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

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).

Answer 2

I'm a little confused: you have a complicated while expression, and a complicated next expression, but they fit nicely into a C for loop? That doesn't make sense to me.

I recommend the custom iterator approach. You will likely find other uses for the iterator, and encapsulating the iteration is good practice anyway.

UPDATE: Using your example, I would definitely make a custom iterator. It seems perfectly natural to me that a calendar would be able to generate a series of quarterly dates:

class Calendar:
    # ...

    def quarters(self, start, end):
        """Generate the quarter-start dates between `start` and `end`."""
        date = start
        while date < end:
            yield date
            date = self.next_quarter_end(date)


for date in calendar.quarters(start, end):
    if another_calendar.is_holiday(date):
       continue
    # ... do stuff...

This seems like a wonderful abstraction for your calendar class to provide, and I bet you'll use it more than once.

Answer 3

I often do

while True:
   val = <awkward/complicated update>
   if not val:
     break

   etc.

Answer 4

What about:

date = start

while date < end:

    if not another_calendar.is_holiday(date):
        # ... do stuff...

    date = calendar.next_quarter_end(date)

But if you use that particular construct often, you're better off defining the generator once and re-using it as you did in your question.

(The fact is, since they're different languages, you can't possibly have every construct in C map to a more compact construct in Python. It's like claiming to have a compression algorithm that works equally well on all random inputs.)

Answer 5

You could use a try/finally clause to execute the update:

val = START_VAL

while <awkward/complicated test case>:
    try:
        # do stuff
        continue

    finally:
        val = <awkward/complicated update>

Caveat: this will also execute the update statement if you do a break.

Answer 6

Heh:

def forre(a,i,c,top,increment,run):
    increment = increment.replace("++","+=1").replace("--","-=1").replace("; ","")
    while i != top:
        try: exec(run)
        except: print "error: "; print run
        try: exec(increment)
        except: print "error: "; print increment

forre("int i=",0,"; i<",6,"; i++", 
    "print i"
    )