Lazy Evaluation - Laziness in Eager Languages

Laziness in Eager Languages

Python

In Python 2.x the range function computes a list of integers (eager or immediate evaluation):

>>> r = range(10) >>> print r >>> print r 3

In Python 3.x the range function returns an iterator which computes elements of the list on demand (lazy or deferred evaluation):

>>> r = range(10) >>> print(r) range(0, 10) >>> print(r) 3
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.

Python manifests lazy evaluation by implementing iterators (lazy sequences) unlike tuple or list sequences. For instance:

>>> list = range(10) >>> iterator = iter(list) >>> print list >>> print iterator >>> print iterator.next 0
The above example shows that lists are evaluated when called, but in case of iterator, the first element '0' is printed when need arises.
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