Merge branch 'no-set-default-session'

10 files changed, 360 insertions, 313 deletions

diff --git a/NEWS b/NEWS
index 42943a4..b87c110 100644
--- a/NEWS
+++ b/NEWS
@@ -35,6 +35,10 @@ New features:
   (:ticket:`#491`).
 - Added ``async_`` as an alias for ``async`` to support Python 3.7 where
   ``async`` will become a keyword (:ticket:`#495`).
+- Unless in autocommit, do not use :sql:`default_transaction_*` settings to
+  control the session characteristics as it may create problems with external
+  connection pools such as pgbouncer; use :sql:`BEGIN` options instead
+  (:ticket:`#503`).
 
 Bug fixes:
 
@@ -46,6 +50,11 @@ Other changes:
 - Dropped support for Python 2.5 and 3.1.
 - Dropped support for client library older than PostgreSQL 9.1 (but older
   server versions are still supported).
+- `~connection.isolation_level` doesn't read from the database but will return
+  `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT` if no value was set on the
+  connection.
+- `~connection.set_isolation_level()` will throw an exception if executed
+  inside a transaction; previously it would have silently rolled it back.
 
 
 What's new in psycopg 2.6.3
diff --git a/doc/src/connection.rst b/doc/src/connection.rst
index 0bc584c..2adad59 100644
--- a/doc/src/connection.rst
+++ b/doc/src/connection.rst
@@ -400,6 +400,32 @@ The ``connection`` class
 
         .. versionadded:: 2.4.2
 
+        .. versionchanged:: 2.7
+            Before this version, the function would have set
+            :sql:`default_transaction_*` attribute in the current session;
+            this implementation has the problem of not playing well with
+            external connection pooling working at transaction level and not
+            resetting the state of the session: changing the default
+            transaction would pollute the connections in the pool and create
+            problems to other applications using the same pool.
+
+            Starting from 2.7, if the connection is not autocommit, the
+            transaction characteristics are issued together with :sql:`BEGIN`
+            and will leave the :sql:`default_transaction_*` settings untouched.
+            For example::
+
+                conn.set_session(readonly=True)
+
+            will not change :sql:`default_transaction_read_only`, but
+            following transaction will start with a :sql:`BEGIN READ ONLY`.
+            Conversely, using::
+
+                conn.set_session(readonly=True, autocommit=True)
+
+            will set :sql:`default_transaction_read_only` to :sql:`on` and
+            rely on the server to apply the read only state to whatever
+            transaction, implicit or explicit, is executed in the connection.
+
 
     .. attribute:: autocommit
 
@@ -428,32 +454,54 @@ The ``connection`` class
         .. versionadded:: 2.4.2
 
 
-    .. attribute:: isolation_level
     .. method:: set_isolation_level(level)
 
         .. note::
 
-            From version 2.4.2, `set_session()` and `autocommit`, offer
+            From version 2.4.2, `set_session()` and `autocommit` offer
             finer control on the transaction characteristics.
 
-        Read or set the `transaction isolation level`_ for the current session.
+        Set the `transaction isolation level`_ for the current session.
         The level defines the different phenomena that can happen in the
         database between concurrent transactions.
 
-        The value set or read is an integer: symbolic constants are defined in
+        The value set is an integer: symbolic constants are defined in
         the module `psycopg2.extensions`: see
         :ref:`isolation-level-constants` for the available values.
 
-        The default level is :sql:`READ COMMITTED`: at this level a
-        transaction is automatically started the first time a database command
-        is executed.  If you want an *autocommit* mode, switch to
-        `~psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT` before
+        The default level is `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT`:
+        at this level a transaction is automatically started the first time a
+        database command is executed.  If you want an *autocommit* mode,
+        switch to `~psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT` before
         executing any command::
 
             >>> conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
 
         See also :ref:`transactions-control`.
 
+        .. versionchanged:: 2.7
+
+            the function must be called outside a transaction; previously it
+            would have executed an implicit :sql:`ROLLBACK`; it will now raise
+            an exception.
+
+
+    .. attribute:: isolation_level
+
+        Read the `transaction isolation level`_ for the current session.  The
+        value is one of the :ref:`isolation-level-constants` defined in the
+        `psycopg2.extensions` module.
+
+        .. versionchanged:: 2.7
+
+            the default value for `!isolation_level` is
+            `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT`; previously the
+            property would have queried the server and returned the real value
+            applied. To know this value you can run a query such as :sql:`show
+            transaction_isolation`. Usually the default value is `READ
+            COMMITTED`, but this may be changed in the server configuration.
+
+
     .. index::
         pair: Client; Encoding
 
diff --git a/doc/src/extensions.rst b/doc/src/extensions.rst
index 8d70ba3..ae40b72 100644
--- a/doc/src/extensions.rst
+++ b/doc/src/extensions.rst
@@ -567,15 +567,16 @@ Isolation level constants
 -------------------------
 
