List Comprehension

In Python, list comprehension is a way to create lists in a declarative way. It is a syntactic construct that allows us to create lists from other lists, Generators, etc. It is mostly used to replace map() and filter() functions when the function we want to apply is a simple lambda function. For more complex functions, it is better to use map() and filter() as (to my opinion) they are more readable.

For example, if we want to create a list of all the even numbers from 1 to 10, we can do it like this:

my_list = [x for x in range(1, 11) if x % 2 == 0]
print(my_list) # [2, 4, 6, 8, 10]

The above is equivalent to the following:

my_list = []
for x in range(1, 11):
    if x % 2 == 0:
        my_list.append(x)
print(my_list) # [2, 4, 6, 8, 10]

or this:

my_list = list(filter(lambda x: x % 2 == 0, range(1, 11)))
print(my_list) # [2, 4, 6, 8, 10]

NOTE 1: You can stack many for and if statements in a list comprehension. They will be executed in the same order as they would be in a normal for loop.

NOTE 2: List comprehension can also be used to create dictionaries and sets. For example:

 my_dict = {x: x * 2 for x in range(1, 11)}
 print(my_dict) # {1: 2, 2: 4, 3: 6, 4: 8, 5: 10, 6: 12, 7: 14, 8: 16, 9: 18, 10: 20}
 
 my_set = {x for x in range(1, 11) if x % 2 == 0}
 print(my_set) # {2, 4, 6, 8, 10}

NOTE 3: List comprehension can also be used to create Generators. For example:

 my_generator = (x for x in range(1, 11) if x % 2 == 0)
 print(list(my_generator)) # [2, 4, 6, 8, 10]

This is useful when we want to create a list of a large number of items, but we don’t want to store them in memory.