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##Links

##Problem Have you ever found yourself writing this:

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

odd_result = []
for i in data:
	if i % 2 == 0:
		odd_result.append(i)

formated = []
for entry in data:
	formated.append("Number: {}".format(entry))

##Solution There is a better way to write this:

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

odd_result = [i for i in data if i % 2 == 0]

formated = ["Number: {}".format(entry) for entry in data]

##Syntax

result = [ < what to put in the list > < iterable > < optional condition > ]

##Nested comprehensive list:

result = [i for i in [j for j in range(1, 200) if j > 100 ] if i % 2 == 0]
matrix = [[1,2,3],[4,5,6],[7,8,9]]
result = [cell for row in matrix for cell in row]
print(result)

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

result = [[cell for cell in row] for row in matrix ]

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

##Let's generate some data for performance testing:

tags = ["test1", "test2", "test3"]
import random
import uuid
big_list = [{"id": i, "uuid": str(uuid.uuid4()), "tag": random.choice(tags)} for i in range(0,1000000)]  #10 million

print(big_list)

[{'id': 0, 'tag': 'test2', 'uuid': 'd290ac0d-9c54-497f-9287-d80f5f74e1a4'},
{'id': 1, 'tag': 'test1', 'uuid': '56150efd-1c0b-455f-9bca-715d37610ddc'},
{'id': 2, 'tag': 'test3', 'uuid': '5bf5c7e0-5665-4fb0-884b-d064f0e03f90'},
{'id': 3, 'tag': 'test2', 'uuid': 'bf0c96b4-c133-43db-8d04-36c8b8931fdb'},
{'id': 4, 'tag': 'test1', 'uuid': 'aa87b549-2334-42ae-b197-008b5df77277'},
{'id': 5, 'tag': 'test1', 'uuid': 'b4afc38e-1f7e-467f-aaa6-f6831efb2c98'},
........
........]

##Performance testing:

  • Using a bare for
In [30]: %%time
    ...: result = []
    ...: for i in big_list:
    ...:     if i['tag'] == "test1":
    ...:         result.append(i)
    ...:

CPU times: user 1.57 s, sys: 53.3 ms, total: 1.62 s
Wall time: 1.58 s

  • Using a comprehensive list
In [35]: %%time
    ...: result = [i for i in big_list if i['tag'] == "test1"]
    ...:

CPU times: user 1.29 s, sys: 40 ms, total: 1.33 s
Wall time: 1.3 s

  • Careful to how you access distionaries
In [52]: %%time
    ...: t = "test1"
    ...: result = [i for i in big_list if i.get('tag', "") == t]
    ...:

CPU times: user 2.54 s, sys: 100 ms, total: 2.64 s
Wall time: 2.54 s

  • Filter and function
In [59]: %%time
    ...: def f(i):
    ...:     if i['tag'] == "test1":
    ...:         return True
    ...:     else:
    ...:         return False
    ...: result = filter(f, big_list)
    ...:

CPU times: user 1.71 s, sys: 76.7 ms, total: 1.78 s
Wall time: 1.73 s

  • Filter and lambda
In [42]: %%time
    ...: result = filter(lambda i:i['tag'] == "test1", big_list)
    ...:

CPU times: user 1.49 s, sys: 107 ms, total: 1.6 s
Wall time: 1.53 s

##Sets and Dictionaries comprehension

  • Set comprehension
result = {i for i in [1,2,3,3,3]}
print(result)

{1, 2, 3}

  • Dictionary comprehension
d = {"key1": "value1", "key2": "value2"}
inverted = {value: key for key, value in d.items()}
print(inverted)

{'value1': 'key1', 'value2': 'key2'}

list1 = ["aa", "bb", "cc"]
list2 = ["11", "22", "33"]
result = {key: value for key, value in zip(list1, list2)}
print(result)

{'aa': '11', 'bb': '22', 'cc': '33'}