Generator expressions#

Generator expressions are a high-performance, memory-efficient generalization of list comprehensions and generators.

Importing libraries and packages#

1# Mathematical operations and data manipulation
2
3# System
4from sys import getsizeof

Generator expressions#

1odd_numbers2 = [x for x in range(100000) if x % 2 != 0]
2# The bytes of memory a generator expression uses
3getsizeof(odd_numbers2)

444376

1# Surrounding the list comprehension statement with round brackets
2# instead of square ones to create a generator expression, so no
3# explicit memory has been allocated for it (lazy evaluation)
4odd_numbers = (x for x in range(100000) if x % 2 != 0)
5getsizeof(odd_numbers)

112

1# Using
2for i, number in enumerate(odd_numbers):
3    print(number)
4    if i > 10:
5        break

1
3
5
7
9
11
13
15
17
19
21
23

Single-line generator expression#

Creating a one-liner generator expression efficiently using a simple for loop

 1words = [
 2    "Evil\n",
 3    "begins",
 4    "when you begin\n",
 5    "to treat people as\n",
 6    "Things\n",
 7]
 8modified_words = (word.strip().lower() for word in words)
 9list_of_words = [word for word in modified_words]
10list_of_words

['evil', 'begins', 'when you begin', 'to treat people as', 'things']

Extracting a list with single words#

1modified_words2 = (
2    w.strip().lower() for word in words for w in word.split(" ")
3)
4list_of_words2 = [word for word in modified_words2]
5list_of_words2

['evil',
 'begins',
 'when',
 'you',
 'begin',
 'to',
 'treat',
 'people',
 'as',
 'things']

1# Equivalent code using a nested for loop
2modified_words3 = []
3for word in words:
4    for w in word.split(" "):
5        modified_words3.append(w.strip().lower())
6modified_words3

['evil',
 'begins',
 'when',
 'you',
 'begin',
 'to',
 'treat',
 'people',
 'as',
 'things']

Independent for loops in a generator expression#

1marbles = ["RED", "BLUE", "GREEN"]
2counts = [1, 5, 13]

 1# Generating all possible combinations of the values in the marbles
 2# array and counts array
 3marble_with_count = ((m, c) for m in marbles for c in counts)
 4
 5# This generator expression creates a tuple in each iteration of the for loops.
 6marble_with_count_as_list_2 = []
 7for m in marbles:
 8    for c in counts:
 9        marble_with_count_as_list_2.append((m, c))
10marble_with_count_as_list_2

[('RED', 1),
 ('RED', 5),
 ('RED', 13),
 ('BLUE', 1),
 ('BLUE', 5),
 ('BLUE', 13),
 ('GREEN', 1),
 ('GREEN', 5),
 ('GREEN', 13)]