Comprehensions

Comprehensions

Constructs that allow sequences to be built from other sequences.

Several types of comprehensions are supported:

• list comprehensions

• dictionary comprehensions

• set comprehensions

• generator comprehensions

list comprehensions

Short way to create lists.

Square brackets

Blueprint

variable = [out_exp for out_exp in input_list if out_exp == 2]

Example:

multiples = [i for i in range(30) if i % 3 == 0]
print(multiples)
# Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27] 

Example with for loop:

squared = []
for x in range(10):
    squared.append(x**2)

Example with list comprehensions:

squared = [x**2 for x in range(10)]

dict comprehensions

Short way to create dicts

Curly braces {}

Example:

mcase = {'a': 10, 'b': 34, 'A': 7, 'Z': 3}

mcase_frequency = {
    k.lower(): mcase.get(k.lower(), 0) + mcase.get(k.upper(), 0)
    for k in mcase.keys()
}

# mcase_frequency == {'a': 17, 'z': 3, 'b': 34}

In the above example we are combining the values of keys which are same but in different typecase.

Example: switch keys and values of a dictionary:

{v: k for k, v in some_dict.items()}

set comprehensions

Short way to create sets

Curly braces {}.

Example:

squared = {x**2 for x in [1, 1, 2]}
print(squared)
# Output: {1, 4}

generator comprehensions

They don’t allocate memory for the whole list but generate one item at a time.

More memory efficient.

Example:

multiples_gen = (i for i in range(30) if i % 3 == 0)
print(multiples_gen)
# Output: <generator object <genexpr> at 0x7fdaa8e407d8>
for x in multiples_gen:
  print(x)
  # Outputs numbers