在開發中遇到了一個奇葩的問題，設計的資料庫其中有Column以雙下劃線開頭，導致在執行中出現錯誤。

現在以例子來複現並解決錯誤。

首先建立資料表

CREATE TABLE student (
    id INT PRIMARY KEY AUTO_INCREMENT,
    __name VARCHAR(16),
    age INT
);

該表為包含主鍵id，和另外兩個column（__name、age）的student資料表，其中一個column的鍵值以雙下劃線開頭。

mysql> DESC student;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| id     | int(11)     | NO   | PRI | NULL    | auto_increment |
| __name | varchar(16) | YES  |     | NULL    |                |
| age    | int(11)     | YES  |     | NULL    |                |
+--------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
 

現在我們向其中插入兩條資料後檢視資料表

mysql> SELECT * FROM student;
+----+--------+------+
| id | __name | age  |
+----+--------+------+
|  1 | zhang3 |   12 |
|  2 | wang2  |   10 |
+----+--------+------+
2 rows in set (0.00 sec)

這個時候我們就可以建立sql.py檔案通過sqlalchemy連線我們的資料庫

from sqlalchemy import Column, String, Integer, create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

# 建立物件的基類:
Base = declarative_base()

# 定義User物件:
class Student(Base):
    # 表的名字:
    __tablename__ = 'student'

    # 表的結構:
    id = Column(Integer, primary_key=True, autoincrement=True)
    __name = Column(String(20))
    age = Column(Integer)

# 初始化資料庫連線:
engine = create_engine('mysql+mysqlconnector://root:
 
[email protected]:3306/test')
# 建立DBSession型別:
DBSession = sessionmaker(bind=engine)

# 建立session物件並查詢
session = DBSession()
students = session.query(Student).filter().all()

print(students)

執行之後我們會發現出錯了

Traceback (most recent call last):
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 1193, in _execute_context
    context)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
  File "C:\Python\Python36\lib\site-packages\mysql\connector\cursor.py", line 566, in execute
    self._handle_result(self._connection.cmd_query(stmt))
  File "C:\Python\Python36\lib\site-packages\mysql\connector\connection.py", line 530, in cmd_query
    result = self._handle_result(self._send_cmd(ServerCmd.QUERY, query))
  File "C:\Python\Python36\lib\site-packages\mysql\connector\connection.py", line 432, in _handle_result
    raise errors.get_exception(packet)
mysql.connector.errors.ProgrammingError: 1054 (42S22): Unknown column 'student._Student__name' in 'field list'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:/Users/A/Desktop/db.py", line 25, in <module>
    students = session.query(Student).filter().all()
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\orm\query.py", line 2737, in all
    return list(self)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\orm\query.py", line 2889, in __iter__
    return self._execute_and_instances(context)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\orm\query.py", line 2912, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\sql\elements.py", line 269, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 1060, in _execute_clauseelement
    compiled_sql, distilled_params
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 1200, in _execute_context
    context)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\util\compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\util\compat.py", line 186, in reraise
    raise value.with_traceback(tb)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\base.py", line 1193, in _execute_context
    context)
  File "C:\Python\Python36\lib\site-packages\sqlalchemy\engine\default.py", line 507, in do_execute
    cursor.execute(statement, parameters)
  File "C:\Python\Python36\lib\site-packages\mysql\connector\cursor.py", line 566, in execute
    self._handle_result(self._connection.cmd_query(stmt))
  File "C:\Python\Python36\lib\site-packages\mysql\connector\connection.py", line 530, in cmd_query
    result = self._handle_result(self._send_cmd(ServerCmd.QUERY, query))
  File "C:\Python\Python36\lib\site-packages\mysql\connector\connection.py", line 432, in _handle_result
    raise errors.get_exception(packet)
sqlalchemy.exc.ProgrammingError: (mysql.connector.errors.ProgrammingError) 1054 (42S22): Unknown column 'student._Student__name' in 'field list' [SQL: 'SELECT student.id AS student_id, student.`_Student__name` AS `student__Student__name`, student.age AS student_age \nFROM student'] (Background on this error at: http://sqlalche.me/e/f405)
 

檢查錯誤，將會發現Unknown column 'student._Student__name'

明明定義的column是student.__name，為什變成了student._Student__name？

這就要從python的類說起了，python的類並沒有關於私有變數的定義，但是有個近似私有變數的處理。

這裡節選一段

>> Any identifier of the form__spam (at least two leading underscores, at most one trailing underscore)is textually replaced with _classname__spam, where classname is thecurrent class name with leading underscore(s) stripped

是說類的定義中任何以雙下劃線開頭的變數都將被重新命名為 _類名__變數名 的形式

所以回到sqlalchemy的問題，就會發現student.__name被重新命名為了student._Student__name，而資料表中並沒有_Student__name，導致查詢出錯。

在不能夠改變資料庫的前提下如何修復這個錯誤呢？其實很簡單，只要將變數放在類定義外面就好了。

重新修改一下sql.py

from sqlalchemy import Column, String, Integer, create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

# 建立物件的基類:
Base = declarative_base()

# 定義User物件:
class Student(Base):
    # 表的名字:
    __tablename__ = 'student'
    # 表的結構:
    id = Column(Integer, primary_key=True, autoincrement=True)


# 修該此處,將帶有雙下劃線的變數放在類外
Student.__name = Column(String(20))
Student.age = Column(Integer)

# 初始化資料庫連線:
engine = create_engine('mysql+mysqlconnector://root:[email protected]:3306/test')
# 建立DBSession型別:
DBSession = sessionmaker(bind=engine)

# 建立session物件並查詢
session = DBSession()
students = session.query(Student).filter().all()

for student in students:
    print(student.__name)

此時再次執行程式碼

zhang3
wang2