No.044 [Python] 문자열 목록 사전 변환:ast.literal_eval()

이번에 우리는 문자열 목록과 사전의 변환을 쓸 것이다.

I'll write about "the conversion from strings to lists or dictionaries

■ 문자열 구분자 등을 통해 분할, 목록

 Split or making a list with such as delimiters

 空白ありのカンマで区切られた例

>>> s = "a, b, c"
>>> 
>>> l = s.split(",")
>>> 
>>> print(l)
['a', ' b', ' c']
>>> 
>>> print(type(l))
<class 'list'>

 数字も数値型ではなく文字列となる
 数値をリストにする場合：int()やfloat()とリスト内包表記を組み合わせる

>>> s = "1-2-3"
>>> 
>>> l = s.split("-")
>>> 
>>> print(l)
['1', '2', '3']
>>> 
>>> print(type(l[0]))
<class 'str'>
>>> 
>>> l = [int(c) for c in s.split("-")]
>>> 
>>> print(l)
[1, 2, 3]
>>> 
>>> print(type(l[0]))
<class 'int'>

■ 문자열을 목록이나 사전으로:ast.literal_eval( )

 The conversion from strings to lists or dictionaries

 astモジュールをインポート
 追加のインストールは必要なし

>>> import ast
>>> 
>>> s = '["a","b","c"]'
>>> 
>>> l = ast.literal_eval(s)
>>> 
>>> print(l)
['a', 'b', 'c']
>>> 
>>> print(type(l))
<class 'list'>

 ast.literal_eval()：文字列をPythonのLiteralとして評価
 数値やブール値等を表現する文字列をそのまま型の値に変換する

 ast.literal_eval()は以下のLiteralを変換する
 文字列／バイト列／数／タプル／リスト／辞書／集合／ブール値／None 

>>> s = '{"key10":10, "key20":20}'
>>> 
>>> d = ast.literal_eval(s)
>>> 
>>> print(d)
{'key10': 10, 'key20': 20}
>>> 
>>> print(type(d))
<class 'dict'>
>>> 
>>> s = '{10, 20, 30}'
>>> 
>>> se = ast.literal_eval(s)
>>> 
>>> print(se)
{10, 20, 30}
>>> 
>>> print(type(se))
<class 'set'>

■eval()와ast.literal_eval ()의 차이

 Difference between eval() and ast.literal_eval()

 ◆ eval()とast.literal_eval()の違い
   ① eval(): リテラル、変数およびそれらの演算を含んだ式を評価
   ② ast.literal_eval(): リテラルのみ含む式を評価
   → eval()：+による加算を評価可能 ⇄ ast.literal_eval()評価不可

>>> s = '["z", 1 + 10]'
>>> 
>>> print(eval(s))
['z', 11]
>>> 
>>> print(ast.literal_eval(s))
ValueError: malformed node or string: <_ast.BinOp object at 0x102ef7a20>

 ast.literal_eval()：対象をリテラルのみに限定。そのため、eval()よりも安全

■ json.loads ()와ast.literal_eval ()의 차이

 Difference between json.loads() and ast.literal_eval()

>>> import json
>>> 
>>> s = '{"key1": [10, 20, 20], "key2": "abcdefg"}'
>>> 
>>> print(json.loads(s))
{'key1': [10, 20, 20], 'key2': 'abcdefg'

>>> import ast
>>> 
>>> s = '{"key1": [10, 20, 20], "key2": "abcdefg"}'
>>> 
>>> print(ast.literal_eval(s))
{'key1': [10, 20, 20], 'key2': 'abcdefg'}

 json.loads():JSON形式の文字列が対象
 json仕様から外の文字列変換は不可

 ast.literal_eval()：True, False, Noneの変換可能
 json.loads()：True, False, Noneの変換不可

>>> s = '[True, False, None]'
>>> print(json.loads(s))
Traceback (most recent call last):
  File "<pyshell#86>", line 1, in <module>
    print(json.loads(s))
NameError: name 'json' is not defined
>>> 
>>> print(ast.literal_eval(s))
[True, False, None]

 json.loads():true, false, nullは逆で変換可能

>>> import json
>>> 
>>> s = "[true, false, null]"
>>> 
>>> print(json.loads(s))
[True, False, None]

 json.loads():文字列は二重引用符（ダブルクォーテーション/"）で囲むことがマスト
 ast.literal_eval()：一重引用符(シングル／')でも三重引用符(トリプル／''')、または"""でも可能

>>> s = "{'key1': 'abcdefg', 'key2': '''vwxyz'''}"
>>> 
>>> print(json.loads(s))
json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
>>> 
>>> print(ast.literal_eval(s))
{'key1': 'abcdefg', 'key2': 'vwxyz'}

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