StopIteration is the signal that an iterator has no more values. A for loop calls iter() to obtain an iterator, repeatedly calls next(), and exits when __next__ raises StopIteration. section 10.0
A custom iterator usually stores its data and current position, returns itself from __iter__(), and raises StopIteration when the position reaches the data’s length. The exception ends iteration rather than representing an unexpected failure. section 10.2
A finite custom iterator
class LoudIterator: def __init__(self, data): self.data = data self.index = 0 def __iter__(self): return self def __next__(self): if self.index >= len(self.data): raise StopIteration value = self.data[self.index] self.index += 1 return value for item in LoudIterator('abc'): print(item) The loop prints a, b, and c, then stops when the next call raises StopIteration. section 10.0 section 10.2
Keep these distinctions in mind when implementing or using iteration:
- Iterables provide an iterator through __iter__(); iterators provide values through __next__(). section 10.0
- A reusable iterable can return a fresh helper iterator instead of returning itself, avoiding exhausted state on a second loop. section 10.2
- Generator functions raise StopIteration automatically when they finish; use return for early termination rather than explicitly raising it. section 10.15
- An iterator can repeat or transform values and may stop only after producing a customized number of results. section 10.6
class LoudIterator():
def __init__(self, data):
print('\tNow in __init__')
self.data = data #1
self.index = 0 #2
def __iter__(self):
print('\tNow in __iter__')
return self #3
def __next__(self):
print('\tNow in __next__')
if self.index >= len(self.data): #4
print(
f'\tself.index ({self.index}) is too big; exiting')
raise StopIteration
value = self.data[self.index] #5
self.index += 1 #6
print('\tGot value {value}, incremented index to {self.index}')
return value
for one_item in LoudIterator('abc'):
print(one_item)
Trace __init__, __iter__, __next__, value production, index updates, and the final StopIteration event in a runnable iterator. section 10.0
