iter and next

Learn iterability with __iter__ and __next__.

iter and next

To make a class usable in a for loop, define the __iter__ and __next__ methods. __iter__ returns an iterator; __next__ gives the next item.

Example: class Counter:\n def __iter__(self):\n return self\n def __next__(self):\n .... Writing for n in c first calls __iter__, then __next__ at each step. When items run out, __next__ raises StopIteration; the loop ends.

The for loop flow

Writing for n in c works like this: first iter(c) calls __iter__; on the returned iterator, next() calls __next__ at each step. When StopIteration is raised, the loop ends.

Example: Counter(3) gives three items. for n in Counter(3): print(n) prints 0, 1, 2. Each next() returns a value; on the fourth call StopIteration is raised. This protocol is the same for all iterators.

Exercise Add an iterator to Counter

Define the Counter class. Add def __init__(self, limit): with self.limit = limit and self.current = 0 in the body. Add def __iter__(self): with return self. Add def __next__(self):; in the body write if self.current >= self.limit:, raise StopIteration, then value = self.current, self.current += 1 and return value. Then call c = Counter(3) and for n in c: print(n).

Show example solution

# Define the Counter class
class Counter:
    def __init__(self, limit):
        self.limit = limit
        self.current = 0
    def __iter__(self):
        return self
    def __next__(self):
        if self.current >= self.limit:
            raise StopIteration
        value = self.current
        self.current += 1
        return value

c = Counter(3)
for n in c:
    print(n)

Test yourself

What does __next__ raise when items run out?
- ValueError
- TypeError
- IndexError
- StopIteration
Show answer

StopIteration — StopIteration signals the end of the loop.

Complete the __iter__ body

    def __iter__(self):
        return ____

Show answer

    def __iter__(self):
        return self

Match the iterator methods

__iter__

Returns iterator
__next__

Next item
StopIteration

Loop end

Next lesson: __contains__

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