Iterator

Iterator is an object representing a stream of data; this object returns the data one element at a time.

Iterator must support a method called __next__() that takes no arguments and always returns the next element of the stream. If there are no more elements in the stream, __next__() must raise the StopIteration exception. Iterators don’t have to be finite, though; it’s perfectly reasonable to write an iterator that produces an infinite stream of data.

The built-in iter() function takes an arbitrary object and tries to return an iterator that will return the object’s contents or elements, raising TypeError if the object doesn’t support iteration. Several of Python’s built-in data types support iteration, the most common being lists and dictionaries. An object is called iterable if you can get an iterator for it.

Iterator protocol

Iterator is:

• Object with defined __next__() (next() for Python2), __iter__()

  • __next__() takes no arguments and always returns the next element of the stream. If there are no more elements in the stream, __next__() must raise the StopIteration exception

    • iterator can be infinite

  • __iter__() return self (to use inside for loop which uses iter() for it's data)

• Object returned by generator function (the one that uses yield)

• Object returned by iter(<iterable>)

Iterator vs generator

Generator is partial case of iterator.

Iterator main methods

• __next__() gives next value or raises StopIteration exception

• __iter__() returns iterator itself (used by for operator)

So, it's exactly how for and iter works! It just used object's __iter__() method. In other case object must support the sequence protocol (the __getitem__() method with integer arguments starting at 0).

The best way to get all elements of iterator - it's to feed it to list()

🪄 Code:

list(iter([1, 2, "a", None, 10.1]))

📟 Output:

[1, 2, 'a', None, 10.1]

🪄 Code:

a = [1, 2, 3, 4]
print( "__next__" in a ) # list doesn't have __next__() --> not iterator! just iterable

print(a.__iter__ )
print(a.__iter__().__next__())
a_iter = iter(a)
print(a_iter )
print(a_iter.__next__(), a_iter.__next__(), a_iter.__next__(), a_iter.__next__())
print(a_iter.__next__())

📟 Output:

False
<method-wrapper '__iter__' of list object at 0x7f57511021c0>
1
<list_iterator object at 0x7f5751f67a00>
1 2 3 4



---------------------------------------------------------------------------

StopIteration                             Traceback (most recent call last)

Input In [1], in <cell line: 9>()
      7 print(a_iter )
      8 print(a_iter.__next__(), a_iter.__next__(), a_iter.__next__(), a_iter.__next__())
----> 9 print(a_iter.__next__())


StopIteration: