+----------+--------------------+------------------------+
| | __iter__ | __next__ |
+----------+--------------------+------------------------+
| iterable | return an iterator | not available |
+----------+--------------------+------------------------+
| iterator | return self | return next item |
| | | or raise StopIteration |
+----------+--------------------+------------------------+
iter(x) is x --> True: x is an iterator
False: x is an iterable
raises Exception: x is neither iterator nor iterable
Once next(it) raises a StopIteration on a well-behaved iterator it must
continue to raise `StopIteration`s on subsequent next(it) calls.
There's an odd outlier that I probably shouldn't tell you about:
classes with a __getitem__() method behave like iterators. An eventual
IndexError exception is propagated as StopIteration:
>>> class A:
... def __getitem__(self, index):
... if index > 2:
... raise IndexError
... return index * index
...
>>> for item in A():
... print(item)
...
0
1
4
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