List Comprehensions in Python

Fill old list on new list using for loop and append :

new_list = []
for i in old_list:
  if filter(i):
  new_list.append(expressions(i)) 

There is a better and shorter way to do the same things using using list comprehension : new_list = [expression(i) for i in old_list if filter(i)]

new_list : The new list (result). expression(i) : Expression is based on the variable used for each element in the old list You can use a function as expression that take I as parameter and return a processing value . if filter(i) : Apply a filter with an If-statement.

Example 1 :

new_list = []
old_list = [1,2,3,4,5,6]
new_list = [i*10 for i in old_list if i%2==0]
print(new_list)

result : [20,20,60]

Example 2 : convert lower case array to upper case letters inside new array :

[x.lower() for x in ["A","B","C"]]

result : ['a', 'b', 'c']