from __future__ import generators

True, False = 1==1, 0==1

import threading
from util.threadinglocal import local as threading_local
import re
import warnings
import atexit
import os
import new
import types
import urllib
import weakref
import inspect
import sqlbuilder
from cache import CacheSet
import col
from joins import sorter
from converters import sqlrepr
import classregistry

warnings.filterwarnings("ignore", "DB-API extension cursor.lastrowid used")

_connections = {}

def _closeConnection(ref):
    conn = ref()
    if conn is not None:
        conn.close()

class DBConnection:

    def __init__(self, name=None, debug=False, debugOutput=False,
                 cache=True, style=None, autoCommit=True,
                 debugThreading=False, registry=None):
        self.name = name
        self.debug = debug
        self.debugOutput = debugOutput
        self.debugThreading = debugThreading
        self.cache = CacheSet(cache=cache)
        self.doCache = cache
        self.style = style
        self._connectionNumbers = {}
        self._connectionCount = 1
        self.autoCommit = autoCommit
        self.registry = registry or None
        classregistry.registry(self.registry).addCallback(
            self.soClassAdded)
        registerConnectionInstance(self)
        atexit.register(_closeConnection, weakref.ref(self))

    def uri(self):
        auth = getattr(self, 'user', None) or ''
        if auth:
            if self.password:
                auth = auth + '@' + self.password
            auth = auth + ':'
        else:
            assert not getattr(self, 'password', None), (
                'URIs cannot express passwords without usernames')
        uri = '%s://%s' % (self.dbName, auth)
        if self.host:
            uri += self.host + '/'
        db = self.db
        if db.startswith('/'):
            db = path[1:]
        return uri + db

    def isSupported(cls):
        raise NotImplemented
    isSupported = classmethod(isSupported)

    def connectionFromURI(cls, uri):
        raise NotImplemented
    connectionFromURI = classmethod(connectionFromURI)

    def _parseURI(uri):
        schema, rest = uri.split(':', 1)
        assert rest.startswith('/'), "URIs must start with scheme:/ -- you did not include a / (in %r)" % rest
        if rest.startswith('/') and not rest.startswith('//'):
            host = None
            rest = rest[1:]
        elif rest.startswith('///'):
            host = None
            rest = rest[3:]
        else:
            rest = rest[2:]
            if rest.find('/') == -1:
                host = rest
                rest = ''
            else:
                host, rest = rest.split('/', 1)
        if host and host.find('@') != -1:
            user, host = host.split('@', 1)
            if user.find(':') != -1:
                user, password = user.split(':', 1)
            else:
                password = None
        else:
            user = password = None
        if host and host.find(':') != -1:
            _host, port = host.split(':')
            try:
                port = int(port)
            except ValueError:
                raise ValueError, "port must be integer, got '%s' instead" % port
            if not (1 <= port <= 65535):
                raise ValueError, "port must be integer in the range 1-65535, got '%d' instead" % port
            host = _host
        else:
            port = None
        path = '/' + rest
        if os.name == 'nt':
            if (len(rest) > 1) and (rest[1] == '|'):
                path = "%s:%s" % (rest[0], rest[2:])
        args = {}
        if path.find('?') != -1:
            path, arglist = path.split('?', 1)
            arglist = arglist.split('&')
            for single in arglist:
                argname, argvalue = single.split('=', 1)
                argvalue = urllib.unquote(argvalue)
                args[argname] = argvalue
        return user, password, host, port, path, args
    _parseURI = staticmethod(_parseURI)

    def soClassAdded(self, soClass):
        """
        This is called for each new class; we use this opportunity
        to create an instance method that is bound to the class
        and this connection.
        """
        name = soClass.__name__
        assert not hasattr(self, name), (
            "Connection %r already has an attribute with the name "
            "%r (and you just created the conflicting class %r)"
            % (self, name, soClass))
        setattr(self, name, ConnWrapper(soClass, self))

class ConnWrapper(object):

