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

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 <module>
    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

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"

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

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()

Quick hack:

def dowhile(func = None, condition = None):
    if not func or not condition:
        return
    else:
        func()
        while condition():
            func()

Use like so:

>>> x = 10
>>> def f():
...     global x
...     x = x - 1
>>> def c():
        global x
        return x > 0
>>> dowhile(f, c)
>>> print x
0

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

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 > 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.

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

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)

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.

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(">>>", 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.

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.

while condition is True: 
  stuff()
else:
  stuff()

The way I've done this is as follows...

condition = True
while condition:
     do_stuff()
     condition = (<something that evaluates to True or False>)

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.

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.)

Why don't you just do

for s in l :
    print s
print "done"

?