__iter__ and __next__
- Python course
- Intermediate
- Lesson 33/116
- 5 min
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.
Practice 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).
Starter code
# Define the Counter classSample 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)Expected output
0 1 2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does __next__ raise when items run out?
- ValueError
- TypeError
- IndexError
- StopIteration Correct answer
Explanation: StopIteration signals the end of the loop.
2 Complete the __iter__ body
def __iter__(self):
return ____ def __iter__(self):
return selfExplanation: When the class is its own iterator, __iter__ returns self.
3 Match the iterator methods
| Term | Meaning |
|---|---|
__iter__ |
Returns iterator |
__next__ |
Next item |
StopIteration |
Loop end |