When a value works in a for loop, Python asks it for an iterator. The iter built-in performs that request by calling __iter__, whose result supplies values through __next__ until StopIteration ends the loop. Strings, lists, dictionaries, files, and custom objects can participate; integers cannot. section 11.1 section 11.2
Implement __iter__ according to the behavior your object needs:
- Return self when the object also implements __next__ and should act as its own stateful iterator. section 11.3
- Return a separate iterator when each new for loop should begin independently, as with strings, lists, tuples, and dictionaries. section 11.5 section 11.7
- Return any iterator object, including a helper-class instance or generator expression; __iter__ does not have to return self. section 11.11
A repeating iterable
class Circle: def __init__(self, data, max_times): self.data = data self.max_times = max_times def __iter__(self): n = len(self.data) return (self.data[x % n] for x in range(self.max_times)) Circle('abc', 5) produces a, b, c, a, b. The generator expression returned by __iter__ controls the iteration count, while modulus wraps the data index. section 11.11
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 the calls made by a for loop: construction, __iter__, repeated __next__ calls, and the final StopIteration. section 11.3
A return-self implementation is stateful: after one complete pass, its index remains exhausted, so a second pass may produce nothing. Use a helper iterator or generator when the containing object must be reusable. section 11.7 section 11.9
