from collections import defaultdict
d = defaultdict(list)  # list, set等を指定
d['a'].append(1)  # setの場合は.add(1)

d = {}  # 普通のディクショナリ
d.setdeault('a', []).append(1)

>>> re.split(r'[;,\s]\s*', line)
['asdf', 'fjdk', 'afed', 'fjek', 'asdf', 'foo']
>>> re.split(r'(;|,|\s)\s*', line)  # captureグループ(結果のリストに出力される)
['asdf', ' ', 'fjdk', ';', 'afed', ',', 'fjek', ',', 'asdf', ',', 'foo']
>>> re.split(r'(?:;|,|\s)\s*', line)  # noncaptureグループ (?: ...)
['asdf', 'fjdk', 'afed', 'fjek', 'asdf', 'foo']

with open(filename) as f:
    lines = (line.strip() for line in f)  # iteratorを作るだけ
    for line in lines:
        ...

>>> data = ['ACME', 50, 91.1]
>>> ','.join(str(d) for d in data)
'ACME,50,91.1'

>>> import random
>>> values = [1,2,3,4,5,6]
>>> random.choice(values)  # 一つ選ぶ
2
>>> random.sample(values, 3)  # 3つ選ぶ
[4, 3, 1]
>>> random.shuffle(values)  # ランダムに並び替える
>>> values
[5, 2, 3, 1, 4, 6]
>>> random.random()  # 0-1のfloat値を得る
0.14043684176026439
>>> random.seed()  # システム時間やos.random()ベースのシード
>>> random.seed(12345)

>>> from datetime import datetime
>>> import pytz
>>> d = datetime(2021, 1, 27, 21, 30,0)  # (1) datetimeで日時を表記
>>> print(d)
2021-01-27 21:30:00
>>> boston = timezone('America/New_York')
>>> bos_d = boston.localize(d)  # (2) 東海岸時間に指定
>>> print(bos_d)
2021-01-27 21:30:00-05:00
>>> utc_d = bos_d.astimezone(pytz.utc)  # (3) UTCに変換
>>> print(utc_d)
2021-01-28 02:30:00+00:00
>>> later_utc_d = utc_d + timedelta(minutes=30)  # (4) UTCで時間計算
>>> japan = timezone('Asia/Tokyo')
>>> print(later_utc_d.astimezone(japan))  # (5) JST時間で表示
2021-01-28 12:00:00+09:00

>>> pytz.country_timezones['JP']
['Asia/Tokyo']
>>> pytz.country_timezones['US']
['America/New_York', 'America/Detroit', 'America/Kentucky/Louisville', 
'America/Kentucky/Monticello', 'America/Indiana/Indianapolis', 
'America/Indiana/Vincennes', 'America/Indiana/Winamac', 
'America/Indiana/Marengo', 'America/Indiana/Petersburg', 
'America/Indiana/Vevay', 'America/Chicago', 'America/Indiana/Tell_City', 
'America/Indiana/Knox', 'America/Menominee', 'America/North_Dakota/Center', 
'America/North_Dakota/New_Salem', 'America/North_Dakota/Beulah', 
'America/Denver', 'America/Boise', 'America/Phoenix', 
'America/Los_Angeles', 'America/Anchorage', 'America/Juneau', 
'America/Sitka', 'America/Metlakatla', 'America/Yakutat', 'America/Nome', 
'America/Adak', 'Pacific/Honolulu']

class Node:
    def __init__(self, value)
        self._value = value
        self._children = []
<snip>
    def __iter__(self):
        return iter(self._children)
    def depth_first(self):  # ジェネレーター関数
        yield self
        for c in self:
            yield from c.depth_first()  # 再帰呼び出し

def reader(s):
    for chunk in iter(lambda: s.recv(CHUNKSIZE), b''):
        process_data(chunk)

import sys
f = open('/etc/passwd')
for chunk in iter(lambda: f.read(10), ''):
    n = sys.stdout.write(chunk)

>>> import array
>>> nums = array.array('i', [1,2,3,4])
>>> with open('tt.bin', 'wb') as f:
...     f.write(nums)
>>> a = array.array('i',[0,0,0,0,0,0])
>>> with open('tt.bin', 'rb') as f:
...     f.readinto(a)
>>> a
array('i', [1, 2, 3, 4, 0, 0])

import gzip
with gzip.open('file.gz', 'rt') as f:
    text = f.read()

import bz2
with bz2.open('file.bz2', 'wb') as f:
    f.write(bytes)

f = open('file.gz', 'rb')
with gzip.open(f, 'rt') as g:
    text = g.read()

import os                                                                              
import mmap                                                                            
def memory_map(filename, access=mmap.ACCESS_WRITE):                                    
    size = os.path.getsize(filename)                                                   
    fd = os.open(filename, os.O_RDWR)                                                  
    return mmap.mmap(fd, size, access=access)

size = 10000
with open('tt.bin', 'wb') as f:
    f.seek(size-1)
    f.write(b'\x00')

>>> m = memory_map('tt.bin')
>>> len(m)
10000
>>> m[:10]
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
>>> m[0]
0
>>> m[:11] = b'Hello World'
>>> m
<mmap.mmap closed=False, access=ACCESS_WRITE, length=10000, pos=0, offset=0>
>>> m.close()  # 必須!
>>> m
<mmap.mmap closed=True>
>>> with open('tt.bin', 'rb') as f:
...     print(f.read(11))
... 
b'Hello World'

with memory_map('tt.bin') as m:
    print(len(m))
    print(m[0:10])

>>> f = open('tt.txt', 'w')
>>> f  # テキストハンドリングレイヤー
<_io.TextIOWrapper name='tt.txt' mode='w' encoding='UTF-8'>
>>> f.buffer  # buffered I/Oレイヤー
<_io.BufferedWriter name='tt.txt'>
>>> f.buffer.raw  # OSの低レベルファイルデスクリプター
<_io.FileIO name='tt.txt' mode='wb' closefd=True>