Simulating C-style for loops in python

Question

(even the title of this is going to cause flames, I realize)

Python made the deliberate design choice to have the for loop use explicit iterables, with the benefit of considerably simplified code in most cases.

However, sometimes it is quite a pain to construct an iterable if your test case and update function are complicated, and so I find myself writing the following while loops:

val = START_VAL while <awkward/complicated test case>:     # do stuff     ...     val = <awkward/complicated update> 

The problem with this is that the update is at the bottom of the while block, meaning that if I want to have a continue embedded somewhere in it I have to:

• use duplicate code for the complicated/awkard update, AND

• run the risk of forgetting it and having my code infinite loop

I could go the route of hand-rolling a complicated iterator:

def complicated_iterator(val):     while <awkward/complicated test case>:          yeild val          val = <awkward/complicated update>  for val in complicated_iterator(start_val):     if <random check>:          continue # no issues here     # do stuff 

This strikes me as waaaaay too verbose and complicated. Do folks in stack overflow have a simpler suggestion?

Response to comments:

@Glenn Maynard: Yes, I dismissed the answer. It's bad to write five lines if there is a way to do it in one... especially in a case that comes up all the time (looping being a common feature of Turing-complete programs).

For the folks looking for a concrete example: let's say I'm working with a custom date library. My question would then be, how would you express this in python:

for (date = start; date < end; date = calendar.next_quarter_end(date)):     if another_calendar.is_holiday(date):        continue     # ... do stuff... 

Answer 1

This is the best I can come up with:

def cfor(first,test,update):     while test(first):         yield first         first = update(first)  def example(blah):     print "do some stuff"     for i in cfor(0,lambda i:i<blah,lambda i:i+1):         print i     print "done" 

I wish python had a syntax for closured expressions.

Edit: Also, note that you only have to define cfor once (as opposed to your complicated_iterator function).