 Psycopg2 `connection` objects hold informations about the PostgreSQL
-`transaction isolation level`_.  The current transaction level can be read
-from the `~connection.isolation_level` attribute.  The default isolation
-level is :sql:`READ COMMITTED`.  A different isolation level con be set
-through the `~connection.set_isolation_level()` method.  The level can be
-set to one of the following constants:
+`transaction isolation level`_.  By default Psycopg doesn't change the default
+configuration of the server (`ISOLATION_LEVEL_DEFAULT`); the default for
+PostgreSQL servers is typically :sql:`READ COMMITTED`, but this may be changed
+in the server configuration files.  A different isolation level can be set
+through the `~connection.set_isolation_level()` or `~connection.set_session()`
+methods.  The level can be set to one of the following constants:
 
 .. data:: ISOLATION_LEVEL_AUTOCOMMIT
 
-    No transaction is started when command are issued and no
+    No transaction is started when commands are executed and no
     `~connection.commit()` or `~connection.rollback()` is required.
     Some PostgreSQL command such as :sql:`CREATE DATABASE` or :sql:`VACUUM`
     can't run into a transaction: to run such command use::
@@ -651,6 +652,16 @@ set to one of the following constants:
 
         .. __: http://www.postgresql.org/docs/current/static/transaction-iso.html#XACT-SERIALIZABLE
 
+.. data:: ISOLATION_LEVEL_DEFAULT
+
+    A new transaction is started at the first `~cursor.execute()` command, but
+    the isolation level is not explicitly selected by Psycopg: the server will
+    use whatever level is defined in its configuration or by statements
+    executed within the session outside Pyscopg control.  If you want to know
+    what the value is you can use a query such as :sql:`show
+    transaction_isolation`.
+
+    .. versionadded:: 2.7
 
 
 .. index::
diff --git a/doc/src/usage.rst b/doc/src/usage.rst
index 1366485..6cb038b 100644
--- a/doc/src/usage.rst
+++ b/doc/src/usage.rst
@@ -676,8 +676,7 @@ commands executed will be immediately committed and no rollback is possible. A
 few commands (e.g. :sql:`CREATE DATABASE`, :sql:`VACUUM`...) require to be run
 outside any transaction: in order to be able to run these commands from
 Psycopg, the connection must be in autocommit mode: you can use the
-`~connection.autocommit` property (`~connection.set_isolation_level()` in
-older versions).
+`~connection.autocommit` property.
 
 .. warning::
 
diff --git a/lib/extensions.py b/lib/extensions.py
index b123e88..f4dc706 100644
--- a/lib/extensions.py
+++ b/lib/extensions.py
@@ -72,6 +72,7 @@ ISOLATION_LEVEL_READ_UNCOMMITTED = 4
 ISOLATION_LEVEL_READ_COMMITTED = 1
 ISOLATION_LEVEL_REPEATABLE_READ = 2
 ISOLATION_LEVEL_SERIALIZABLE = 3
+ISOLATION_LEVEL_DEFAULT = 5
 
 
 """psycopg connection status values."""
diff --git a/psycopg/connection.h b/psycopg/connection.h
index 2e2d51d..65efcaf 100644
--- a/psycopg/connection.h
+++ b/psycopg/connection.h
@@ -38,6 +38,12 @@ extern "C" {
 #define ISOLATION_LEVEL_READ_COMMITTED      1
 #define ISOLATION_LEVEL_REPEATABLE_READ     2
 #define ISOLATION_LEVEL_SERIALIZABLE        3
+#define ISOLATION_LEVEL_DEFAULT             5
+
+/* 3-state values on/off/default */
+#define STATE_OFF               0
+#define STATE_ON                1
+#define STATE_DEFAULT           2
 
 /* connection status */
 #define CONN_STATUS_SETUP       0
@@ -129,6 +135,11 @@ struct connectionObject {
      * codecs.getdecoder('utf8') */
     PyObject *pyencoder;        /* python codec encoding function */
     PyObject *pydecoder;        /* python codec decoding function */
+
+    /* Values for the transactions characteristics */
+    int isolevel;
+    int readonly;
+    int deferrable;
 };
 
 /* map isolation level values into a numeric const */
@@ -155,11 +166,8 @@ HIDDEN void conn_close(connectionObject *self);
 HIDDEN void conn_close_locked(connectionObject *self);
 RAISES_NEG HIDDEN int  conn_commit(connectionObject *self);
 RAISES_NEG HIDDEN int  conn_rollback(connectionObject *self);
-RAISES_NEG HIDDEN int  conn_set_session(connectionObject *self, const char *isolevel,
-                                 const char *readonly, const char *deferrable,
-                                 int autocommit);
-HIDDEN int  conn_set_autocommit(connectionObject *self, int value);
-RAISES_NEG HIDDEN int  conn_switch_isolation_level(connectionObject *self, int level);
+RAISES_NEG HIDDEN int conn_set_session(connectionObject *self, int autocommit,
+        int isolevel, int readonly, int deferrable);
 RAISES_NEG HIDDEN int  conn_set_client_encoding(connectionObject *self, const char *enc);
 HIDDEN int  conn_poll(connectionObject *self);
 RAISES_NEG HIDDEN int  conn_tpc_begin(connectionObject *self, xidObject *xid);
diff --git a/psycopg/connection_int.c b/psycopg/connection_int.c
index f92a658..e6906eb 100644
--- a/psycopg/connection_int.c
+++ b/psycopg/connection_int.c
@@ -34,18 +34,36 @@
 
