Is generator.next() visible in Python 3?

Question

I have a generator that generates a series, for example:

def triangle_nums():
    '''Generates a series of triangle numbers'''
    tn = 0
    counter = 1
    while True:
        tn += counter
        yield tn
        counter += + 1

In Python 2 I am able to make the following calls:

g = triangle_nums()  # get the generator
g.next()             # get the next value

however in Python 3 if I execute the same two lines of code I get the following error:

AttributeError: 'generator' object has no attribute 'next'

but, the loop iterator syntax does work in Python 3

for n in triangle_nums():
    if not exit_cond:
       do_something()...

I haven't been able to find anything yet that explains this difference in behavior for Python 3.

score 472 · Accepted Answer · edited Jun 18 '20 at 01:44

472

g.next() has been renamed to g.__next__(). The reason for this is consistency: special methods like __init__() and __del__() all have double underscores (or "dunder" in the current vernacular), and .next() was one of the few exceptions to that rule. This was fixed in Python 3.0. [*]

But instead of calling g.__next__(), use next(g).

[*] There are other special attributes that have gotten this fix; func_name, is now __name__, etc.

edited Jun 18 '20 at 01:44

Boris Verkhovskiy

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answered Jul 02 '09 at 10:15

Lennart Regebro

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any idea why python 2 eschewed the dunder convention for these methods in the first place? – Rick Mar 29 '16 at 14:47
That's probably just an oversight. – Lennart Regebro Mar 31 '16 at 11:49
What about when you overwrite __ str __ in classes? Does it change str(obj) or __str__(obj) ? – NoName Oct 27 '19 at 04:00
@NoName There is no such thing as `__str__(obj)`, so I don't really understand the question. – Lennart Regebro Oct 29 '19 at 18:52
@LennartRegebro In `instance1`'s class, I modify `__str__(self)`, in main function, I call `str(instance1)`. – NoName Oct 29 '19 at 22:21
1

@NoName Yes, you do. – Lennart Regebro Oct 30 '19 at 09:56
Thank you. It would be nice if this change in syntax was documented in PEP 289, where generator expressions are proposed. – Matthew Milone Sep 12 '20 at 18:23
1

@MatthewMilone It's mentioned in the PEP where the change was proposed, which was five years later. https://www.python.org/dev/peps/pep-3114/ – Lennart Regebro Oct 08 '20 at 16:56

score 157 · Answer 2 · edited Jul 27 '13 at 03:52

157

Try:

next(g)

Check out this neat table that shows the differences in syntax between 2 and 3 when it comes to this.

edited Jul 27 '13 at 03:52

Lennart Regebro

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answered Jul 02 '09 at 09:31

Paolo Bergantino

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@MaikuMori I fixed the link (waiting for peer revision) (The site http://diveintopython3.org seems to be down. Mirror site http://diveintopython3.ep.io is still alive) – gecco Jan 05 '12 at 20:59
1

Fixed the link again. http://python3porting.com/differences.html is more complete, btw. – Lennart Regebro Jul 27 '13 at 03:53
Is there any justification for the switch from a method to a function, beyond `g.next()` should really be `g.__next__()`, and we need to have something that isn't a dunder method with the functionality of `g.next()`? – T.C. Proctor Dec 17 '19 at 23:25
Works for the error ` 'generator' object has no attribute 'next' `. – Yugendran Apr 13 '21 at 11:51

score 12 · Answer 3 · answered Sep 17 '15 at 17:09

12

If your code must run under Python2 and Python3, use the 2to3 six library like this:

import six

six.next(g)  # on PY2K: 'g.next()' and onPY3K: 'next(g)'

answered Sep 17 '15 at 17:09

danius

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There's not much need for this unless you need to support Python versions earlier than 2.6. Python 2.6 and 2.7 have the `next` built-in function. – Mark Dickinson Sep 17 '15 at 17:15

Is generator.next() visible in Python 3?

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