sqlite single transaction without foreign key or triggers

[gert]

I am trying to do this in a single transaction, the 3 separate
statements work fine, but i am screwed if they are not executed
together.

 ########### db.execute('BEGIN') #############
 db.execute('UPDATE users SET uid=? WHERE uid=?',(v['uid'],s.UID))
 db.execute('UPDATE sessions SET uid=? WHERE sid=?',(v['uid'],s.SID))
 # only do this if there is no primary key conflict in the above
 if db.ERROR == None: db.execute('UPDATE groups SET uid=? WHERE uid=?',
(v['uid'],s.UID))
 ########### db.execute('END') #####################

My tables are as follows

CREATE TABLE users (
    uid     VARCHAR(64) PRIMARY KEY,
    name    VARCHAR(64) DEFAULT '',
    adress  VARCHAR(64) DEFAULT '',
    city    VARCHAR(64) DEFAULT '',
    country VARCHAR(64) DEFAULT '',
    phone   VARCHAR(64) DEFAULT '',
    picture BLOB
);

CREATE TABLE groups (
    gid     VARCHAR(64),
    uid     VARCHAR(64),
    PRIMARY KEY(gid,uid),
    FOREIGN KEY(uid) REFERENCES users(uid) ON UPDATE CASCADE ON DELETE
CASCADE
);

CREATE TABLE sessions (
    uid     VARCHAR(64) UNIQUE,
    pwd     VARCHAR(64) DEFAULT '',
    sid     VARCHAR(64) PRIMARY KEY,
    exp     DATETIME,
    FOREIGN KEY(uid) REFERENCES users(uid) ON UPDATE CASCADE ON DELETE
CASCADE
);

What is the python or sql way of doing this kind of things ?