 #include <string.h>
 
-/* Mapping from isolation level name to value exposed by Python.
- *
- * Note: ordering matters: to get a valid pre-PG 8 level from one not valid,
- * we increase a pointer in this list by one position. */
-const IsolationLevel conn_isolevels[] = {
-    {"",                    ISOLATION_LEVEL_AUTOCOMMIT},
-    {"read uncommitted",    ISOLATION_LEVEL_READ_UNCOMMITTED},
-    {"read committed",      ISOLATION_LEVEL_READ_COMMITTED},
-    {"repeatable read",     ISOLATION_LEVEL_REPEATABLE_READ},
-    {"serializable",        ISOLATION_LEVEL_SERIALIZABLE},
-    {"default",            -1},     /* never to be found on the server */
-    { NULL }
+/* String indexes match the ISOLATION_LEVEL_* consts */
+const char *srv_isolevels[] = {
+    NULL, /* autocommit */
+    "READ COMMITTED",
+    "REPEATABLE READ",
+    "SERIALIZABLE",
+    "READ UNCOMMITTED",
+    "default"       /* only to set GUC, not for BEGIN */
+};
+
+/* Read only false, true */
+const char *srv_readonly[] = {
+    " READ WRITE",
+    " READ ONLY",
+    ""      /* default */
+};
+
+/* Deferrable false, true */
+const char *srv_deferrable[] = {
+    " NOT DEFERRABLE",
+    " DEFERRABLE",
+    ""      /* default */
+};
+
+/* On/Off/Default GUC states
+ */
+const char *srv_state_guc[] = {
+    "off",
+    "on",
+    "default"
 };
 
 
@@ -553,50 +571,13 @@ exit:
 RAISES_NEG int
 conn_get_isolation_level(connectionObject *self)
 {
-    PGresult *pgres = NULL;
-    char *error = NULL;
-    int rv = -1;
-    char *lname;
-    const IsolationLevel *level;
-
     /* this may get called by async connections too: here's your result */
     if (self->autocommit) {
-        return 0;
+        return ISOLATION_LEVEL_AUTOCOMMIT;
     }
-
-    Py_BEGIN_ALLOW_THREADS;
-    pthread_mutex_lock(&self->lock);
-
-    if (!(lname = pq_get_guc_locked(self, "default_transaction_isolation",
-            &pgres, &error, &_save))) {
-        goto endlock;
-    }
-
-    /* find the value for the requested isolation level */
-    level = conn_isolevels;
-    while ((++level)->name) {
-        if (0 == strcasecmp(level->name, lname)) {
-            rv = level->value;
-            break;
-        }
-    }
-    if (-1 == rv) {
-        error = malloc(256);
-        PyOS_snprintf(error, 256,
-            "unexpected isolation level: '%s'", lname);
-    }
-
-    free(lname);
-
-endlock:
-    pthread_mutex_unlock(&self->lock);
-    Py_END_ALLOW_THREADS;
-
-    if (rv < 0) {
-        pq_complete_error(self, &pgres, &error);
+    else {
+        return self->isolevel;
     }
-
-    return rv;
 }
 
 
@@ -1208,156 +1189,98 @@ conn_rollback(connectionObject *self)
     return res;
 }
 
-RAISES_NEG int
-conn_set_session(connectionObject *self,
-        const char *isolevel, const char *readonly, const char *deferrable,
-        int autocommit)
-{
-    PGresult *pgres = NULL;
-    char *error = NULL;
-    int res = -1;
-
-    Py_BEGIN_ALLOW_THREADS;
-    pthread_mutex_lock(&self->lock);
-
-    if (isolevel) {
-        Dprintf("conn_set_session: setting isolation to %s", isolevel);
-        if ((res = pq_set_guc_locked(self,
-                "default_transaction_isolation", isolevel,
-                &pgres, &error, &_save))) {
-            goto endlock;
-        }
-    }
-
-    if (readonly) {
-        Dprintf("conn_set_session: setting read only to %s", readonly);
-        if ((res = pq_set_guc_locked(self,
-                "default_transaction_read_only", readonly,
-                &pgres, &error, &_save))) {
-            goto endlock;
-        }
-    }
-
-    if (deferrable) {
-        Dprintf("conn_set_session: setting deferrable to %s", deferrable);
-        if ((res = pq_set_guc_locked(self,
-                "default_transaction_deferrable", deferrable,
-                &pgres, &error, &_save))) {
-            goto endlock;
-        }
-    }
-
-    if (self->autocommit != autocommit) {
-        Dprintf("conn_set_session: setting autocommit to %d", autocommit);
-        self->autocommit = autocommit;
-    }
-
-    res = 0;
-
-endlock:
-    pthread_mutex_unlock(&self->lock);
-    Py_END_ALLOW_THREADS;
-
-    if (res < 0) {
-        pq_complete_error(self, &pgres, &error);
-    }
-
-    return res;
-}
-
-int
-conn_set_autocommit(connectionObject *self, int value)
-{
-    Py_BEGIN_ALLOW_THREADS;
-    pthread_mutex_lock(&self->lock);
-
-    self->autocommit = value;
-
-    pthread_mutex_unlock(&self->lock);
-    Py_END_ALLOW_THREADS;
-
-    return 0;
-}
-
-/* conn_switch_isolation_level - switch isolation level on the connection */
 