    """
    This represents a SQLObject class that is bound to a specific
    connection (instances have a connection instance variable, but
    classes are global, so this is binds the connection variable
    lazily when a class method is accessed)
    """
    # @@: methods that take connection arguments should be explicitly
    # marked up instead of the implicit use of a connection argument
    # and inspect.getargspec()

    def __init__(self, soClass, connection):
        self._soClass = soClass
        self._connection = connection

    def __call__(self, *args, **kw):
        kw['connection'] = self._connection
        return self._soClass(*args, **kw)

    def __getattr__(self, attr):
        meth = getattr(self._soClass, attr)
        if not isinstance(meth, types.MethodType):
            # We don't need to wrap non-methods
            return meth
        try:
            takes_conn = meth.takes_connection
        except AttributeError:
            args, varargs, varkw, defaults = inspect.getargspec(meth)
            assert not varkw and not varargs, (
                "I cannot tell whether I must wrap this method, "
                "because it takes **kw: %r"
                % meth)
            takes_conn = 'connection' in args
            meth.im_func.takes_connection = takes_conn
        if not takes_conn:
            return meth
        return ConnMethodWrapper(meth, self._connection)

class ConnMethodWrapper(object):

    def __init__(self, method, connection):
        self._method = method
        self._connection = connection

    def __getattr__(self, attr):
        return getattr(self._method, attr)

    def __call__(self, *args, **kw):
        kw['connection'] = self._connection
        return self._method(*args, **kw)

    def __repr__(self):
        return '<Wrapped %r with connection %r>' % (
            self._method, self._connection)

class DBAPI(DBConnection):

    """
    Subclass must define a `makeConnection()` method, which
    returns a newly-created connection object.

    ``queryInsertID`` must also be defined.
    """

    dbName = None

    def __init__(self, **kw):
        self._pool = []
        self._poolLock = threading.Lock()
        DBConnection.__init__(self, **kw)
        self._binaryType = type(self.module.Binary(''))

    def _runWithConnection(self, meth, *args):
        conn = self.getConnection()
        try:
            val = meth(conn, *args)
        finally:
            self.releaseConnection(conn)
        return val

    def getConnection(self):
        self._poolLock.acquire()
        try:
            if not self._pool:
                conn = self.makeConnection()
                self._connectionNumbers[id(conn)] = self._connectionCount
                self._connectionCount += 1
            else:
                conn = self._pool.pop()
            if self.debug:
                s = 'ACQUIRE'
                if self._pool is not None:
                    s += ' pool=[%s]' % ', '.join([str(self._connectionNumbers[id(v)]) for v in self._pool])
                self.printDebug(conn, s, 'Pool')
            return conn
        finally:
            self._poolLock.release()

    def releaseConnection(self, conn, explicit=False):
        if self.debug:
            if explicit:
                s = 'RELEASE (explicit)'
            else:
                s = 'RELEASE (implicit, autocommit=%s)' % self.autoCommit
            if self._pool is None:
                s += ' no pooling'
            else:
                s += ' pool=[%s]' % ', '.join([str(self._connectionNumbers[id(v)]) for v in self._pool])
            self.printDebug(conn, s, 'Pool')
        if self.supportTransactions and not explicit:
            if self.autoCommit == 'exception':
                if self.debug:
                    self.printDebug(conn, 'auto/exception', 'ROLLBACK')
                conn.rollback()
                raise Exception, 'Object used outside of a transaction; implicit COMMIT or ROLLBACK not allowed'
            elif self.autoCommit:
                if self.debug:
                    self.printDebug(conn, 'auto', 'COMMIT')
                if not getattr(conn, 'autocommit', False):
                    conn.commit()
            else:
                if self.debug:
                    self.printDebug(conn, 'auto', 'ROLLBACK')
                conn.rollback()
        if self._pool is not None:
            if conn not in self._pool:
                # @@: We can get duplicate releasing of connections with
                # the __del__ in Iteration (unfortunately, not sure why
                # it happens)
                self._pool.insert(0, conn)
        else:
            conn.close()

    def printDebug(self, conn, s, name, type='query'):
        if name == 'Pool' and self.debug != 'Pool':
            return
        if type == 'query':
            sep = ': '
        else:
            sep = '->'
            s = repr(s)
        n = self._connectionNumbers[id(conn)]
        spaces = ' '*(8-len(name))
        if self.debugThreading:
            threadName = threading.currentThread().getName()
            threadName = (':' + threadName + ' '*(8-len(threadName)))
        else:
            threadName = ''
        print '%(n)2i%(threadName)s/%(name)s%(spaces)s%(sep)s %(s)s' % locals()

