Insert into table in sqlite3 and python. The magic of the “with” keyword

Question

I am following this sqlite3 for python tutorial.

I am trying to insert a row into a table with this script:

conn = sqlite3.connect(database)

sql = ''' INSERT INTO projects(name,begin_date,end_date)
              VALUES(?,?,?) '''    
project = ('X', '2015-01-01', '2015-01-30');

cur = conn.cursor()
cur.execute(sql,project)

conn.close()

I can run it with no errors but the row is not inserted.

Although, the row is inserted when using with :

with conn:
    sql = ''' INSERT INTO projects(name,begin_date,end_date)
                  VALUES(?,?,?) '''    
    project = ('X', '2015-01-01', '2015-01-30');

    cur = conn.cursor()
    cur.execute(sql,project)

Does anyone know whats happening?

UPDATE

I found in docs.python.org that after one or more execute statements one does commit as in

cur.execute(sql, project)
conn.commit()

and everything goes fine.

But still, I would much appreciate some technical explanation of with keyword

Answer 1

But still, I would much appreciate some technical explanation of with keyword

The reason why you should not commit() when using the database connection as a context manager ( with keyword), is because there's a mechanism that auto-commit in such case, if the transaction succeed (meaning that there's no exception raised).

It is explained in the sqlite3 API doc :

Using the connection as a context manager

Connection objects can be used as context managers that automatically commit or rollback transactions. In the event of an exception, the transaction is rolled back; otherwise, the transaction is committed:

 import sqlite3 con = sqlite3.connect(":memory:") con.execute("create table person (id integer primary key, firstname varchar unique)") # Successful, con.commit() is called automatically afterwards with con: con.execute("insert into person(firstname) values (?)", ("Joe",)) # con.rollback() is called after the with block finishes with an exception, the # exception is still raised and must be caught try: with con: con.execute("insert into person(firstname) values (?)", ("Joe",)) except sqlite3.IntegrityError: print("couldn't add Joe twice")