+/* Change the state of the session */
 RAISES_NEG int
-conn_switch_isolation_level(connectionObject *self, int level)
+conn_set_session(connectionObject *self, int autocommit,
+        int isolevel, int readonly, int deferrable)
 {
+    int rv = -1;
     PGresult *pgres = NULL;
     char *error = NULL;
-    int curr_level;
-    int ret = -1;
 
-    /* use only supported levels on older PG versions */
+    /* Promote an isolation level to one of the levels supported by the server */
     if (self->server_version < 80000) {
-        if (level == ISOLATION_LEVEL_READ_UNCOMMITTED)
-            level = ISOLATION_LEVEL_READ_COMMITTED;
-        else if (level == ISOLATION_LEVEL_REPEATABLE_READ)
-            level = ISOLATION_LEVEL_SERIALIZABLE;
-    }
-
-    if (-1 == (curr_level = conn_get_isolation_level(self))) {
-        return -1;
-    }
-
-    if (curr_level == level) {
-        /* no need to change level */
-        return 0;
+        if (isolevel == ISOLATION_LEVEL_READ_UNCOMMITTED) {
+            isolevel = ISOLATION_LEVEL_READ_COMMITTED;
+        }
+        else if (isolevel == ISOLATION_LEVEL_REPEATABLE_READ) {
+            isolevel = ISOLATION_LEVEL_SERIALIZABLE;
+        }
     }
 
-    /* Emulate the previous semantic of set_isolation_level() using the
-     * functions currently available. */
-
     Py_BEGIN_ALLOW_THREADS;
     pthread_mutex_lock(&self->lock);
 
-    /* terminate the current transaction if any */
-    if ((ret = pq_abort_locked(self, &pgres, &error, &_save))) {
-        goto endlock;
-    }
-
-    if (level == 0) {
-        if ((ret = pq_set_guc_locked(self,
-                "default_transaction_isolation", "default",
-                &pgres, &error, &_save))) {
-            goto endlock;
+    if (autocommit) {
+        /* we are in autocommit state, so no BEGIN will be issued:
+         * configure the session with the characteristics requested */
+        if (isolevel != self->isolevel) {
+            if (0 > pq_set_guc_locked(self,
+                    "default_transaction_isolation", srv_isolevels[isolevel],
+                    &pgres, &error, &_save)) {
+                goto endlock;
+            }
         }
-        self->autocommit = 1;
-    }
-    else {
-        /* find the name of the requested level */
-        const IsolationLevel *isolevel = conn_isolevels;
-        while ((++isolevel)->name) {
-            if (level == isolevel->value) {
-                break;
+        if (readonly != self->readonly) {
+            if (0 > pq_set_guc_locked(self,
+                    "default_transaction_read_only", srv_state_guc[readonly],
+                    &pgres, &error, &_save)) {
+                goto endlock;
             }
         }
-        if (!isolevel->name) {
-            ret = -1;
-            error = strdup("bad isolation level value");
+        if (deferrable != self->deferrable && self->server_version >= 90100) {
+            if (0 > pq_set_guc_locked(self,
+                    "default_transaction_deferrable", srv_state_guc[deferrable],
+                    &pgres, &error, &_save)) {
+                goto endlock;
+            }
+        }
+    }
+    else if (self->autocommit) {
+        /* we are moving from autocommit to not autocommit, so revert the
+         * characteristics to defaults to let BEGIN do its work */
+        if (0 > pq_set_guc_locked(self,
+                "default_transaction_isolation", "default",
+                &pgres, &error, &_save)) {
             goto endlock;
         }
-
-        if ((ret = pq_set_guc_locked(self,
-                "default_transaction_isolation", isolevel->name,
-                &pgres, &error, &_save))) {
+        if (0 > pq_set_guc_locked(self,
+                "default_transaction_read_only", "default",
+                &pgres, &error, &_save)) {
             goto endlock;
         }
-        self->autocommit = 0;
+        if (self->server_version >= 90100) {
+            if (0 > pq_set_guc_locked(self,
+                    "default_transaction_deferrable", "default",
+                    &pgres, &error, &_save)) {
+                goto endlock;
+            }
+        }
     }
 
-    Dprintf("conn_switch_isolation_level: switched to level %d", level);
+    self->autocommit = autocommit;
+    self->isolevel = isolevel;
+    self->readonly = readonly;
+    self->deferrable = deferrable;
+    rv = 0;
 
 endlock:
     pthread_mutex_unlock(&self->lock);
     Py_END_ALLOW_THREADS;
 
-    if (ret < 0) {
+    if (rv < 0) {
         pq_complete_error(self, &pgres, &error);
+        goto exit;
     }
 
-    return ret;
+    Dprintf(
+        "conn_set_session: autocommit %d, isolevel %d, readonly %d, deferrable %d",
+        autocommit, isolevel, readonly, deferrable);
+
+
+exit:
+    return rv;
 }
 
 
diff --git a/psycopg/connection_type.c b/psycopg/connection_type.c
index 2066579..7647056 100644
--- a/psycopg/connection_type.c
+++ b/psycopg/connection_type.c
@@ -36,6 +36,9 @@
 #include <string.h>
 #include <ctype.h>
 
