division //

if any of the number is float, the result will be float need int() conversion to get int. (How is it done?)

defaultdict vs setdefault

setdefault is a bad name, the accurate name should be ‘get_or_set’

key = "list_a"
dict.setdefault(key, []).append('element')

For defaultdicts, merely check whether a key exists unexpectedly creates the key.

how does a defaultdict interact with .get()? Safe, it doesn’t creates a new value.

Use the start argument to control the start index of the enumeration change from 0 index to 1 index

items = ['a', 'b', 'c']
for index, item in enumerate(items, 1):
    print(index, item)

1 a
2 b
3 c

在 python 裏面構建索引，搞錯寫法的話，讀取索引反而花時間。例如為了讓代碼更好讀，每一個數據點創建物件的做法，在 python 裏面是顧此失彼。

Python 沒有編譯，沒法優化創建物件的開銷。想要寫得快，就要用接近 C 語言的寫法，用沒有名字的數組。

所以算法根本不適合用 python 教。算法的本質就是過早優化。在慢的語言裏面追求速度是本末倒置。

rf"" 需要注意什麼？

error: externally-managed-environment

python -m venv myenv
source myenv/bin/activate

note:

use myenv/bin/{executable} (e.g. myenv/bin/pip)

don’t change the folder path after creating the virtual env

conda create --name myenv python=3.X
conda activate myenv
conda env list

for first, second, third in zip(string, string[1:], string[2:]): print(first + second + third)