List comprehension technique is similar to mathematical set builder notation. A classical for/while loop is generally used to traverse and process each ITEM in an iterable. List comprehension is considerably more efficient than processing a list by ‘for’ loop. It is a very concise mechanism of creating new list by performing a certain process on each item of existing list.

Suppose we want to compute square of each number in a list and store squares in another list object. We can do it by a ‘for’ loop as shown below:

The squares list object is DISPLAYED as follows:

Same result is achieved by list comprehension technique MUCH more efficiently. List comprehension statement USES following syntax:

We use above format to construct list of squares using list comprehension.

We can even generate a dictionary or tuple object as a result of list comprehension.

Nested loops can also be used in a list comprehension expression. To obtain list of all combinations of items from two lists (Cartesian product):

The resulting list stores all combinations of one number from each list

We can even have if condition in list comprehension. Following statement will result in list of all non-vowel alphabets in a string.