+extern HIDDEN const char *srv_isolevels[];
+extern HIDDEN const char *srv_readonly[];
+extern HIDDEN const char *srv_deferrable[];
 
 /** DBAPI methods **/
 
@@ -444,18 +447,17 @@ exit:
 
 /* parse a python object into one of the possible isolation level values */
 
-extern const IsolationLevel conn_isolevels[];
-
-static const char *
-_psyco_conn_parse_isolevel(connectionObject *self, PyObject *pyval)
+RAISES_NEG static int
+_psyco_conn_parse_isolevel(PyObject *pyval)
 {
-    const IsolationLevel *isolevel = NULL;
+    int rv = -1;
+    long level;
 
     Py_INCREF(pyval);   /* for ensure_bytes */
 
     /* parse from one of the level constants */
     if (PyInt_Check(pyval)) {
-        long level = PyInt_AsLong(pyval);
+        level = PyInt_AsLong(pyval);
         if (level == -1 && PyErr_Occurred()) { goto exit; }
         if (level < 1 || level > 4) {
             PyErr_SetString(PyExc_ValueError,
@@ -463,65 +465,80 @@ _psyco_conn_parse_isolevel(connectionObject *self, PyObject *pyval)
             goto exit;
         }
 
-        isolevel = conn_isolevels;
-        while ((++isolevel)->value != level)
-            ; /* continue */
+        rv = level;
     }
 
     /* parse from the string -- this includes "default" */
+
     else {
-        isolevel = conn_isolevels;
-        while ((++isolevel)->name) {
-            if (!(pyval = psycopg_ensure_bytes(pyval))) {
-                goto exit;
-            }
-            if (0 == strcasecmp(isolevel->name, Bytes_AS_STRING(pyval))) {
+        if (!(pyval = psycopg_ensure_bytes(pyval))) {
+            goto exit;
+        }
+        for (level = 1; level <= 4; level++) {
+            if (0 == strcasecmp(srv_isolevels[level], Bytes_AS_STRING(pyval))) {
+                rv = level;
                 break;
             }
         }
-        if (!isolevel->name) {
-            char msg[256];
-            snprintf(msg, sizeof(msg),
-                "bad value for isolation_level: '%s'", Bytes_AS_STRING(pyval));
-            PyErr_SetString(PyExc_ValueError, msg);
+        if (rv < 0 && 0 == strcasecmp("default", Bytes_AS_STRING(pyval))) {
+            rv = ISOLATION_LEVEL_DEFAULT;
         }
-    }
-
-    /* use only supported levels on older PG versions */
-    if (isolevel && self->server_version < 80000) {
-        if (isolevel->value == ISOLATION_LEVEL_READ_UNCOMMITTED
-            || isolevel->value == ISOLATION_LEVEL_REPEATABLE_READ) {
-            ++isolevel;
+        if (rv < 0) {
+            PyErr_Format(PyExc_ValueError,
+                "bad value for isolation_level: '%s'", Bytes_AS_STRING(pyval));
+            goto exit;
         }
     }
 
 exit:
     Py_XDECREF(pyval);
 
-    return isolevel ? isolevel->name : NULL;
+    return rv;
 }
 
-/* convert True/False/"default" into a C string */
+/* convert False/True/"default" -> 0/1/2 */
 
-static const char *
+RAISES_NEG static int
 _psyco_conn_parse_onoff(PyObject *pyval)
 {
-    int istrue = PyObject_IsTrue(pyval);
-    if (-1 == istrue) { return NULL; }
-    if (istrue) {
-        int cmp;
-        PyObject *pydef;
-        if (!(pydef = Text_FromUTF8("default"))) { return NULL; }
-        cmp = PyObject_RichCompareBool(pyval, pydef, Py_EQ);
-        Py_DECREF(pydef);
-        if (-1 == cmp) { return NULL; }
-        return cmp ? "default" : "on";
+    int rv = -1;
+
+    Py_INCREF(pyval);   /* for ensure_bytes */
+
+    if (PyUnicode_CheckExact(pyval) || Bytes_CheckExact(pyval)) {
+        if (!(pyval = psycopg_ensure_bytes(pyval))) {
+            goto exit;
+        }
+        if (0 == strcasecmp("default", Bytes_AS_STRING(pyval))) {
+            rv = STATE_DEFAULT;
+        }
+        else {
+            PyErr_Format(PyExc_ValueError,
+                "the only string accepted is 'default'; got %s",
+                Bytes_AS_STRING(pyval));
+            goto exit;
+        }
     }
     else {
-        return "off";
+        int istrue;
+        if (0 > (istrue = PyObject_IsTrue(pyval))) { goto exit; }
+        rv = istrue ? STATE_ON : STATE_OFF;
     }
+
+exit:
+    Py_XDECREF(pyval);
+
+    return rv;
 }
 
+#define _set_session_checks(self,what) \
+do { \
+    EXC_IF_CONN_CLOSED(self); \
+    EXC_IF_CONN_ASYNC(self, what); \
+    EXC_IF_IN_TRANSACTION(self, what); \
+    EXC_IF_TPC_PREPARED(self, what); \
+} while(0)
+
 /* set_session - set default transaction characteristics */
 
