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The iter() Built In Method

The iter() method has 2 functionalities depending on how it is called. When calling iter() with only one argument, that argument must be a collection object which supports iteration protocol (the __iter__() method). The object returned is a list_iterator type object built from the collection object that was passed. For the sake of simplicity we can use a list.

>>> # Instantiate a standard Python list object
>>> list_object = [1, 2, 3, 4, "cat", "dog", "purple"]
>>> # Use iter() to return a list_iterator object to the variable
>>> list_iter_object = iter(list_object)
>>> # Demonstrate that the returned object is indeed a different type
>>> type(list_object)
<class 'list'>
>>> type(list_iter_object)
<class 'list_iterator'>

Effectively this converts our list object into a generator object which yields the items from the list that was passed to iter()

When we pass 2 arguments to the iter() method the first argument must be a callable. The iterable created in this instance will call the object callable with no arguments for each call to it's __next__ method. If the value returned is equal to the second argument (sentinel), StopIteration will be raised, if it does not match then the value returned by the callable will be returnd.

>>> # Create a list object with some identifiable data
>>> i = [f'{x}TR{x+3}' for x in range(30)]
>>> i
['0TR3', '1TR4', '2TR5', '3TR6', '4TR7', '5TR8', '6TR9', '7TR10', '8TR11', '9TR12', '10TR13', '11TR14', '12TR15', '13TR16', '14TR17', '15TR18', '16TR19', '17TR20', '18TR21', '19TR22', '20TR23', '21TR24', '22TR25', '23TR26', '24TR27', '25TR28', '26TR29', '27TR30', '28TR31', '29TR32']
>>> # Create a list_iterable from this list (any object with a callable would work)
>>> i = iter(i)
>>> # Loop over the iterable created
>>> # The object created by iter() in this case is created by calling the __next__ method on the 
>>> # generator until the sentinel (second arg) is matched by the output
>>> # iteration is stopped at that point
>>> for _ in iter(i.__next__, '5TR8'):
...     print(_)
...
0TR3
1TR4
2TR5
3TR6
4TR7
>>> 

Let's break this down. I create a list here using some string formatting to make the items a little more unique. I use the single argument form of iter() to convert that list into a list_iterator. I then write a little for loop. The iterator I create for it to loop over is using the 2 argument form. The first argument is my callable which is the next method of my generator (called without argument). The sentinal argument is a value I know to be in the iterable. It loops calling next on the generator until that next yields the value of my sentinel argument. Upon yielding the sentinel value it raises StopIteration and does not return the sentinel value.