英文:
Closing cursors and exception
问题
在处理SQL游标中的异常时，关闭游标和数据库是一种正确的做法。以下是一种处理异常的方法：
try:my_cursor = my_database.cursor(buffered=True)my_sql_query = "SELECT MY_FIELDS FROM MY_TABLE"my_cursor.execute(my_sql_query)my_selected_rows = my_cursor.fetchall()for rows in my_selected_rows:# 进行一些操作my_cursor.close()my_database.close()except Exception as e:if my_cursor is not None:my_cursor.close()if my_database is not None:my_database.close()
在PyCharm中，您提到的警告 "Local variable might be referenced before assignment" 是因为IDE检测到在异常处理块之外引用了my_cursor和my_database，即使它们在异常情况下可能未分配值。为了解决这个警告，您可以在try块之前初始化这些变量，例如：
my_cursor = Nonemy_database = Nonetry:my_cursor = my_database.cursor(buffered=True)my_sql_query = "SELECT MY_FIELDS FROM MY_TABLE"my_cursor.execute(my_sql_query)my_selected_rows = my_cursor.fetchall()for rows in my_selected_rows:# 进行一些操作my_cursor.close()my_database.close()except Exception as e:if my_cursor is not None:my_cursor.close()if my_database is not None:my_database.close()
这样可以避免警告，并且在异常情况下也能够正常关闭游标和数据库连接。
英文:
What is the "correct" way to deal with exceptions in a code block using a SQL cursor ?
Is it necessary/recommended to close the cursor and the database? If so, what is the correct way to do it?
I use to do it like this:
try:my_cursor = my_database.cursor(buffered=True)my_sql_query = "SELECT MY_FIELDS FROM MY_TABLE"my_cursor.execute(my_sql_query )my_selected_rows = my_cursor.fetchall()for rows in my_selected_rows :do_somethingmy_cursor.close()my_database.close()except Exception as e:if my_cursor is not None:my_cursor.close()if my_database is not None:my_database.close()
In PyCharm, this code rise two warnings "Local variable might be referenced before assignment" for the line if my_cursor is not None and if my_database is not None
答案1
得分: 1
一种解决方法是使用contextlib.ExitStack：
from contextlib import ExitStackwith ExitStack() as stack:my_database = connect(...)stack.callback(my_database.close)my_cursor = my_database.cursor(buffered=True)stack.callback(my_cursor.close)...一旦退出 with 块（无论是正常退出还是由于异常），如果数据库连接或游标的创建没有引发异常，将调用`close`方法。
英文:
One way to solve this is the use of contextlib.ExitStack:
from contextlib import ExitStackwith ExitStack() as stack:my_database = connect(...)stack.callback(my_database.close)my_cursor = my_database.cursor(buffered=True)stack.callback(my_cursor.close)...
As soon as the with-block is left (regardless if regularly or by exception) the close methods are called if the creation of db connection or cursor didn't raise an exception themselves.
答案2
得分: 0
我认为你非常接近了。但是我认为在创建数据库或游标时不应该期望引发异常，所以它们应该在try - except之前创建。这样你就不会收到你提到的警告。
回到第一个问题：最安全的方法是在不再需要它们之后始终关闭游标和连接。
英文:
I think you're very close. However I assume that you should not expect Exception to be raised while creating database or a cursor, so those should be created before try - except. This way you won't get warnings that you've mentioned.
Back to the first question: the most secure approach is to always close both - cursor and connection - after you don't need them anymore.
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