have it right now   for item in sequence      process item  else     no break     suite        Bonus quote from that part of the video   Just like if we called lambda makefunction  nobody would ask   What does lambda do

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for i in range 3       print i       if i    2          print  Too big - I m giving up            break  else      print  Completed successfully      else  here is crazily simple  just mean     1   if for clause is completed   for i in range 3       print i       if i    2          print  Too big - I m giving up            break  if  for clause is completed       print  Completed successfully     It s wielding to write such long statements as  for clause is completed   so they introduce  else    else here is a if in its nature   2  However  How about for clause is not run at all  In  331   for i in range 0                 print i                           if i    9                    print  Too big - I m giving up                      break           else                print  Completed successfully                  Completed successfully   So it s completely statement is logic combination   if  for clause is completed  or  not run at all        do else stuff   or put it this way   if  for clause is not partially run       do else stuff   or this way   if  for clause not encounter a break       do else stuff

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I was just trying to make sense of it again myself  I found that the following helps         Think of the else as being paired with the if inside the loop  instead of with the for  - if condition is met then break the loop  else do this - except it s one else paired with multiple ifs       If no ifs were satisfied at all  then do the else       The multiple ifs can also actually be thought of as if-elifs

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I read it something like   If still on the conditions to run the loop  do stuff  else do something else

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I think documentation has a great explanation of  else  continue           it is executed when the loop terminates through exhaustion of the list  with for  or when the condition becomes false  with while   but not when the loop is terminated by a break statement     Source  Python 2 docs  Tutorial on control flow

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To make it simple  you can think of it like that    If it encounters the break command in the for loop  the else part will not be called  If it does not encounter the break command in the for loop  the else part will be called    In other words  if for loop iteration is not  broken  with break  the else part will be called

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Here s another idiomatic use case besides searching  Let s say you wanted to wait for a condition to be true  e g  a port to be open on a remote server  along with some timeout  Then you could utilize a while   else construct like so   import socket import time  sock   socket socket   timeout   time time     15 while time time    lt  timeout      if sock connect ex   127 0 0 1   80   is 0          print  Port is open now            break     print  Still waiting      else      raise TimeoutError

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It s a strange construct even to seasoned Python coders  When used in conjunction with for-loops it basically means  find some item in the iterable  else if none was found do       As in   found obj   None for obj in objects      if obj key    search key          found obj   obj         break else      print  No object found      But anytime you see this construct  a better alternative is to either encapsulate the search in a function   def find obj search key       for obj in objects          if obj key    search key              return obj   Or use a list comprehension   matching objs    o for o in objects if o key    search key  if matching objs      print  Found     format matching objs 0    else      print  No object found      It is not semantically equivalent to the other two versions  but works good enough in non-performance critical code where it doesn t matter whether you iterate the whole list or not  Others may disagree  but I personally would avoid ever using the for-else or while-else blocks in production code    See also  Python-ideas  Summary of for   else threads

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Great answers are    this which explain the history  and this gives the right citation to ease yours translation understanding    My note here comes from what Donald Knuth once said  sorry can t find reference  that there is a construct where while-else is indistinguishable from if-else  namely  in Python    x   2 while x  gt  3      print  foo       break else      print  boo     has the same flow  excluding low level differences  as   x   2 if x  gt  3      print  foo   else      print  boo     The point is that if-else can be considered as syntactic sugar for while-else which has implicit break at the end of its if block  The opposite implication  that while loop is extension to if  is more common  it s just repeated looped conditional check   because if is often taught before while  However that isn t true because that would mean else block in while-else would be executed each time when condition is false   To ease your understanding think of it that way      Without break  return  etc   loop ends only when condition is no longer true and in such case else block will also execute once  In case of Python for you must consider C-style for loops  with conditions  or translate them to while    Another note      Premature break  return  etc  inside loop makes impossible for condition to become false because execution jumped out of the loop while condition was true and it would never come back to check it again

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The easiest way I found to  get  what the for else did  and more importantly  when to use it  was to concentrate on where the break statement jumps to  The For else construct is a single block  The break jumps out of the block  and so jumps  over  the else clause  If the contents of the else clause simply followed the for clause  it would never be jumped over  and so the equivalent logic would have to be provided by putting it in an if  This has been said before  but not quite in these words  so it may help somebody else  Try running the following code fragment  I m wholeheartedly in favour of the  no break  comment for clarity   for a in range 3       print a      if a  4    change value to force break or not         break else   no break   10 for whoever thought of this decoration     print  for completed OK    print  statement after for loop

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A common construct is to run a loop until something is found and then to break out of the loop  The problem is that if I break out of the loop or the loop ends I need to determine which case happened  One method is to create a flag or store variable that will let me do a second test to see how the loop was exited   For example assume that I need to search through a list and process each item until a flag item is found and then stop processing  If the flag item is missing then an exception needs to be raised   Using the Python for   else construct you have  for i in mylist      if i    theflag          break     process i  else      raise ValueError  List argument missing terminal flag      Compare this to a method that does not use this syntactic sugar   flagfound   False for i in mylist      if i    theflag          flagfound   True         break     process i   if not flagfound      raise ValueError  List argument missing terminal flag      In the first case the raise is bound tightly to the for loop it works with  In the second the binding is not as strong and errors may be introduced during maintenance

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Because they didn t want to introduce a new keyword to the language  Each one steals an identifier and causes backwards compatibility problems  so it s usually a last resort

