Python 2列上的数组_agg，输出未识别为列表

我在Python中使用psycopg2的一个Postgresql查询中使用了array_agg

我发现的问题是当循环遍历结果行时。 查询生成的列不理解一列是列表列表。相反，它认为这是一个str列表

以下是数据库的一个示例：

+---------+-------------+-----------------+
| student | grade_value |  grade_comment  |
+---------+-------------+-----------------+
| Paul    | 1           | Very good       |
| John    | 1           | Very good       |
| John    | 2           | Average         |
| Mark    | 1           | Very good       |
| Mark    | 3           | Could do better |
| Mark    | 1           | Very good       |
+---------+-------------+-----------------+

我的问题是：

    connection = psycopg2.connect(ls.CONNECTION_STRING)

    cursor = connection.cursor(cursor_factory=RealDictCursor)

    cursor.execute(
        "SELECT student, array_agg('(' || grade_value || ',' || grade_comment || ')') as grades"
        "FROM random_table"
        "GROUP BY student"
    )

    students_grades = cursor.fetchall()
    # This returns something like: RealDictRow([('student', John), ('grades', ['(1,Very good)', '(2,Average)'])]), RealDictRow([('student', Paul), ('grades', ['(1,Very good)'])])

    for student in students_grades:
       for grade in student['grades']:
           print(grade)
           print(type(grade))

末尾的打印（等级）值的格式如下：（1，非常好） 但是查询说类型是字符串。因此，我无法通过键入grade[1]来访问成绩注释。它认为分数是一根弦

---

您知道如何修复此问题吗？

拆分字符串，使其成为一个列表。根据需要应用预处理

for grade in student['grades']:
    # Let's say grade = "(A,Very good)"
    g = grade.replace('(', '').replace(')','') # g --> "A,Very good"
    l = g.split(',') # l --> ["A", "Very good"]
    print(l[1]) # --> "Very good"

在IPython中测试：

In [1]: grade = "(A,Very good)"

In [2]: g = grade.replace('(', '').replace(')', '')

In [3]: l = g.split(',')

In [4]: print(l[0])
A

In [5]: print(l[1])
Very good

编辑：

In [4]: grade = "(A,Very good, but needs some improvement.)"

In [5]: g = grade.replace('(', '').replace(')', '')

In [6]: l = g.split(',', 1)

In [7]: print(l[0])
A

In [8]: print(l[1])
Very good, but needs some improvement.

---

您不需要将字符串集中在array\u agg中，只需将列传递到数组中即可。
Psycopg2将在postgres和python之间转换正确的类型，正如您在输出中看到的那样

students\u grade['grades']

作为列表获取：

cursor.execute("""
select
    student,
    array_agg(array[grade_value, grade_comment]) as grades
from random_table
group by student""")

students_grades = cursor.fetchall()

for students_grade in students_grades:
    print(students_grade['student'])

    for grade in students_grade['grades']:
        print("%s %s" % (type(grade), grade))

输出：

Tom
<class 'list'> ['2', 'Good']
<class 'list'> ['3', 'Very good']
John
<class 'list'> ['2', 'Very good']

Tom
<class 'dict'> {'grade': 2, 'comment': ['Good']}
<class 'dict'> {'grade': 3, 'comment': ['Very good']}
John
<class 'dict'> {'grade': 2, 'comment': ['Very good']}

输出：

Tom
<class 'list'> ['2', 'Good']
<class 'list'> ['3', 'Very good']
John
<class 'list'> ['2', 'Very good']

Tom
<class 'dict'> {'grade': 2, 'comment': ['Good']}
<class 'dict'> {'grade': 3, 'comment': ['Very good']}
John
<class 'dict'> {'grade': 2, 'comment': ['Very good']}

Tom
{'grade'：2，'comment'：['Good']}
{'grade'：3，'comment'：['Very good']}
约翰
{'grade'：2，'comment'：['Very good']}

谢谢你的建议。这个问题是grade_评论可以包含“，”本身，所以我认为它会中断。如果列表中只有两个元素，那么在第一次出现时就拆分。请参阅编辑。这似乎可以完美地处理相同类型的两列。但是，我有以下问题：“连接到PostgreSQL数组类型时出错整数和文本无法匹配”这主要是因为grade_值为int，grade_注释为字符串。经过编辑，在不同类型的情况下，可以使用“json_agg（）”。或者将int强制转换为text/varchar:

array\u agg（array[grade\u value:：text，grade\u comment]）作为grades

。