Iter

This built-in is used to create custom looping logic. We can use iter in a for-loop. We continue looping while values are returned by iter.

We usually will call iter in a for-loop, as it returns values we can loop over. But sometimes we may call iter and store the result in a local variable.

Example, two arguments

With two arguments, iter continually calls the method passed as argument 1. It stops when the second argument, a value, is reached.

import random

elements = ["cat", "dog", "horse", None, "gerbil"]

def random_element():
    # Return random element from list.
    return random.choice(elements)

# Use iter until a None element is returned.
for element in iter(random_element, None):
    print(element)
cat
horse
dog
dog
gerbil

Example, one argument

Here iter does something different. It acts upon a collection and returns each element in order. This usage is not often needed—we can just reference the collection.

elements = ["cat", "dog", "horse", None, "gerbil"]

# Iter returns each element, one after another.
for element in iter(elements):
    print(element)
cat
dog
horse
None
gerbil

Iter, next method

With iter we get an iterator variable. We must call next() on this iterator to use it. Here we call next() to get successive values from a list—no loop is needed.

values = [1, 10, 100, 1000]
i = iter(values)

# Call the next built-in on an iter.
# ... This style of code is not often useful.
value = next(i)
print(value)

value = next(i)
print(value)

value = next(i)
print(value)
1
10
100

With iter we can create custom logic in for-loops. We can invoke next() to get the next element from the iterator in a statement.