Else clause on Python while statement

Question

I ve noticed the following code is legal in Python  My question is why  Is there a specific reason   n   5 while n    0      print n     n -  1 else      print  what the

User · Answer

The else clause is executed if you exit a block normally  by hitting the loop condition or falling off the bottom of a try block  It is not executed if you break or return out of a block  or raise an exception  It works for not only while and for loops  but also try blocks   You typically find it in places where normally you would exit a loop early  and running off the end of the loop is an unexpected unusual occasion  For example  if you re looping through a list looking for a value   for value in values      if value    5          print  Found it           break else      print  Nowhere to be found   -

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As far as I know the main reason for adding else to loops in any language is in cases when the iterator is not on in your control  Imagine the iterator is on a server and you just give it a signal to fetch the next 100 records of data  You want the loop to go on as long as the length of the data received is 100  If it is less  you need it to go one more times and then end it  There are many other situations where you have no control over the last iteration  Having the option to add an else in these cases makes everything much easier

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Else is executed if while loop did not break   I kinda like to think of it with a  runner  metaphor     The  else  is like crossing the finish line  irrelevant of whether you started at the beginning or end of the track   else  is only not executed if you break somewhere in between   runner at   0   or 10 makes no difference  if unlucky sector is not 0-10 unlucky sector   6 while runner at  lt  10      print  Runner at     runner at      if runner at    unlucky sector          print  Runner fell and broke his foot  Will not reach finish            break     runner at    1 else      print  Runner has finished the race      Not executed if runner broke his foot    Main use cases is using this breaking out of nested loops or if you want to run some statements only if loop didn t break somewhere  think of breaking being an unusual situation    For example  the following is a mechanism on how to break out of an inner loop without using variables or try catch   for i in  1 2 3       for j in   a    unlucky    c            print i  j          if j     unlucky               break     else           continue    Only executed if inner loop didn t break      break           This is only reached if inner loop  breaked  out since continue didn t run    print  Finished     1 a   1 b   Finished

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In reply to Is there a specific reason   here is one interesting application  breaking out of multiple levels of looping   Here is how it works  the outer loop has a break at the end  so it would only be executed once  However  if the inner loop completes  finds no divisor   then it reaches the else statement and the outer break is never reached  This way  a break in the inner loop will break out of both loops  rather than just one   for k in  2  3  5  7  11  13  17  25       for m in range 2  10           if k    m              continue         print  trying  s     s     k  m          if k   m    0              print  found a divisor   d     d  breaking out of loop     k  m              break     else          continue     print  breaking another level of loop      break else      print  no divisor could be found     For both while and for loops  the else statement is executed at the end  unless break was used   In most cases there are better ways to do this  wrapping it into a function or raising an exception   but this works

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The better use of  while  else   construction in Python should be if no loop is executed in  while  then the  else  statement is executed  The way it works today doesn t make sense because you can use the code below with the same results     n   5 while n    0      print n     n -  1 print  what the

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I know this is old question but       As Raymond Hettinger said  it should be called while no break instead of while else  I find it easy to understeand if you look at this snippet   n   5 while n  gt  0      print n     n -  1     if n    2          break if n    0      print n   Now instead of checking condition after while loop we can swap it with else and get rid of that check   n   5 while n  gt  0      print n     n -  1     if n    2          break else     read it as  no break      print n   I always read it as while no break to understand the code and that syntax makes much more sense to me

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The else clause is only executed when the while-condition becomes false   Here are some examples   Example 1  Initially the condition is false  so else-clause is executed   i   99999999  while i  lt  5      print i      i    1 else      print  this     OUTPUT   this   Example 2  The while-condition i  lt  5 never became false because i    3 breaks the loop  so else-clause was not executed   i   0  while i  lt  5      print i      if i    3          break     i    1 else      print  this     OUTPUT   0 1 2 3   Example 3  The while-condition i  lt  5 became  false when i was 5  so else-clause was executed   i   0  while i  lt  5      print i      i    1 else      print  this     OUTPUT   0 1 2 3 4 this

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My answer will focus on WHEN we can use while for-else   At the first glance  it seems there is no different when using   while CONDITION      EXPRESSIONS print  ELSE  print  The next statement    and   while CONDITION      EXPRESSIONS else      print  ELSE  print  The next statement    Because the print  ELSE  statement seems always executed in both cases  both when the while loop finished or not run    Then  it s only different when the statement print  ELSE  will not be executed   It s when there is a breakinside the code block under while   In  17   i   0  In  18   while i  lt  5      print i     if i    2          break     i   i  1 else      print  ELSE  print  The next statement           0 1 2 The next statement   If differ to   In  19   i   0  In  20   while i  lt  5      print i     if i    2          break     i   i  1 print  ELSE  print  The next statement           0 1 2 ELSE The next statement   return is not in this category  because it does the same effect for two above cases   exception raise also does not cause difference  because when it raises  where the next code will be executed is in exception handler  except block   the code in else clause or right after the while clause will not be executed

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The else-clause is executed when the while-condition evaluates to false  From the documentation   The while statement is used for repeated execution as long as an expression is true  while stmt       quot while quot  expression  quot   quot  suite                   quot else quot   quot   quot  suite   This repeatedly tests the expression and  if it is true  executes the first suite  if the expression is false  which may be the first time it is tested  the suite of the else clause  if present  is executed and the loop terminates  A break statement executed in the first suite terminates the loop without executing the else clause   s suite  A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression

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The else  statement is executed when and only when the while loop no longer meets its condition  in your example  when n    0 is false    So the output would be this   5 4 3 2 1 what the

User · Answer

The else clause is only executed when your while condition becomes false   If you break out of the loop  or if an exception is raised  it won t be executed     One way to think about it is as an if else construct with respect to the condition   if condition      handle true   else      handle false     is analogous to the looping construct   while condition      handle true   else        condition is false now  handle and go on with the rest of the program     handle false     An example might be along the lines of   while value  lt  threshold      if not process acceptable value value             something went wrong  exit the loop  don t pass go  don t collect 200         break     value   update value  else        value  gt   threshold  pass go  collect 200     handle threshold reached

[python] Else clause on Python while statement

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