How to use iter and next for iteration in python?

Question

I am trying to learn how iterators, generators and decorators works in python based on the tutorials in this website.

In the first example, he/she demonstrate a simple example as below:

class Count:
    def __init__(self, low, high):
        self.low = low
        self.high = high

    def __iter__(self):
        return self

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            self.current +=1
            return self.current -1

The problem is, I can't iterate over objects of this class:

>>> ================================ RESTART ================================
>>> 
>>> c = Count(1, 10)
>>> for i in c:
    print i



Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    for i in c:
TypeError: instance has no next() method
>>>

What's wrong?

vaultah · Accepted Answer · 2015-09-12 04:45:25Z

4

That tutorial appears to be for Python 3.

In Python 2 iterators must have the next() method (PEP 3114 - Renaming iterator.next() to iterator.__next__()):

class Count:
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        return self

    def next(self):
        if self.current > self.high:
            raise StopIteration
        else:
            self.current +=1
            return self.current -1

edited Sep 12, 2015 at 4:45

answered Sep 12, 2015 at 4:34

vaultah

46.9k12 gold badges120 silver badges145 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Ebrahim Ghasemi Over a year ago

Thank you. Anyway, why I don't receive Undefined Variable error for self.current? what is its value when the line self.current +=1 run for first time?

vaultah Over a year ago

@Abraham I think it should be self.current = low

BrenBarn Over a year ago

@Abraham: You will indeed get an error. You should initialize self.current in __init__.

wwii Over a year ago

@Abraham - what do you get if you print self.current as the first statement of next()?

Blckknght Over a year ago

An alternative solution to renaming __next__ to next in your function would be to switch Python interpreters so that you're using Python 3 like your tutorial expects. Many aspects of Python 3 are better than Python 2, most notably its handling of Unicode text. There are some modules that are only available for Python 2, but that number is diminishing steadily, so unless you're sure you need one of them already, I'd suggest learning Python 3 first, rather than learning Python 2 and then learning how to port your code to Python 3 later on.

|

Collectives™ on Stack Overflow

How to use iter and next for iteration in python?

1 Answer 1

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related