How to emulate a do-while loop in Python

Question

I need to emulate a do-while loop in a Python program  Unfortunately  the following straightforward code does not work   list of ints     1  2  3   iterator   list of ints   iter     element   None  while True    if element      print element    try      element   iterator next     except StopIteration      break  print  done    Instead of  1 2 3 done   it prints the following output    stdout  1  stdout  2  stdout  3 None  Traceback  most recent call last         File  test python py   line 8  in  lt module gt      s   i next       StopIteration      What can I do in order to catch the  stop iteration  exception and break a while loop properly   An example of why such a thing may be needed is shown below as pseudocode   State machine   s      while True     if state is STATE CODE       if      in s         tokens add  TOKEN COMMENT  s split         1          state   STATE COMMENT     else         tokens add  TOKEN CODE  s     if state is STATE COMMENT       if      in s         tokens append  TOKEN COMMENT  s split         1        else       state   STATE CODE         Re-evaluate same line       continue   try       s   i next     except StopIteration       break

User · Answer

Exception will break the loop  so you might as well handle it outside the loop   try    while True      if s        print s     s   i next   except StopIteration       pass   I guess that the problem with your code is that behaviour of break inside except is not defined  Generally break goes only one level up  so e g  break inside try goes directly to finally  if it exists  an out of the try  but not out of the loop   Related PEP  http   www python org dev peps pep-3136  Related question  Breaking out of nested loops

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Quick hack   def dowhile func   None  condition   None       if not func or not condition          return     else          func           while condition                func     Use like so    gt  gt  gt  x   10  gt  gt  gt  def f            global x         x   x - 1  gt  gt  gt  def c            global x         return x  gt  0  gt  gt  gt  dowhile f  c   gt  gt  gt  print x 0

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Here s a very simple way to emulate a do-while loop   condition   True while condition        loop body here     condition   test loop condition     end of loop   The key features of a do-while loop are that the loop body always executes at least once  and that the condition is evaluated at the bottom of the loop body   The control structure show here accomplishes both of these with no need for exceptions or break statements   It does introduce one extra Boolean variable

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do     stuff     while  condition      -   while True    stuff     if not condition        break   You can do a function   def do while stuff  condition     while condition stuff         pass   But 1  It s ugly  2  Condition should be a function with one parameter  supposed to be filled by stuff  it s the only reason not to use the classic while loop

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Python 3 8 has the answer  It s called assignment expressions  from the documentation    Loop over fixed length blocks while  block    f read 256              process block

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while condition is True     stuff   else    stuff

User · Answer

My code below might be a useful implementation  highlighting the main difference between do-while vs while as I understand it    So in this one case  you always go through the loop at least once   first pass   True while first pass or condition      first pass   False     do stuff

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The built-in iter function does specifically that  for x in iter YOUR FN  TERM VAL            E g   tested in Py2 and 3   class Easy    X   0    classmethod   def com cls       cls X    1     return cls X  for x in iter Easy com  10     print  quot  gt  gt  gt  quot   x   If you want to give a condition to terminate instead of a value  you always can set an equality  and require that equality to be True

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For me a typical while loop will be something like this   xBool   True   A counter to force a condition  eg  yCount   some integer value   while xBool        set up the condition  eg  if yCount  gt  0            Do something          yCount   yCount - 1     else             condition is not met  set xBool False          xBool   False   I could include a for  loop within the while loop as well  if situation so warrants  for looping through another set of condition

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Here is a crazier solution of a different pattern -- using coroutines  The code is still very similar  but with one important difference  there are no exit conditions at all  The coroutine  chain of coroutines really  just stops when you stop feeding it with data   def coroutine func          Coroutine decorator      Coroutines must be started  advanced to their first  yield  point      and this decorator does this automatically              def startcr  ar    kw           cr   func  ar    kw          cr next           return cr     return startcr   coroutine def collector storage          Act as  sink  and collect all sent in  storage        while True          storage append  yield     coroutine       def state machine sink            send   new parts to be tokenized by the state machine      tokens are passed on to  sink              s          state   STATE CODE     while True           if state is STATE CODE               if      in s                   sink send  TOKEN COMMENT  s split         1                     state   STATE COMMENT             else                   sink send   TOKEN CODE  s            if state is STATE COMMENT               if      in s                   sink send   TOKEN COMMENT  s split         1                 else                 state   STATE CODE                   re-evaluate same line                 continue         s    yield   tokens      sm   state machine collector tokens   for piece in i      sm send piece    The code above collects all tokens as tuples in tokens and I assume there is no difference between  append   and  add   in the original code

User · Answer

I am not sure what you are trying to do  You can implement a do-while loop like this   while True    stuff     if fail condition      break   Or   stuff   while not fail condition    stuff     What are you doing trying to use a do while loop to print the stuff in the list  Why not just use   for i in l    print i print  done    Update   So do you have a list of lines  And you want to keep iterating through it  How about    for s in l     while True       stuff          use a  break  instead of s   i next     Does that seem like something close to what you would want  With your code example  it would be   for s in some list    while True      if state is STATE CODE        if      in s          tokens add  TOKEN COMMENT  s split         1            state   STATE COMMENT       else           tokens add  TOKEN CODE  s       if state is STATE COMMENT        if      in s          tokens append  TOKEN COMMENT  s split         1            break   get next s       else          state   STATE CODE           re-evaluate same line           continues automatically

User · Answer

The way I ve done this is as follows     condition   True while condition       do stuff        condition     lt something that evaluates to True or False gt     This seems to me to be the simplistic solution  I m surprised I haven t seen it here already  This can obviously also be inverted to  while not condition    etc

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for a do - while loop containing try statements  loop   True while loop      generic stuff       try          questionable stuff           to break from successful completion         loop   False       except          optional stuff           to break from unsuccessful completion -          the case referenced in the OP s question         loop   False    finally          more generic stuff     alternatively  when there s no need for the  finally  clause  while True      generic stuff       try          questionable stuff           to break from successful completion         break       except          optional stuff           to break from unsuccessful completion -          the case referenced in the OP s question         break

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Why don t you just do  for s in l       print s print  done

User · Answer

If you re in a scenario where you are looping while a resource is unavaliable or something similar that throws an exception  you could use something like  import time  while True      try         f   open  some path    r       except IOError         print  File could not be read  Retrying in 5 seconds             time sleep 5      else         break

User · Answer

See if this helps    Set a flag inside the exception handler and check it before working on the s   flagBreak   false  while True        if flagBreak   break      if s           print s     try           s   i next       except StopIteration           flagBreak   true  print  done

[python] How to emulate a do-while loop in Python?

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