    def _executeRetry(self, conn, cursor, query):
        if self.debug:
            self.printDebug(conn, query, 'QueryR')
        return cursor.execute(query)


    def _query(self, conn, s):
        if self.debug:
            self.printDebug(conn, s, 'Query')
        self._executeRetry(conn, conn.cursor(), s)

    def query(self, s):
        return self._runWithConnection(self._query, s)

    def _queryAll(self, conn, s):
        if self.debug:
            self.printDebug(conn, s, 'QueryAll')
        c = conn.cursor()
        self._executeRetry(conn, c, s)
        value = c.fetchall()
        if self.debugOutput:
            self.printDebug(conn, value, 'QueryAll', 'result')
        return value

    def queryAll(self, s):
        return self._runWithConnection(self._queryAll, s)

    def _queryAllDescription(self, conn, s):
        """
        Like queryAll, but returns (description, rows), where the
        description is cursor.description (which gives row types)
        """
        if self.debug:
            self.printDebug(conn, s, 'QueryAllDesc')
        c = conn.cursor()
        self._executeRetry(conn, c, s)
        value = c.fetchall()
        if self.debugOutput:
            self.printDebug(conn, value, 'QueryAll', 'result')
        return c.description, value

    def queryAllDescription(self, s):
        return self._runWithConnection(self._queryAllDescription, s)

    def _queryOne(self, conn, s):
        if self.debug:
            self.printDebug(conn, s, 'QueryOne')
        c = conn.cursor()
        self._executeRetry(conn, c, s)
        value = c.fetchone()
        if self.debugOutput:
            self.printDebug(conn, value, 'QueryOne', 'result')
        return value

    def queryOne(self, s):
        return self._runWithConnection(self._queryOne, s)

    def _insertSQL(self, table, names, values):
        return ("INSERT INTO %s (%s) VALUES (%s)" %
                (table, ', '.join(names),
                 ', '.join([self.sqlrepr(v) for v in values])))

    def transaction(self):
        return Transaction(self)

    def queryInsertID(self, soInstance, id, names, values):
        return self._runWithConnection(self._queryInsertID, soInstance, id, names, values)

    def iterSelect(self, select):
        return select.IterationClass(self, self.getConnection(),
                         select, keepConnection=False)

    def accumulateSelect(self, select, *expressions):
        """ Apply an accumulate function(s) (SUM, COUNT, MIN, AVG, MAX, etc...)
            to the select object.
        """
        ops = select.ops
        join = ops.get('join')
        if join:
            tables = self._fixTablesForJoins(select)
        else:
            tables = select.tables
        q = "SELECT %s" % ", ".join([str(expression) for expression in expressions])
        q += " FROM %s" % ", ".join(tables)
        if join:
            if tables: q += ','
            q += self._addJoins(select)
        q += " WHERE"
        q = self._addWhereClause(select, q, limit=0, order=0)
        val = self.queryOne(q)
        if len(expressions) == 1:
            val = val[0]
        return val

    def queryForSelect(self, select):
        ops = select.ops
        join = ops.get('join')
        cls = select.sourceClass
        if join:
            tables = self._fixTablesForJoins(select)
        else:
            tables = select.tables
        if ops.get('distinct', False):
            q = 'SELECT DISTINCT '
        else:
            q = 'SELECT '
        if ops.get('lazyColumns', 0):
            q += "%s.%s FROM %s" % \
                 (cls.sqlmeta.table, cls.sqlmeta.idName,
                  ", ".join(tables))
        else:
            columns = ", ".join(["%s.%s" % (cls.sqlmeta.table, col.dbName)
                                 for col in cls.sqlmeta.columnList])
            if columns:
                q += "%s.%s, %s FROM %s" % \
                     (cls.sqlmeta.table, cls.sqlmeta.idName, columns,
                      ", ".join(tables))
            else:
                q += "%s.%s FROM %s" % \
                     (cls.sqlmeta.table, cls.sqlmeta.idName,
                      ", ".join(tables))

        if join:
            if tables: q += ','
            q += self._addJoins(select)
        q += " WHERE"
        return self._addWhereClause(select, q)

