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@ishigero

【備忘】Effective Python 1章(スライス、リスト内包表記、lambda)

Pythonicな箇所をピックアップして残そうと思います。
これからPythonを勉強しようと思っている方にも役立つかもしれないです。

シーケンスのスライス

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] 

print('first four:', nums[:4])      # first four: [0, 1, 2, 3]
print('last four:', nums[-4:])      # last four: [7, 8, 9, 10]
print('middle two:', nums[3:-3])    # middle two: [3, 4, 5, 6, 7]

print(nums[:])        # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(nums[:5])       # [0, 1, 2, 3, 4]
print(nums[:-1])      # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
print(nums[4:])       # [4, 5, 6, 7, 8, 9, 10]
print(nums[-3:])      # [8, 9, 10]
print(nums[2:5])      # [2, 3, 4]
print(nums[2:-1])     # [2, 3, 4, 5, 6, 7, 8, 9]
print(nums[-3:-1])    # [8, 9]

スッキリ書ける。

リスト内包表記とlambda

nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

リスト内包表記

[x**2 for x in nums]
# [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

lambda

list(map(lambda x: x**2, nums))
# [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

条件式も使える

[x**2 for x in nums if x % 2 == 0] 
# [4, 16, 36, 64, 100]

filterを使いlambdaでも表現できる

list(map(lambda x: x**2, filter(lambda x: x % 2 == 0, nums)))
# [4, 16, 36, 64, 100]

内包表記は辞書にも使える

chile_ranks = {'ghost': 1, 'habanero': 2, 'cayenne': 3}
{rank: name for name, rank in chile_ranks.items()}
# {1: 'ghost', 2: 'habanero', 3: 'cayenne'}

{len(name) for name in rank_dict.values()}
# {8, 5, 7}

lambdaは少し可読性が下がる。
内包表記はfor文より速い。
・appendメソッドの呼び出し部分のオーバーヘッド
・for文はループする度にリストオブジェクトのappendを参照する
・for文はappendをpythonの関数として実行する

速くて見やすいって最高。

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