Editing Iterator (section)

===Python===<!-- This section is linked from [[Associative array]] -->

Iterators in [[Python (programming language)|Python]] are a fundamental part of the language and in many cases go unseen as they are implicitly used in the <code>for</code> ([[foreach]]) statement, in [[list comprehension]]s, and in [[Python syntax and semantics#Generator expressions|generator expressions]]. All of Python's standard built-in [[Collection (abstract data type)|collection]] types support iteration, as well as many classes that are part of the standard library. The following example shows typical implicit iteration over a sequence:

<syntaxhighlight lang="python">
for value in sequence:
    print(value)
</syntaxhighlight>

Python dictionaries (a form of [[associative array]]) can also be directly iterated over, when the dictionary keys are returned; or the <code>items()</code> method of a dictionary can be iterated over where it yields corresponding key,value pairs as a tuple:

<syntaxhighlight lang="python">
for key in dictionary:
    value = dictionary[key]
    print(key, value)
</syntaxhighlight>

<syntaxhighlight lang="python">
for key, value in dictionary.items():
    print(key, value)
</syntaxhighlight>

Iterators however can be used and defined explicitly. For any iterable sequence type or class, the built-in function <code>iter()</code> is used to create an iterator object. The iterator object can then be iterated with the <code>next()</code> function, which uses the <code>__next__()</code> method internally, which returns the next element in the container. (The previous statement applies to Python 3.x. In Python 2.x, the <code>next()</code> method is equivalent.) A <code>StopIteration</code> exception will be raised when no more elements are left. The following example shows an equivalent iteration over a sequence using explicit iterators:

<syntaxhighlight lang="python">
it = iter(sequence)
while True:
    try:
        value = it.next()  # in Python 2.x
        value = next(it)  # in Python 3.x
    except StopIteration:
        break
    print(value)
</syntaxhighlight>

Any user-defined class can support standard iteration (either implicit or explicit) by defining an <code>__iter__()</code> method that returns an iterator object. The iterator object then needs to define a <code>__next__()</code> method that returns the next element.

Python's [[#Generators|generators]] implement this iteration [[Protocol (object-oriented programming)|protocol]].