    def _fixTablesForJoins(self, select):
        ops = select.ops
        join = ops.get('join')
        tables = select.tables
        if type(join) is str:
            return tables
        else:
            tables = tables[:] # maka a copy for modification
            if isinstance(join, sqlbuilder.SQLJoin):
                if join.table1 in tables: tables.remove(join.table1)
                if join.table2 in tables: tables.remove(join.table2)
            else:
                for j in join:
                    if j.table1 in tables: tables.remove(j.table1)
                    if j.table2 in tables: tables.remove(j.table2)
            return tables

    def _addJoins(self, select):
        ops = select.ops
        join = ops.get('join')
        if type(join) is str:
            pass
        elif isinstance(join, sqlbuilder.SQLJoin):
            join_str = self.sqlrepr(join)
        else:
            join_str = ", ".join([self.sqlrepr(j) for j in join])
        return ' ' + join_str

    def _addWhereClause(self, select, startSelect, limit=1, order=1):

        q = select.clause
        if type(q) not in [type(''), type(u'')]:
            q = self.sqlrepr(q)
        ops = select.ops

        def clauseList(lst, desc=False):
            if type(lst) not in (type([]), type(())):
                lst = [lst]
            lst = [clauseQuote(i) for i in lst]
            if desc:
                lst = [sqlbuilder.DESC(i) for i in lst]
            return ', '.join([self.sqlrepr(i) for i in lst])

        def clauseQuote(s):
            if type(s) is type(""):
                if s.startswith('-'):
                    desc = True
                    s = s[1:]
                else:
                    desc = False
                assert sqlbuilder.sqlIdentifier(s), "Strings in clauses are expected to be column identifiers.  I got: %r" % s
                if s in select.sourceClass.sqlmeta.columns:
                    s = select.sourceClass.sqlmeta.columns[s].dbName
                if desc:
                    return sqlbuilder.DESC(sqlbuilder.SQLConstant(s))
                else:
                    return sqlbuilder.SQLConstant(s)
            else:
                return s

        if order and ops.get('dbOrderBy'):
            q = "%s ORDER BY %s" % (q, clauseList(ops['dbOrderBy'], ops.get('reversed', False)))

        start = ops.get('start', 0)
        end = ops.get('end', None)

        q = startSelect + ' ' + q

        if limit and (start or end):
            # @@: Raising an error might be an annoyance, but some warning is
            # in order.
            #assert ops.get('orderBy'), "Getting a slice of an unordered set is unpredictable!"
            q = self._queryAddLimitOffset(q, start, end)

        return q

    def _SO_createJoinTable(self, join):
        self.query(self._SO_createJoinTableSQL(join))

    def _SO_createJoinTableSQL(self, join):
        return ('CREATE TABLE %s (\n%s %s,\n%s %s\n)' %
                (join.intermediateTable,
                 join.joinColumn,
                 self.joinSQLType(join),
                 join.otherColumn,
                 self.joinSQLType(join)))

    def _SO_dropJoinTable(self, join):
        self.query("DROP TABLE %s" % join.intermediateTable)

    def _SO_createIndex(self, soClass, index):
        self.query(self.createIndexSQL(soClass, index))

    def createIndexSQL(self, soClass, index):
        assert 0, 'Implement in subclasses'

    def createTable(self, soClass):
        createSql, constraints = self.createTableSQL(soClass)
        self.query(createSql)

        return constraints

    def createReferenceConstraints(self, soClass):
        refConstraints = [self.createReferenceConstraint(soClass, column) \
                          for column in soClass.sqlmeta.columnList \
                          if isinstance(column, col.SOForeignKey)]
        refConstraintDefs = [constraint \
                             for constraint in refConstraints \
                             if constraint]
        return refConstraintDefs

    def createTableSQL(self, soClass):
        constraints = self.createReferenceConstraints(soClass)
        createSql = ('CREATE TABLE %s (\n%s\n)' %
                (soClass.sqlmeta.table, self.createColumns(soClass)))
        return createSql, constraints

    def createColumns(self, soClass):
        columnDefs = [self.createIDColumn(soClass)] \
                     + [self.createColumn(soClass, col)
                        for col in soClass.sqlmeta.columnList]
        return ",\n".join(["    %s" % c for c in columnDefs])

