Python __next__() Magic Method

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Syntax

object.__next__(self)

The Python __next__ method returns an arbitrary element that represents the “next” element when you iterate over the object on which it is called. For example, if you iterate over my_object using for x in my_object, Python internally calls my_object.__next__() in each loop iteration to determine the next element.

Formally, the __next__() method implements the built-in next() function. For example, if you call next(x) an object x, Python internally calls x.__next__() to determine the next element of the iterable object x.

We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Background next()

The next(iterator) function is one of Python’s built-in functions—so, you can use it without importing any library.

Return Value: It returns the next value from the iterator you pass as a required first argument. An optional second argument default returns the passed default value in case the iterator doesn’t provide a next value.

Before we learn more about the __next__() dunder method, let’s have a look at a couple of basic next() examples:

users = ['Alice', 'Bob', 'Carl', 'David']

# convert the list to an iterator
users_iterator = iter(users)

x = next(users_iterator)
print(x)
# Output: 'Alice'

x = next(users_iterator)
print(x)
# Output: 'Bob'

x = next(users_iterator)
print(x)
# Output: 'Carl'

x = next(users_iterator)
print(x)
# Output: 'David'

Example Custom __next__()

In the following example, you create a custom class Data and overwrite the __init__(), __iter__(), and __next__() methods so that you can create your own iterator over a Data object.

class Data:
    def __init__(self, data):
        self.data = data # an iterable

    def __iter__(self):
        self.current_index = 0
        return self

    def __next__(self):
        if self.current_index < len(self.data):
            x = self.data[self.current_index]
            self.current_index += 1
            return x
        raise StopIteration
  • __init__() initializes the data attribute that is expected to be an iterable.
  • __iter__() returns the iterator object — the one that implements the __next__() method. In our case, this is the Data object on which it is called itself. We initialize current_index with zero, so we start iterating with the first index of data.
  • __next__() returns the next value after one iteration. We increment the current_index attribute to keep track of the current index of the element in data.

Let’s create a Data object d and an iterator over the data object using the built-in iter() function (that internally calls __iter__())—and start iterating over the object using the built-in next() function (that internally calls __next__()):

d = Data([1, 'Alice', 42, 'finxter'])

# Create an iterator
iterator = iter(d)

# Dynamically generate the next values - iterate!
print(next(iterator))
print(next(iterator))
print(next(iterator))
print(next(iterator))
print(next(iterator))

The output is as follows: The first four calls result in the expected elements of the data attribute, i.e., 1, 'Alice', 42, and 'finxter'. The fifth call of next() results in a StopIteration error because we have finished iterating over all elements.

1
Alice
42
finxter
Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 34, in <module>
    print(next(iterator))
  File "C:\Users\xcent\Desktop\code.py", line 14, in __next__
    raise StopIteration
StopIteration

If you hadn’t defined the __iter__() method, Python would’ve raised an error:

TypeError: ‘…’ object is not iterable

If you call iter(x) on an object on which the x.__iter__() dunder method is not defined, Python will raise a TypeError: '...' object is not iterable.

To fix this error, simply define the __iter__() method in the class definition before calling iter() on an object—and make sure that __iter__() returns an iterator object on which the dunder method __next__() is defined!

Here’s an example:

class Data:
    def __init__(self, data):
        self.data = data # an iterable

    
        
d = Data([1, 'Alice', 42, 'finxter'])

# Create an iterator
iterator = iter(d)

Here’s the error message:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 10, in <module>
    iterator = iter(d)
TypeError: 'Data' object is not iterable

References:

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