looking for a more pythonic way to access the database

Question

I have a bunch of python methods that follow this pattern:

def delete_session(guid):
    conn = get_conn()
    cur = conn.cursor()

    cur.execute("delete from sessions where guid=%s", guid)

    conn.commit()
    conn.close()

Is there a more pythonic way to execute raw sql. The 2 lines at the beginning and end of every method are starting to bother me.

I'm not looking for an orm, I want to stick with raw sql.

Answer 1

You could write a context manager and use the with statement. For example, see this blog post:

http://jessenoller.com/2009/02/03/get-with-the-program-as-contextmanager-completely-different/

Also the python documentation has a sample that pretty much matches your needs. See section 8.1 on this page, in particular the snippet that begins:

db_connection = DatabaseConnection()
with db_connection as cursor:
    cursor.execute('insert into ...')
    cursor.execute('delete from ...')
    # ... more operations ...

• http://www.python.org/doc/2.5.1/whatsnew/pep-343.html

Answer 2

Careful about that execute, the second argument needs to be [guid] (a list with just one item). As for your question, I normally just use a class encapsulating connection and cursor, but it looks like you may prefer to use an execution context object whose __enter__ method gives you a cursor while __leave__ commits or rollbacks depending on whether the termination was normal or by exception; this would make your code

def delete_session():
    with get_cursor() as cur:
        cur.execute(etc etc)

If you like this style, let us know and I'll show you how to write get_cursor. Others will no doubt propose a decorator instead, so you'd write:

@withcursor
def delete_session(cur):
    cur.execute(etc etc)

but I think this makes commit/rollback, among other issues, a bit murkier. Still, if this is your preference, again let us know and I can show you how to write that form, too.

Answer 3

It doesn't have to be more pythonic, just more structured:

def execSql(statement):
    conn = get_conn()
    cur = conn.cursor()
    cur.execute(statement)
    conn.commit()
    conn.close()

def delete_session(guid):
    execSql("delete from sessions where guid=%s"%(guid))

Answer 4

A decorator?

class SqlExec:
   def __init__ (self, f):
      self.f = f
   def __call__ (self, *args):
      conn = get_conn() 
      cur = conn.cursor()
      cur.execute(self.f (*args))
      conn.commit()
      conn.close()

@SqlExec
def delete_session(guid):
      return "delete from sessions where guid=%s" % guid

Answer 5

"I have a bunch of python methods that follow this pattern:"

This is confusing.

Either you have a bunch of functions, or you have a bunch of methods of a class.

Bunch of Functions.

Do this instead.

class SQLFunction( object ):
    def __init__( self, connection ):
        self.connection = connection
    def __call__( self, args=None ):
        self.cursor= self.connection.cursor()
        self.run( args )
        self.cursor.commit()
        self.cursor.close()

class DeleteSession( SQLFunction ):
    def run( self, args ):
        self.cursor.execute( "statement" )

delete_session = DeleteSession( connection )

Your function declarations are two lines longer, but essentially the same. You can do func1( args ) because it's a callable object. The rest of your program should remain unchanged.

Bunch of Methods in One Class.

class SomeClass( object ):
    def __init__( self, connection ):
        self.connection= connection
    def sql_execute( self, statement, args= None )
        self.cursor= self.connection.cursor() 
        self.cursor.execute( statement, args if args is not None else [] )
        self.connection.commit()
        self.cursor.close()
    def delete_session( self ):
        self.sql_execute( "statement" )

All your methods can look like delete_session and make use of a common sql_execute method.