    def createReferenceConstraint(self, soClass, col):
        assert 0, "Implement in subclasses"

    def createColumn(self, soClass, col):
        assert 0, "Implement in subclasses"

    def dropTable(self, tableName, cascade=False):
        self.query("DROP TABLE %s" % tableName)

    def clearTable(self, tableName):
        # 3-03 @@: Should this have a WHERE 1 = 1 or similar
        # clause?  In some configurations without the WHERE clause
        # the query won't go through, but maybe we shouldn't override
        # that.
        self.query("DELETE FROM %s" % tableName)

    def createBinary(self, value):
        """
        Create a binary object wrapper for the given database.
        """
        # Default is Binary() function from the connection driver.
        return self.module.Binary(value)

    # The _SO_* series of methods are sorts of "friend" methods
    # with SQLObject.  They grab values from the SQLObject instances
    # or classes freely, but keep the SQLObject class from accessing
    # the database directly.  This way no SQL is actually created
    # in the SQLObject class.

    def _SO_update(self, so, values):
        self.query("UPDATE %s SET %s WHERE %s = %s" %
                   (so.sqlmeta.table,
                    ", ".join(["%s = %s" % (dbName, self.sqlrepr(value))
                               for dbName, value in values]),
                    so.sqlmeta.idName,
                    self.sqlrepr(so.id)))

    def _SO_selectOne(self, so, columnNames):
        columns = ", ".join(columnNames)
        if columns:
            return self.queryOne(
                "SELECT %s FROM %s WHERE %s = %s" %
                (columns,
                 so.sqlmeta.table,
                 so.sqlmeta.idName,
                 self.sqlrepr(so.id)))
        else:
            return self.queryOne(
                "SELECT NULL FROM %s WHERE %s = %s" %
                (so.sqlmeta.table,
                 so.sqlmeta.idName,
                 self.sqlrepr(so.id)))

    def _SO_selectOneAlt(self, cls, columnNames, column, value):
        return self.queryOne("SELECT %s FROM %s WHERE %s = %s" %
                             (", ".join(columnNames),
                              cls.sqlmeta.table,
                              column,
                              self.sqlrepr(value)))

    def _SO_delete(self, so):
        self.query("DELETE FROM %s WHERE %s = %s" %
                   (so.sqlmeta.table,
                    so.sqlmeta.idName,
                    self.sqlrepr(so.id)))

    def _SO_selectJoin(self, soClass, column, value):
        return self.queryAll("SELECT %s FROM %s WHERE %s = %s" %
                             (soClass.sqlmeta.idName,
                              soClass.sqlmeta.table,
                              column,
                              self.sqlrepr(value)))

    def _SO_intermediateJoin(self, table, getColumn, joinColumn, value):
        return self.queryAll("SELECT %s FROM %s WHERE %s = %s" %
                             (getColumn,
                              table,
                              joinColumn,
                              self.sqlrepr(value)))

    def _SO_intermediateDelete(self, table, firstColumn, firstValue,
                               secondColumn, secondValue):
        self.query("DELETE FROM %s WHERE %s = %s AND %s = %s" %
                   (table,
                    firstColumn,
                    self.sqlrepr(firstValue),
                    secondColumn,
                    self.sqlrepr(secondValue)))

    def _SO_intermediateInsert(self, table, firstColumn, firstValue,
                               secondColumn, secondValue):
        self.query("INSERT INTO %s (%s, %s) VALUES (%s, %s)" %
                   (table,
                    firstColumn,
                    secondColumn,
                    self.sqlrepr(firstValue),
                    self.sqlrepr(secondValue)))

    def _SO_columnClause(self, soClass, kw):
        ops = {None: "IS"}
        data = {}
        if 'id' in kw:
            data[soClass.sqlmeta.idName] = kw['id']
        for key, col in soClass.sqlmeta.columns.items():
            if key in kw:
                data[col.dbName] = kw[key]
            elif col.foreignName in kw:
                obj = kw[col.foreignName]
                if obj is None:
                    data[col.dbName] = None
                else:
                    data[col.dbName] = obj.id
        return ' AND '.join(
            ['%s %s %s' %
             (dbName, ops.get(value, "="), self.sqlrepr(value))
             for dbName, value
             in data.items()])

    def sqlrepr(self, v):
        return sqlrepr(v, self.dbName)

