TypeError: iter() returned non-iterator of type 'NoneType'

Question

from collections.abc import Iterable
from collections.abc import Iterator


class MyList(object):
    def __init__(self): self.Container = [11, 22, 33]
    def add(self, item): self.Container.append(item)
    def __iter__(self): return MyIterator


class MyIterator(object):
    def __init__(self): pass
    def __next__(self): pass
    def __iter__(self): pass


mylist = MyList()
mylist_iter = iter(mylist)

print(isinstance(mylist, Iterable))
print(isinstance(mylist, Iterator))
print(isinstance(mylist_iter, Iterable))
print(isinstance(mylist_iter, Iterator))

---

ERRO! Traceback (most recent call last): File "D:\Python\t.py", line 19, in mylist_iter = iter(mylist) TypeError: iter() returned non-iterator of type 'type'

Why & How to solve this problem?

Answer 1

You need parenthesis after MyIterator here in order to instantiate a MyIterator object:

def __iter__(self): return MyIterator()

You were returning a "type" object (MyIterator) instead of an actual instance of the MyIterator class.

A simple example:

# init an actual int type variable
>>> d = int()
>>> type(d)
<class 'int'>  # its type is "int"
>>> d
0

# init what is essentially an alias for the int class
>>> d = int
>>> type(d)
<class 'type'>  # its type is "type"
>>> d
<class 'int'>

# we can even use "d" in place of "int" now to initialize an integer
>>> d(234)
234