Comprehensions

Comprehensions are considered as most "Pythonic" way of constructing needed data on-the-fly.

• List comprehension

• Dictionary comprehension

• Set comprehensions

• Generator expression*

List comprehension

Bread and butter of day-to-day Python programming

🪄 Code:

[x for x in range(0, 10)]

📟 Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

🪄 Code:

[x for x in range(0,10) if x%2 == 0]

📟 Output:

[0, 2, 4, 6, 8]

🪄 Code:

[(x, y) for x in range(0,10) if x%2 == 0 for y in range(x) if y%2 != 0]

📟 Output:

[(2, 1),
 (4, 1),
 (4, 3),
 (6, 1),
 (6, 3),
 (6, 5),
 (8, 1),
 (8, 3),
 (8, 5),
 (8, 7)]

Dictionary comprehension

Useful to create a dictionary with the same (default) value or predefined by some logic

🪄 Code:

{x: str(x) for x in range(5)}

📟 Output:

{0: '0', 1: '1', 2: '2', 3: '3', 4: '4'}

🪄 Code:

{x: y for x in range(3) for y in range(3)}

📟 Output:

{0: 2, 1: 2, 2: 2}

Set comprehension

Not so widely used but still can be quite helpful. For example if you read lines from the file you can collect unqiue ones.

🪄 Code:

list_with_duplicated = [1, 1, 2, 3, 2, 1, 4, 2]
print({x for x in list_with_duplicated})
print(set(list_with_duplicated)) # recommended way

print({x for x in list_with_duplicated if x % 2}) # more logical usage

📟 Output:

{1, 2, 3, 4}
{1, 2, 3, 4}
{1, 3}

Generator expression

"Kind of" comprehension but instead of returning sequence as other do, generator expression returns generator object.

🪄 Code:

(x * x for x in range(10))

📟 Output:

<generator object <genexpr> at 0x0000023DF7253678>

🪄 Code:

for x in (x * x for x in range(10)):
    print(x, end=" ")

0 1 4 9 16 25 36 49 64 81