 #define psyco_conn_set_session_doc \
@@ -536,17 +553,15 @@ psyco_conn_set_session(connectionObject *self, PyObject *args, PyObject *kwargs)
     PyObject *deferrable = Py_None;
     PyObject *autocommit = Py_None;
 
-    const char *c_isolevel = NULL;
-    const char *c_readonly = NULL;
-    const char *c_deferrable = NULL;
+    int c_isolevel = self->isolevel;
+    int c_readonly = self->readonly;
+    int c_deferrable = self->deferrable;
     int c_autocommit = self->autocommit;
 
     static char *kwlist[] =
         {"isolation_level", "readonly", "deferrable", "autocommit", NULL};
 
-    EXC_IF_CONN_CLOSED(self);
-    EXC_IF_CONN_ASYNC(self, set_session);
-    EXC_IF_IN_TRANSACTION(self, set_session);
+    _set_session_checks(self, set_session);
 
     if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|OOOO", kwlist,
             &isolevel, &readonly, &deferrable, &autocommit)) {
@@ -554,13 +569,13 @@ psyco_conn_set_session(connectionObject *self, PyObject *args, PyObject *kwargs)
     }
 
     if (Py_None != isolevel) {
-        if (!(c_isolevel = _psyco_conn_parse_isolevel(self, isolevel))) {
+        if (0 > (c_isolevel = _psyco_conn_parse_isolevel(isolevel))) {
             return NULL;
         }
     }
 
     if (Py_None != readonly) {
-        if (!(c_readonly = _psyco_conn_parse_onoff(readonly))) {
+        if (0 > (c_readonly = _psyco_conn_parse_onoff(readonly))) {
             return NULL;
         }
     }
@@ -571,17 +586,17 @@ psyco_conn_set_session(connectionObject *self, PyObject *args, PyObject *kwargs)
                 " from PostgreSQL 9.1");
             return NULL;
         }
-        if (!(c_deferrable = _psyco_conn_parse_onoff(deferrable))) {
+        if (0 > (c_deferrable = _psyco_conn_parse_onoff(readonly))) {
             return NULL;
         }
     }
+
     if (Py_None != autocommit) {
-        c_autocommit = PyObject_IsTrue(autocommit);
-        if (-1 == c_autocommit) { return NULL; }
+        if (-1 == (c_autocommit = PyObject_IsTrue(autocommit))) { return NULL; }
     }
 
-    if (0 > conn_set_session(self,
-            c_isolevel, c_readonly, c_deferrable, c_autocommit)) {
+    if (0 > conn_set_session(
+                self, c_autocommit, c_isolevel, c_readonly, c_deferrable)) {
         return NULL;
     }
 
@@ -606,9 +621,7 @@ _psyco_conn_autocommit_set_checks(connectionObject *self)
 {
     /* wrapper to use the EXC_IF macros.
      * return NULL in case of error, else whatever */
-    EXC_IF_CONN_CLOSED(self);
-    EXC_IF_CONN_ASYNC(self, autocommit);
-    EXC_IF_IN_TRANSACTION(self, autocommit);
+    _set_session_checks(self, autocommit);
     return Py_None;     /* borrowed */
 }
 
@@ -619,7 +632,10 @@ psyco_conn_autocommit_set(connectionObject *self, PyObject *pyvalue)
 
     if (!_psyco_conn_autocommit_set_checks(self)) { return -1; }
     if (-1 == (value = PyObject_IsTrue(pyvalue))) { return -1; }
-    if (0 != conn_set_autocommit(self, value)) { return -1; }
+    if (0 > conn_set_session(self, value,
+                self->isolevel, self->readonly, self->deferrable)) {
+        return -1;
+    }
 
     return 0;
 }
@@ -651,20 +667,27 @@ psyco_conn_set_isolation_level(connectionObject *self, PyObject *args)
 {
     int level = 1;
 
-    EXC_IF_CONN_CLOSED(self);
-    EXC_IF_CONN_ASYNC(self, set_isolation_level);
-    EXC_IF_TPC_PREPARED(self, set_isolation_level);
+    _set_session_checks(self, set_isolation_level);
 
     if (!PyArg_ParseTuple(args, "i", &level)) return NULL;
 
-    if (level < 0 || level > 4) {
+    if (level < 0 || level > 5) {
         PyErr_SetString(PyExc_ValueError,
             "isolation level must be between 0 and 4");
         return NULL;
     }
 
-    if (conn_switch_isolation_level(self, level) < 0) {
-        return NULL;
+    if (level == 0) {
+        if (0 > conn_set_session(self, 1,
+                ISOLATION_LEVEL_DEFAULT, self->readonly, self->deferrable)) {
+            return NULL;
+        }
+    }
+    else {
+        if (0 > conn_set_session(self, 0,
+                level, self->readonly, self->deferrable)) {
+            return NULL;
+        }
     }
 