    def __del__(self):
        self.close()

    def close(self):
        if not hasattr(self, '_pool'):
            # Probably there was an exception while creating this
            # instance, so it is incomplete.
            return
        if not self._pool:
            return
        self._poolLock.acquire()
        try:
            conns = self._pool[:]
            self._pool[:] = []
            for conn in conns:
                try:
                    conn.close()
                except self.module.Error:
                    pass
            del conn
            del conns
        finally:
            self._poolLock.release()

    def createEmptyDatabase(self):
        """
        Create an empty database.
        """
        raise NotImplementedError

class Iteration(object):

    def __init__(self, dbconn, rawconn, select, keepConnection=False):
        self.dbconn = dbconn
        self.rawconn = rawconn
        self.select = select
        self.keepConnection = keepConnection
        self.cursor = rawconn.cursor()
        self.query = self.dbconn.queryForSelect(select)
        if dbconn.debug:
            dbconn.printDebug(rawconn, self.query, 'Select')
        self.dbconn._executeRetry(self.rawconn, self.cursor, self.query)

    def __iter__(self):
        return self

    def next(self):
        result = self.cursor.fetchone()
        if result is None:
            self._cleanup()
            raise StopIteration
        if result[0] is None:
            return None
        if self.select.ops.get('lazyColumns', 0):
            obj = self.select.sourceClass.get(result[0], connection=self.dbconn)
            return obj
        else:
            obj = self.select.sourceClass.get(result[0], selectResults=result[1:], connection=self.dbconn)
            return obj

    def _cleanup(self):
        if getattr(self, 'query', None) is None:
            # already cleaned up
            return
        self.query = None
        if not self.keepConnection:
            self.dbconn.releaseConnection(self.rawconn)
        self.dbconn = self.rawconn = self.select = self.cursor = None

    def __del__(self):
        self._cleanup()

class Transaction(object):

    def __init__(self, dbConnection):
        self._dbConnection = dbConnection
        self._connection = dbConnection.getConnection()
        self._dbConnection._setAutoCommit(self._connection, 0)
        self.cache = CacheSet(cache=dbConnection.doCache)
        self._obsolete = False

    def assertActive(self):
        assert not self._obsolete, "This transaction has already gone through ROLLBACK; begin another transaction"

    def query(self, s):
        self.assertActive()
        return self._dbConnection._query(self._connection, s)

    def queryAll(self, s):
        self.assertActive()
        return self._dbConnection._queryAll(self._connection, s)

    def queryOne(self, s):
        self.assertActive()
        return self._dbConnection._queryOne(self._connection, s)

    def queryInsertID(self, soInstance, id, names, values):
        self.assertActive()
        return self._dbConnection._queryInsertID(
            self._connection, soInstance, id, names, values)

    def iterSelect(self, select):
        self.assertActive()
        # We can't keep the cursor open with results in a transaction,
        # because we might want to use the connection while we're
        # still iterating through the results.
        # @@: But would it be okay for psycopg, with threadsafety
        # level 2?
        return iter(list(select.IterationClass(self, self._connection,
                                   select, keepConnection=True)))

    def commit(self):
        if self._obsolete:
            # @@: is it okay to get extraneous commits?
            return
        if self._dbConnection.debug:
            self._dbConnection.printDebug(self._connection, '', 'COMMIT')
        self._connection.commit()

    def rollback(self):
        if self._obsolete:
            # @@: is it okay to get extraneous rollbacks?
            return
        if self._dbConnection.debug:
            self._dbConnection.printDebug(self._connection, '', 'ROLLBACK')
        subCaches = [(sub, sub.allIDs()) for sub in self.cache.allSubCaches()]
        self._connection.rollback()

        for subCache, ids in subCaches:
            for id in ids:
                inst = subCache.tryGet(id)
                if inst is not None:
                    inst.expire()
        self._makeObsolete()

    def __getattr__(self, attr):
        """
        If nothing else works, let the parent connection handle it.
        Except with this transaction as 'self'.  Poor man's
        acquisition?  Bad programming?  Okay, maybe.
        """
        self.assertActive()
        attr = getattr(self._dbConnection, attr)
        try:
            func = attr.im_func
        except AttributeError:
            if isinstance(attr, ConnWrapper):
                return ConnWrapper(attr._soClass, self)
            else:
                return attr
        else:
            meth = new.instancemethod(func, self, self.__class__)
            return meth

