How to stop iteration with __iter__() ?

[Terry Reedy]

ssecorp wrote:
> I want a parse a file of the format:
> movieId
> customerid, grade, date
> customerid, grade, date
> customerid, grade, date
> etc.
> 
> so I could do with open file as reviews and then for line in reviews.
> 
> but first I want to take out the movie id so I use an iterator.
> 
> then i want to iterate through all the rows, but how can I do:
> while movie_iter != None:
> 
> because that doesn't work, itraises an exception, StopItreation, which
> according to the documentation it should. But catching an exception
> can't be the standard way to stop iterating right?

Catching StopIteration *is* the usual way to stop iterating with an 
iterator.  Using while, one must be explicit:

it = iter(iterable)
try:
   while True:
     item = next(iter) # 2.6,3.0
     f(item)
except StopIteration:
   pass

but the main reason to write the above is to show the advantage of 
simply writing the equivalent (and implicit)

for item in iterable:
   f(item)

;-)

In your case, the standard Python idiom, as Jon said, is

it = iter(iterable)
next(it) # 2.6, 3.0
for for item in iterable:
   f(item)

The alternative is a flag variable and test

first = True
for for item in iterable:
   if first:
     first = False
   else:
     f(item)

This takes two more lines and does an unnecessary test for every line 
after the first.  But this approach might be useful if, for instance, 
you needed to skip every other line (put 'first = True' after f(item)).

Terry Jan Reedy