     Py_RETURN_NONE;
@@ -1107,6 +1130,9 @@ connection_setup(connectionObject *self, const char *dsn, long int async)
     self->async_status = ASYNC_DONE;
     if (!(self->string_types = PyDict_New())) { goto exit; }
     if (!(self->binary_types = PyDict_New())) { goto exit; }
+    self->isolevel = ISOLATION_LEVEL_DEFAULT;
+    self->readonly = STATE_DEFAULT;
+    self->deferrable = STATE_DEFAULT;
     /* other fields have been zeroed by tp_alloc */
 
     pthread_mutex_init(&(self->lock), NULL);
diff --git a/psycopg/pqpath.c b/psycopg/pqpath.c
index 328a2b2..50bd520 100644
--- a/psycopg/pqpath.c
+++ b/psycopg/pqpath.c
@@ -53,7 +53,9 @@
 #endif
 
 extern HIDDEN PyObject *psyco_DescriptionType;
-
+extern HIDDEN const char *srv_isolevels[];
+extern HIDDEN const char *srv_readonly[];
+extern HIDDEN const char *srv_deferrable[];
 
 /* Strip off the severity from a Postgres error message. */
 static const char *
@@ -479,6 +481,8 @@ int
 pq_begin_locked(connectionObject *conn, PGresult **pgres, char **error,
                 PyThreadState **tstate)
 {
+    const size_t bufsize = 256;
+    char buf[bufsize];
     int result;
 
     Dprintf("pq_begin_locked: pgconn = %p, autocommit = %d, status = %d",
@@ -489,7 +493,24 @@ pq_begin_locked(connectionObject *conn, PGresult **pgres, char **error,
         return 0;
     }
 
-    result = pq_execute_command_locked(conn, "BEGIN", pgres, error, tstate);
+    if (conn->isolevel == ISOLATION_LEVEL_DEFAULT
+            && conn->readonly == STATE_DEFAULT
+            && conn->deferrable == STATE_DEFAULT) {
+        strcpy(buf, "BEGIN");
+    }
+    else {
+        snprintf(buf, bufsize,
+            conn->server_version >= 80000 ?
+                "BEGIN%s%s%s%s" : "BEGIN;SET TRANSACTION%s%s%s%s",
+            (conn->isolevel >= 1 && conn->isolevel <= 4)
+                ? " ISOLATION LEVEL " : "",
+            (conn->isolevel >= 1 && conn->isolevel <= 4)
+                ? srv_isolevels[conn->isolevel] : "",
+            srv_readonly[conn->readonly],
+            srv_deferrable[conn->deferrable]);
+    }
+
+    result = pq_execute_command_locked(conn, buf, pgres, error, tstate);
     if (result == 0)
         conn->status = CONN_STATUS_BEGIN;
 
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 4b72be1..703d8f1 100755
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -488,7 +488,7 @@ class IsolationLevelsTestCase(ConnectingTestCase):
         conn = self.connect()
         self.assertEqual(
             conn.isolation_level,
-            psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
+            psycopg2.extensions.ISOLATION_LEVEL_DEFAULT)
 
     def test_encoding(self):
         conn = self.connect()
@@ -522,14 +522,33 @@ class IsolationLevelsTestCase(ConnectingTestCase):
             got_name = curs.fetchone()[0]
 
             if name is None:
-                curs.execute('show default_transaction_isolation;')
+                curs.execute('show transaction_isolation;')
                 name = curs.fetchone()[0]
 
             self.assertEqual(name, got_name)
             conn.commit()
 
         self.assertRaises(ValueError, conn.set_isolation_level, -1)
-        self.assertRaises(ValueError, conn.set_isolation_level, 5)
+        self.assertRaises(ValueError, conn.set_isolation_level, 6)
+
+    def test_set_isolation_level_default(self):
+        conn = self.connect()
+        curs = conn.cursor()
+
+        conn.autocommit = True
+        curs.execute("set default_transaction_isolation to 'read committed'")
+
+        conn.autocommit = False
+        conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
+        self.assertEqual(conn.isolation_level,
+            psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
+        curs.execute("show transaction_isolation")
+        self.assertEqual(curs.fetchone()[0], "serializable")
+
+        conn.rollback()
+        conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_DEFAULT)
+        curs.execute("show transaction_isolation")
+        self.assertEqual(curs.fetchone()[0], "read committed")
 
     def test_set_isolation_level_abort(self):
         conn = self.connect()
@@ -541,32 +560,14 @@ class IsolationLevelsTestCase(ConnectingTestCase):
         self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_INTRANS,
             conn.get_transaction_status())
 
-        conn.set_isolation_level(
+        # changed in psycopg 2.7
+        self.assertRaises(psycopg2.ProgrammingError,
+            conn.set_isolation_level,
             psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
-        self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_IDLE,
-            conn.get_transaction_status())
-        cur.execute("select count(*) from isolevel;")
-        self.assertEqual(0, cur.fetchone()[0])
-
-        cur.execute("insert into isolevel values (10);")
         self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_INTRANS,
             conn.get_transaction_status())
-        conn.set_isolation_level(
-            psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
-        self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_IDLE,
-            conn.get_transaction_status())
-        cur.execute("select count(*) from isolevel;")
-        self.assertEqual(0, cur.fetchone()[0])
-
-        cur.execute("insert into isolevel values (10);")
-        self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_IDLE,
-            conn.get_transaction_status())
-        conn.set_isolation_level(
-            psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
-        self.assertEqual(psycopg2.extensions.TRANSACTION_STATUS_IDLE,
-            conn.get_transaction_status())
-        cur.execute("select count(*) from isolevel;")
-        self.assertEqual(1, cur.fetchone()[0])
+        self.assertEqual(conn.isolation_level,
+            psycopg2.extensions.ISOLATION_LEVEL_DEFAULT)
 