    def _makeObsolete(self):
        self._obsolete = True
        self._dbConnection.releaseConnection(self._connection,
                                             explicit=True)
        self._connection = None

    def begin(self):
        # @@: Should we do this, or should begin() be a no-op when we're
        # not already obsolete?
        assert self._obsolete, "You cannot begin a new transaction session without rolling back this one"
        self._obsolete = False
        self._connection = self._dbConnection.getConnection()

    def __del__(self):
        if self._obsolete:
            return
        self.rollback()

class ConnectionHub(object):

    """
    This object serves as a hub for connections, so that you can pass
    in a ConnectionHub to a SQLObject subclass as though it was a
    connection, but actually bind a real database connection later.
    You can also bind connections on a per-thread basis.

    You must hang onto the original ConnectionHub instance, as you
    cannot retrieve it again from the class or instance.

    To use the hub, do something like::

        hub = ConnectionHub()
        class MyClass(SQLObject):
            _connection = hub

        hub.threadConnection = connectionFromURI('...')

    """

    def __init__(self):
        self.threadingLocal = threading_local()

    def __get__(self, obj, type=None):
        # I'm a little surprised we have to do this, but apparently
        # the object's private dictionary of attributes doesn't
        # override this descriptor.
        if obj and obj.__dict__.has_key('_connection'):
            return obj.__dict__['_connection']
        return self.getConnection()

    def __set__(self, obj, value):
        obj.__dict__['_connection'] = value

    def getConnection(self):
        try:
            return self.threadingLocal.connection
        except AttributeError:
            try:
                return self.processConnection
            except AttributeError:
                raise AttributeError(
                    "No connection has been defined for this thread "
                    "or process")

    def _set_threadConnection(self, value):
        self.threadingLocal.connection = value

    def _get_threadConnection(self):
        return self.threadingLocal.connection

    def _del_threadConnection(self):
        del self.threadingLocal.connection

    threadConnection = property(_get_threadConnection,
                                _set_threadConnection,
                                _del_threadConnection)

class ConnectionURIOpener(object):

    def __init__(self):
        self.schemeBuilders = {}
        self.schemeSupported = {}
        self.instanceNames = {}
        self.cachedURIs = {}

    def registerConnection(self, schemes, builder, isSupported):
        for uriScheme in schemes:
            assert not self.schemeBuilders.has_key(uriScheme) \
                   or self.schemeBuilders[uriScheme] is builder, \
                   "A driver has already been registered for the URI scheme %s" % uriScheme
            self.schemeBuilders[uriScheme] = builder
            self.schemeSupported = isSupported

    def registerConnectionInstance(self, inst):
        if inst.name:
            assert not self.instanceNames.has_key(inst.name) \
                   or self.instanceNames[inst.name] is cls, \
                   "A instance has already been registered with the name %s" % inst.name
            assert inst.name.find(':') == -1, "You cannot include ':' in your class names (%r)" % cls.name
            self.instanceNames[inst.name] = inst

    def connectionForURI(self, uri, **args):
        if args:
            if '?' not in uri:
                uri += '?'
            uri += urllib.urlencode(args)
        if self.cachedURIs.has_key(uri):
            return self.cachedURIs[uri]
        if uri.find(':') != -1:
            scheme, rest = uri.split(':', 1)
            assert self.schemeBuilders.has_key(scheme), (
                   "No SQLObject driver exists for %s (only %s)"
                   % (scheme, ', '.join(self.schemeBuilders.keys())))
            conn = self.schemeBuilders[scheme]().connectionFromURI(uri)
        else:
            # We just have a name, not a URI
            assert self.instanceNames.has_key(uri), \
                   "No SQLObject driver exists under the name %s" % uri
            conn = self.instanceNames[uri]
        # @@: Do we care if we clobber another connection?
        self.cachedURIs[uri] = conn
        return conn

TheURIOpener = ConnectionURIOpener()

registerConnection = TheURIOpener.registerConnection
registerConnectionInstance = TheURIOpener.registerConnectionInstance
connectionForURI = TheURIOpener.connectionForURI