The previous example creates a list from a sequence of values based on some selection criteria. An easy way to do this type of processing in Python is to use a list comprehension. List comprehensions are concise ways to create lists. The general syntax is:
[<expression> for <item> in <sequence> if <condition>]
where the if clause is optional. For example,
The expression describes each element of the list that is being built. The for
clause iterates through each item in a sequence. The items are filtered by the if
clause if there is one. In the example above, the for
statement lets item
take on all the values in the list mylist
. Each item is then squared before it is added to the list that is being built. The result is a list of squares of the values in mylist
.
To write the primes_upto
function we will use the is_prime
function to filter the sequence of integers coming from the range
function. In other words, for every integer from 2 up to but not including n
, if the integer is prime, keep it in the list.
def primes_upto(n): """ Return a list of all prime numbers less than n using a list comprehension. """ result = [num for num in range(2,n) if is_prime(num)] return result