Using an iterator in python?

Question

I have just learned about iterators in Python however I am having a hard time implementing them.

I am trying to write a class to so that this loop works:

  odds = OddNumbers(13)
  for i in odds:
      print(i)

I want to write an iter() function and next() function to do this.

So far I have:

class OddNumbers:

    def __init__(self, number):
        self.number = number

    def __iter__(self):
        return self

    def __next__(self):
        current = self.number
        if self.number%2 == 0:
            return current
        else:
            raise StopIteration

But at the moment this is returning nothing. I expect the output to be

Help?

What are you trying to do? You have only one element. Why do you need this class to be an iterable? — thefourtheye
– thefourtheye, Commented May 14, 2015 at 5:55
If you start with an odd number, then __next__ will only raise StopIteration. If you start with an even number, it will only return that number. Did you mean for __next__ to decrement self.number each time? — Brent Washburne
– Brent Washburne, Commented May 14, 2015 at 5:58
@user2357112 Apologies for not putting that in my original post, I forgot! I have edited it in now though. — ASm
– ASm, Commented May 14, 2015 at 5:58
In your case, __next__ will raise an exception in the first loop, so the iterator stopped, that's why there is nothing print. — WKPlus
– WKPlus, Commented May 14, 2015 at 5:59

augurar · Accepted Answer · 2015-05-14 06:02:16Z

3

Your object needs to keep track of its state and update it when __next__ is called.

class OddNumbers(object):
    def __init__(self, number):
        self.current = ...
        self.number = number

    def __iter__(self):
        return self

    def __next__(self):
        # Update self.current
        # If the limit has been reached, raise StopIteration
        # Otherwise, return something

answered May 14, 2015 at 6:02

augurar

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Comments

Brent Washburne · Accepted Answer · 2015-05-14 06:05:24Z

1

You need another variable to track the current number:

def __init__(self, number):
    self.number = number
    self.current = 1

Then you need to compare it with the ending number, and maybe increment it:

def __next__(self):
    if self.current > self.number:
        raise StopIteration
    current = self.current
    self.current += 2
    return current

answered May 14, 2015 at 6:05

Brent Washburne

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4 Comments

ASm Over a year ago

Thank you very much! This solved my issue! At one stage I did have another variable to track the number of iterations, except I didn't initialize it with the init method, I only introduced it in the next. Thank you again!

augurar Over a year ago

You shouldn't tell the OP the whole answer, this doesn't teach them anything.

Brent Washburne Over a year ago

@augurar, the OP showed effort and posted real code so an answer is the right response. The comment showed he learned from this.

augurar Over a year ago

@BrentWashburne You just told him the answer. So he learned what the correct answer was. Good job, I guess.

sberry · Accepted Answer · 2015-05-14 06:05:43Z

0

There is probably a much cleaner way of doing this, but here is a quick stab:

class OddNumbers:

    def __init__(self, number):
        self.number = number

    def __iter__(self):
        self.current = self.number
        return self

    def __next__(self):
        if self.current > 0:
            if self.current % 2 == 0:
                self.current -= 1
            self.current -= 1
            return self.current + 1
        raise StopIteration

answered May 14, 2015 at 6:05

sberry

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Comments

Jason DeBolt · Accepted Answer · 2017-08-12 00:56:55Z

This will give you an iterator-like object which provides even or odd numbers. However, it won't satisfy your for loop semantics as it is not a true iterator.

class NumberIterator(object):
    """Returns simple even/odd number iterators."""
    def __init__(self, current):
        self.current = current
    def next(self):
        self.current += 2
        return self.current
    @classmethod
    def getOddIterator(cls):
        return cls(-1) # start at 1
    @classmethod
    def getEvenIterator(cls):
        return cls(0) # start at 2


    odd_numbers = NumberIterator.getOddIterator()
    even_numbers = NumberIterator.getEvenIterator()

    odd_numbers.next() # Returns 1
    odd_numbers.next() # Returns 3

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