     def test_isolation_level_autocommit(self):
         cnn1 = self.connect()
@@ -1042,13 +1043,13 @@ class TransactionControlTests(ConnectingTestCase):
         cur = self.conn.cursor()
         self.conn.set_session(
             psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         self.assertEqual(cur.fetchone()[0], 'serializable')
         self.conn.rollback()
 
         self.conn.set_session(
             psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ)
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         if self.conn.server_version > 80000:
             self.assertEqual(cur.fetchone()[0], 'repeatable read')
         else:
@@ -1057,13 +1058,13 @@ class TransactionControlTests(ConnectingTestCase):
 
         self.conn.set_session(
             isolation_level=psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         self.assertEqual(cur.fetchone()[0], 'read committed')
         self.conn.rollback()
 
         self.conn.set_session(
             isolation_level=psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED)
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         if self.conn.server_version > 80000:
             self.assertEqual(cur.fetchone()[0], 'read uncommitted')
         else:
@@ -1073,12 +1074,12 @@ class TransactionControlTests(ConnectingTestCase):
     def test_set_isolation_level_str(self):
         cur = self.conn.cursor()
         self.conn.set_session("serializable")
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         self.assertEqual(cur.fetchone()[0], 'serializable')
         self.conn.rollback()
 
         self.conn.set_session("repeatable read")
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         if self.conn.server_version > 80000:
             self.assertEqual(cur.fetchone()[0], 'repeatable read')
         else:
@@ -1086,12 +1087,12 @@ class TransactionControlTests(ConnectingTestCase):
         self.conn.rollback()
 
         self.conn.set_session("read committed")
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         self.assertEqual(cur.fetchone()[0], 'read committed')
         self.conn.rollback()
 
         self.conn.set_session("read uncommitted")
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         if self.conn.server_version > 80000:
             self.assertEqual(cur.fetchone()[0], 'read uncommitted')
         else:
@@ -1106,57 +1107,57 @@ class TransactionControlTests(ConnectingTestCase):
     def test_set_read_only(self):
         cur = self.conn.cursor()
         self.conn.set_session(readonly=True)
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')
         self.conn.rollback()
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')
         self.conn.rollback()
 
         cur = self.conn.cursor()
         self.conn.set_session(readonly=None)
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')
         self.conn.rollback()
 
         self.conn.set_session(readonly=False)
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'off')
         self.conn.rollback()
 
     def test_set_default(self):
         cur = self.conn.cursor()
-        cur.execute("SHOW default_transaction_isolation;")
-        default_isolevel = cur.fetchone()[0]
-        cur.execute("SHOW default_transaction_read_only;")
-        default_readonly = cur.fetchone()[0]
+        cur.execute("SHOW transaction_isolation;")
+        isolevel = cur.fetchone()[0]
+        cur.execute("SHOW transaction_read_only;")
+        readonly = cur.fetchone()[0]
         self.conn.rollback()
 
         self.conn.set_session(isolation_level='serializable', readonly=True)
         self.conn.set_session(isolation_level='default', readonly='default')
 
-        cur.execute("SHOW default_transaction_isolation;")
-        self.assertEqual(cur.fetchone()[0], default_isolevel)
-        cur.execute("SHOW default_transaction_read_only;")
-        self.assertEqual(cur.fetchone()[0], default_readonly)
+        cur.execute("SHOW transaction_isolation;")
+        self.assertEqual(cur.fetchone()[0], isolevel)
+        cur.execute("SHOW transaction_read_only;")
+        self.assertEqual(cur.fetchone()[0], readonly)
 
     @skip_before_postgres(9, 1)
     def test_set_deferrable(self):
         cur = self.conn.cursor()
         self.conn.set_session(readonly=True, deferrable=True)
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')
-        cur.execute("SHOW default_transaction_deferrable;")
+        cur.execute("SHOW transaction_deferrable;")
         self.assertEqual(cur.fetchone()[0], 'on')
         self.conn.rollback()
-        cur.execute("SHOW default_transaction_deferrable;")
+        cur.execute("SHOW transaction_deferrable;")
         self.assertEqual(cur.fetchone()[0], 'on')
         self.conn.rollback()
 
         self.conn.set_session(deferrable=False)
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')
-        cur.execute("SHOW default_transaction_deferrable;")
+        cur.execute("SHOW transaction_deferrable;")
         self.assertEqual(cur.fetchone()[0], 'off')
         self.conn.rollback()
 
@@ -1258,9 +1259,9 @@ class AutocommitTests(ConnectingTestCase):
         self.assertEqual(self.conn.status, psycopg2.extensions.STATUS_READY)
         self.assertEqual(self.conn.get_transaction_status(),
             psycopg2.extensions.TRANSACTION_STATUS_IDLE)
-        cur.execute("SHOW default_transaction_isolation;")
+        cur.execute("SHOW transaction_isolation;")
         self.assertEqual(cur.fetchone()[0], 'serializable')
-        cur.execute("SHOW default_transaction_read_only;")
+        cur.execute("SHOW transaction_read_only;")
         self.assertEqual(cur.fetchone()[0], 'on')