Comprehensions in python

Comprehensions in python are a great tool to simplify iterating and creating lists. They can increase readability when used right. But, be on the lookout for when they might reduce readability. Tip: Readability > Shortcuts

The key to comprehensions is the syntax. Practice to become familiar with it.

Table of contents

• List comprehension
• Set comprehension
• Dict comprehension
• Additional tips

List

Using a for loop to double value of each number in the list:

my_list = [2, 4, 8]
new_list = []

for number in my_list:
    new_list.append(number * 2)

print(new_list)

Output:

[4, 8, 16]

Using list comprehension:

my_list = [2, 4, 8]
new_list = [ number * 2 for number in my_list ]
print(new_list)

Breaking down the syntax:

[ number * 2 for number in my_list ]

We enclose within ‘[]’ to return a list

[ number * 2 for number in my_list ]

The first part is the expression. In a regular for loop expression comes afterwards. In list comprehension it comes first. This is the key to remembering the syntax.

[ number * 2 for number in my_list ]

The rest of the syntax looks very much like regular for loop: for item in list

Set

Set comprehension follows same principal as lists. This will return a set (notice the curly braces):

my_list = [2, 4, 8]
new_list = { number * 2 for number in my_list }
print(new_list)

Output:

{4, 8, 16}

Dictionary

This will return a dict (notice the curly braces and key:value being returned)

my_dict = [2, 4, 8]
new_list = { number:number * 2 for number in my_dict }
print(my_dict)

Output:

{2: 4, 4: 8, 8: 16}

Additional tips

• Mix and match comprehensions with iterators:
  • We saw the example of iterating over a list and returning a list, set or dictionary.
  • You can mix n match. For example, iterate over dictionary and return a list.
  • You are able to iterate over any object with an iterator. How to make an iterator
• Nesting comprehensions
  • You can nest comprehensions. Best way to get comfortable with the syntax is practice and more practice.

      my_list = [[1, 2], [3], [5, 6]]
      new_list = [ num for my_sub_list in my_list for num in my_sub_list ]
      print(new_list)
    

    Output

    [1, 2, 3, 5, 6]

  • It isn’t the easiest thing to read. Key is breaking down its components.
• Comprehension with if
  • Pairing if with your comprehension can be a very powerful tool

  Only multiply by 2 numbers greater than 2:

    my_list = [2, 4, 8]
    new_list = [ number * 2 for number in my_list if number > 2 ]
    print(new_list)
  

  Output

  [8, 16]

• Comprehension with ternary operator ( if else )
  • You can combine comprehensions with ternary operator.
  • This is different from the above ‘if’
  • More on the ternary operator in the upcoming blog.
• Using _ (underscore) for item
  • It is convention to use _ for item variable name if you are not using it.

      n = 100
      list_of_zeros = [ 0 for _ in range(n) ]
    

  • There is an easier and better way to do this without comprehensions. Above is just an example.

      list_of_zeros = [0] * n