Why do you need to create a cursor when querying a

问题:

I'm completely new to Python's sqlite3 module (and SQL in general for that matter), and this just completely stumps me. The abundant lack of descriptions of cursor objects (rather, their necessity) also seems odd.

This snippet of code is the preferred way of doing things:

import sqlite3
conn = sqlite3.connect("db.sqlite")
c = conn.cursor()
c.execute('''insert into table "users" values ("Jack Bauer", "555-555-5555")''')
conn.commit()
c.close()

This one isn't, even though it works just as well and without the (seemingly pointless) cursor:

import sqlite3
conn = sqlite3.connect("db.sqlite")
conn.execute('''insert into table "users" values ("Jack Bauer", "555-555-5555")''')
conn.commit()

Can anyone tell me why I need a cursor?
It just seems like pointless overhead. For every method in my script that accesses a database, I'm supposed to create and destroy a cursor?
Why not just use the connection object?

回答1:

Just a misapplied abstraction it seems to me. A db cursor is an abstraction, meant for data set traversal.

From Wikipedia article on subject:

In computer science and technology, a database cursor is a control structure that enables traversal over the records in a database. Cursors facilitate subsequent processing in conjunction with the traversal, such as retrieval, addition and removal of database records. The database cursor characteristic of traversal makes cursors akin to the programming language concept of iterator.

And:

Cursors can not only be used to fetch data from the DBMS into an application but also to identify a row in a table to be updated or deleted. The SQL:2003 standard defines positioned update and positioned delete SQL statements for that purpose. Such statements do not use a regular WHERE clause with predicates. Instead, a cursor identifies the row. The cursor must be opened and already positioned on a row by means of FETCH statement.

If you check the docs on Python sqlite module, you can see that a python module cursor is needed even for a CREATE TABLE statement, so it's used for cases where a mere connection object should suffice - as correctly pointed out by the OP. Such abstraction is different from what people understand a db cursor to be and hence, the confusion/frustration on the part of users. Regardless of efficiency, it's just a conceptual overhead. Would be nice if it was pointed out in the docs that the python module cursor is bit different than what a cursor is in SQL and databases.

回答2:

You need a cursor object to fetch results. Your example works because it's an INSERT and thus you aren't trying to get any rows back from it, but if you look at the sqlite3 docs, you'll notice that there aren't any .fetchXXXX methods on connection objects, so if you tried to do a SELECT without a cursor, you'd have no way to get the resulting data.

Cursor objects allow you to keep track of which result set is which, since it's possible to run multiple queries before you're done fetching the results of the first.

回答3:

According to the official docs connection.execute() is a nonstandard shortcut that creates an intermediate cursor object:

Connection.execute
This is a nonstandard shortcut that creates a cursor object by calling the cursor() method, calls the cursor’s execute() method with the parameters given, and returns the cursor.

回答4:

12.6.8. Using sqlite3 efficiently

12.6.8.1. Using shortcut methods

Using the nonstandard execute(), executemany() and executescript() methods of the Connection object, your code can be written more concisely because you don’t have to create the (often superfluous) Cursor objects explicitly. Instead, the Cursor objects are created implicitly and these shortcut methods return the cursor objects. This way, you can execute a SELECT statement and iterate over it directly using only a single call on the Connection object.

(sqlite3 documentation; emphasis mine.)

Why not just use the connection object?

Because those methods of the connection object are nonstandard, i.e. they are not part of Python Database API Specification v2.0 (PEP 249).

As long as you use the standard methods of the Cursor object, you can be sure that if you switch to another database implementation that follows the above specification, your code will be fully portable. Perhaps you will only need to change the import line.

But if you use the connection.execute there is a chance that switching won't be that straightforward. That's the main reasons you might want to use cursor.execute instead.

However if you are certain that you're not going to switch, I'd say it's completely OK to take the connection.execute shortcut and be "efficient".

回答5:

It gives us the ability to have multiple separate working environments through the same connection to the database.