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You could think of it like  else as in the rest of the stuff  or the other stuff  that wasn t done in the loop

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Here s a way to think about it that I haven t seen anyone else mention above   First  remember that for-loops are basically just syntactic sugar around while-loops  For example  the loop  for item in sequence      do something item    can be rewritten  approximately  as  item   None while sequence hasnext        item   sequence next       do something item    Second  remember that while-loops are basically just repeated if-blocks  You can always read a while-loop as  if this condition is true  execute the body  then come back and check again    So while else makes perfect sense  It s the exact same structure as if else  with the added functionality of looping until the condition becomes false instead of just checking the condition once   And then for else makes perfect sense too  because all for-loops are just syntactic sugar on top of while-loops  you just need to figure out what the underlying while-loop s implicit conditional is  and then the else corresponds to when that condition becomes False

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Codes in else statement block will be executed when the for loop was not be broke  for x in xrange 1 5       if x    5          print  find 5          break else      print  can not find 5    can not find 5   From the docs   break and continue Statements  and else Clauses on Loops  Loop statements may have an else clause  it is executed when the loop terminates through exhaustion of the list  with for  or when the condition becomes false  with while   but not when the loop is terminated by a break statement  This is exemplified by the following loop  which searches for prime numbers   gt  gt  gt  for n in range 2  10           for x in range 2  n               if n   x    0                  print n   equals   x       n  x                  break         else                loop fell through without finding a factor             print n   is a prime number       2 is a prime number 3 is a prime number 4 equals 2   2 5 is a prime number 6 equals 2   3 7 is a prime number 8 equals 2   4 9 equals 3   3   Yes  this is the correct code  Look closely  the else clause belongs to the for loop  not the if statement   When used with a loop  the else clause has more in common with the else clause of a try statement than it does that of if statements  a try statement   s else clause runs when no exception occurs  and a loop   s else clause runs when no break occurs  For more on the try statement and exceptions  see Handling Exceptions  The continue statement  also borrowed from C  continues with the next iteration of the loop   gt  gt  gt  for num in range 2  10           if num   2    0              print  quot Found an even number quot   num              continue         print  quot Found a number quot   num  Found an even number 2 Found a number 3 Found an even number 4 Found a number 5 Found an even number 6 Found a number 7 Found an even number 8 Found a number 9

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I consider the structure as for  if  A else B  and for if -else is a special if-else  roughly  It may help to understand else   A and B is executed at most once  which is the same as if-else structure   for if  can be considered as a special if  which does a loop to try to meet the if condition  Once the if condition is met  A and break  Else  B

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I agree  it s more like an  elif not  condition s  raising break     I know this is an old thread  but I am looking into the same question right now   and I m not sure anyone has captured the answer to this question in the way I understand it   For me  there are three ways of  reading  the else in For    else or While    else statements  all of which are equivalent  are    else    if the loop completes normally  without a break or error  else    if the loop does not encounter a break else    else not  condition raising break   presumably there is such a condition  or you wouldn t have a loop    So  essentially  the  else  in a loop is really an  elif      where       is  1  no break  which is equivalent to  2  NOT  condition s  raising break    I think the key is that the else is pointless without the  break   so a for   else includes   for      do stuff     conditional break   implied by else else not break      do more stuff   So  essential elements of a for   else loop are as follows  and you would read them in plainer English as   for      do stuff     condition          break else    read as  else not break  or  else not condition      do more stuff   As the other posters have said  a break is generally raised when you are able to locate what your loop is looking for  so the else  becomes  what to do if target item not located    Example  You can also use exception handling  breaks  and for loops all together   for x in range 0 3       print  x      format x       if x    2          try              raise AssertionError  ASSERTION ERROR  x is     format x           except              print AssertionError  ASSERTION ERROR  x is     format x                break else      print  X loop complete without error     Result  x  0 x  1 x  2 ASSERTION ERROR  x is 2 ----------   loop not completed  hit break   so else didn t run   Example  Simple example with a break being hit   for y in range 0 3       print  y      format y       if y    2    will be executed         print  BREAK  y is    n----------  format y           break else    not executed because break is hit     print  y loop completed without break---------- n     Result  y  0 y  1 y  2 BREAK  y is 2 ----------   loop not completed  hit break   so else didn t run   Example  Simple example where there no break  no condition raising a break  and no error are encountered   for z in range 0 3        print  z      format z        if z    4    will not be executed          print  BREAK  z is    n  format y            break      if z    4    will not be executed          raise AssertionError  ASSERTION ERROR  x is     format x   else       print  z loop complete without break or error n---------- n     Result  z  0 z  1 z  2 z loop complete without break or error ----------

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The else keyword can be confusing here  and as many people have pointed out  something like nobreak  notbreak is more appropriate   In order to understand for     else     logically  compare it with try   except   else  not if   else     most of python programmers are familiar with the following code   try      do something   except      print  Error happened      The try block threw an exception else      print  Everything is find      The try block does things just find    Similarly  think of break as a special kind of Exception   for x in iterable      do something x  except break      pass   Implied by Python s loop semantics else      print  no break encountered      No break statement was encountered   The difference is python implies except break and you can not write it out  so it becomes   for x in iterable      do something x  else      print  no break encountered      No break statement was encountered   Yes  I know this comparison can be difficult and tiresome  but it does clarify the confusion

[python] Why does python use 'else' after for and while loops?

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