Comprehensions in Python: Explained

What is comprehension?

Comprehensions are a concise notation for performing some operation for a collection of elements, and/or selecting a subset of them that meet some condition.

Types of Comprehensions:

• List Comprehensions.

• Dict Comprehensions.

• Set Comprehensions.

Simply Put, you can use comprehensions to define new Sequences(List, Dict, Set) from existing once in an elegant way.

Let's see comprehensions in Action.

List Comprehensions

• Suppose, you want to create a list with the squares of the first 10 natural numbers.

  You can do this using a for loop like this:

  squares = []
  for n in range(10):
      squares.append(n * n)
  print(squares)    # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
  

  You can perform the same thing using list comprehensions in a much more elegant and concise way:

  squares = [n * n for n in range(10)]
  print(squares)    # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
  

  Same result!!

  Basically we have put a for loop within square brackets.

• You can also use an if condition inside of comprehension.

  Let's select the even numbers from the square list we just created:

  squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
  even_nums = [num for num in squares if not num % 2]
  print(even_nums)    # [0, 4, 16, 36, 64]
  

  Quite neat! isn't it?

• Nested Comprehensions: You can use multiple for loop inside a comprehension.

  Suppose you want to create a matrix from two lists:

  Using for loop:

  matrix = []
  for a in range(3):
      for b in range(3):
          matrix.append([a, b])
  print(matrix)
  """ Output: 
  [[0, 0], [0, 1], [0, 2],
   [1, 0], [1, 1], [1, 2], 
   [2, 0], [2, 1], [2, 2]]
  """
  

  Using list comprehension:

  matrix = [[a, b] for a in range(3) for b in range(3)]
  print(matrix)
  """ Output: 
  [[0, 0], [0, 1], [0, 2],
   [1, 0], [1, 1], [1, 2], 
   [2, 0], [2, 1], [2, 2]]
  """
  

  From 4 lines to 1 line!!

  Note: for loop over b has a dependency on a, it must follow the for loop over a in the comprehension.

Set Comprehensions

Set comprehension work exactly like list comprehension, only there is a little difference in the syntax.

name = "Harry"
letters1 = set(c for c in name)    # First way using set()
letters2 = {c for c in name}    # Second way using {}
print(letters1)        # {'H', 'y', 'a', 'r'}
print(letters2)        # {'H', 'y', 'a', 'r'}

Dict Comprehensions

Dict comprehension is similar to list and set comprehensions but it requires a key: value pair instead of single values.

d1 = dict((k, v) for k, v in enumerate("Hello", 1))
print(d1)    # {1: 'H', 2: 'e', 3: 'l', 4: 'l', 5: 'o'}
d2 = {k: v for k, v in enumerate("Bye", 1)}
print(d2)    # {1: 'B', 2: 'y', 3: 'e'}

Note:

• enumerate function returns a key: value pair using the iterable passed to it and the start value.
• Dictionaries do not allow duplication in the keys.

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