Editing Lazy evaluation (section)

====Python====
In [[Python (programming language)|Python]] 2.x the <code>range()</code> function<ref>{{cite web|url=https://docs.python.org/library/functions.html#range|title=2. Built-in Functions — Python 2.7.11 documentation}}</ref> computes a list of integers. The entire list is stored in memory when the first assignment statement is evaluated, so this is an example of eager or immediate evaluation:
<syntaxhighlight lang="pycon">
>>> r = range(10)
>>> print r
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print r[3]
3
</syntaxhighlight>

In Python 3.x the <code>range()</code> function<ref>{{cite web|url=https://docs.python.org/py3k/library/functions.html#range|title=2. Built-in Functions — Python 3.5.1 documentation}}</ref> returns a [[Generator (computer programming)|generator]] which computes elements of the list on demand. Elements are only generated when they are needed (e.g., when <code>print(r[3])</code> is evaluated in the following example), so this is an example of lazy or deferred evaluation:

<syntaxhighlight lang="pycon">
>>> r = range(10)
>>> print(r)
range(0, 10)
>>> print(r[3])
3
</syntaxhighlight>
:This change to lazy evaluation saves execution time for large ranges which may never be fully referenced and memory usage for large ranges where only one or a few elements are needed at any time.

In Python 2.x is possible to use a function called <code>xrange()</code> which returns an object that generates the numbers in the range on demand. The advantage of <code>xrange</code> is that generated object will always take the same amount of memory.

<syntaxhighlight lang="pycon">
>>> r = xrange(10)
>>> print(r)
xrange(10)
>>> lst = [x for x in r]
>>> print(lst)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
</syntaxhighlight>

From version 2.2 forward, Python manifests lazy evaluation by implementing iterators (lazy sequences) unlike tuple or list sequences. For instance (Python 2):
<syntaxhighlight lang="pycon">
>>> numbers = range(10)
>>> iterator = iter(numbers)
>>> print numbers
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print iterator
<listiterator object at 0xf7e8dd4c>
>>> print iterator.next()
0
</syntaxhighlight>
:The above example shows that lists are evaluated when called, but in case of iterator, the first element '0' is printed when need arises.