pep-0249.txt

PEP: 249
Title: Python Database API Specification v2.0
Version: $Revision$
Last-Modified: $Date$
Author: mal@lemburg.com (Marc-André Lemburg)
Discussions-To: db-sig@python.org
Status: Final
Type: Informational
Content-Type: text/x-rst
Created:
Post-History:
Replaces: 248


`Introduction`_
===============

This API has been defined to encourage similarity between the Python
modules that are used to access databases.  By doing this, we hope to
achieve a consistency leading to more easily understood modules, code
that is generally more portable across databases, and a broader reach
of database connectivity from Python.

Comments and questions about this specification may be directed to the
`SIG for Database Interfacing with Python <db-sig@python.org>`__.

For more information on database interfacing with Python and available
packages see the `Database Topic Guide
<http://www.python.org/topics/database/>`__.

This document describes the Python Database API Specification 2.0 and
a set of common optional extensions.  The previous version 1.0 version
is still available as reference, in :PEP:`248`. Package writers are
encouraged to use this version of the specification as basis for new
interfaces.


`Module Interface`_
===================

`Constructors`_
---------------

Access to the database is made available through connection
objects. The module must provide the following constructor for these:

.. _connect:

`connect`_\ ( *parameters...* )
    Constructor for creating a connection to the database.

    Returns a Connection_ Object. It takes a number of parameters
    which are database dependent. [1]_


`Globals`_
----------

These module globals must be defined:

.. _apilevel:

`apilevel`_
    String constant stating the supported DB API level.

    Currently only the strings "``1.0``" and "``2.0``" are allowed.
    If not given, a DB-API 1.0 level interface should be assumed.


.. _threadsafety:

`threadsafety`_
    Integer constant stating the level of thread safety the interface
    supports.  Possible values are:

    ============ =======================================================
    threadsafety Meaning
    ============ =======================================================
               0 Threads may not share the module.
               1 Threads may share the module, but not connections.
               2 Threads may share the module and connections.
               3 Threads may share the module, connections and cursors.
    ============ =======================================================

    Sharing in the above context means that two threads may use a
    resource without wrapping it using a mutex semaphore to implement
    resource locking.  Note that you cannot always make external
    resources thread safe by managing access using a mutex: the
    resource may rely on global variables or other external sources
    that are beyond your control.


.. _paramstyle:

`paramstyle`_
    String constant stating the type of parameter marker formatting
    expected by the interface. Possible values are [2]_:

    ============ ==============================================================
    paramstyle   Meaning
    ============ ==============================================================
    ``qmark``    Question mark style, e.g. ``...WHERE name=?``
    ``numeric``  Numeric, positional style, e.g. ``...WHERE name=:1``
    ``named``    Named style, e.g. ``...WHERE name=:name``
    ``format``   ANSI C printf format codes, e.g. ``...WHERE name=%s``
    ``pyformat`` Python extended format codes, e.g.  ``...WHERE name=%(name)s``
    ============ ==============================================================


`Exceptions`_
-------------

The module should make all error information available through these
exceptions or subclasses thereof:

.. _Warning:

`Warning`_
    Exception raised for important warnings like data truncations
    while inserting, etc. It must be a subclass of the Python
    ``StandardError`` (defined in the module exceptions).


.. _Error:

`Error`_
    Exception that is the base class of all other error
    exceptions. You can use this to catch all errors with one single
    ``except`` statement. Warnings are not considered errors and thus
    should not use this class as base. It must be a subclass of the
    Python ``StandardError`` (defined in the module exceptions).


.. _InterfaceError:

`InterfaceError`_
    Exception raised for errors that are related to the database
    interface rather than the database itself.  It must be a subclass
    of Error_.


.. _DatabaseError:

`DatabaseError`_
    Exception raised for errors that are related to the database.  It
    must be a subclass of Error_.


.. _DataError:

`DataError`_
    Exception raised for errors that are due to problems with the
    processed data like division by zero, numeric value out of range,
    etc. It must be a subclass of DatabaseError_.


.. _OperationalError:

`OperationalError`_
    Exception raised for errors that are related to the database's
    operation and not necessarily under the control of the programmer,
    e.g. an unexpected disconnect occurs, the data source name is not
    found, a transaction could not be processed, a memory allocation
    error occurred during processing, etc.  It must be a subclass of
    DatabaseError_.


.. _IntegrityError:

`IntegrityError`_
    Exception raised when the relational integrity of the database is
    affected, e.g. a foreign key check fails.  It must be a subclass
    of DatabaseError_.


.. _InternalError:

`InternalError`_
    Exception raised when the database encounters an internal error,
    e.g. the cursor is not valid anymore, the transaction is out of
    sync, etc.  It must be a subclass of DatabaseError_.


.. _ProgrammingError:

`ProgrammingError`_
    Exception raised for programming errors, e.g. table not found or
    already exists, syntax error in the SQL statement, wrong number of
    parameters specified, etc.  It must be a subclass of
    DatabaseError_.


.. _NotSupportedError:

`NotSupportedError`_
    Exception raised in case a method or database API was used which
    is not supported by the database, e.g. requesting a
    `.rollback()`_ on a connection that does not support transaction
    or has transactions turned off.  It must be a subclass of
    DatabaseError_.

This is the exception inheritance layout::

    StandardError
    |__Warning
    |__Error
       |__InterfaceError
       |__DatabaseError
          |__DataError
          |__OperationalError
          |__IntegrityError
          |__InternalError
          |__ProgrammingError
          |__NotSupportedError

.. Note::
    The values of these exceptions are not defined. They should give the user
    a fairly good idea of what went wrong, though.


.. _Connection:

`Connection Objects`_
=====================

Connection objects should respond to the following methods.


`Connection methods`_
---------------------

.. .close():
.. _Connection.close:

`.close() <#Connection.close>`_
    Close the connection now (rather than whenever ``.__del__()`` is
    called).

    The connection will be unusable from this point forward; an Error_
    (or subclass) exception will be raised if any operation is
    attempted with the connection. The same applies to all cursor
    objects trying to use the connection.  Note that closing a
    connection without committing the changes first will cause an
    implicit rollback to be performed.


.. _.commit:
.. _.commit():

`.commit`_\ ()
    Commit any pending transaction to the database.

    Note that if the database supports an auto-commit feature, this must be
    initially off. An interface method may be provided to turn it back on.

    Database modules that do not support transactions should implement this
    method with void functionality.


.. _.rollback:
.. _.rollback():

`.rollback`_\ ()
    This method is optional since not all databases provide transaction
    support. [3]_

    In case a database does provide transactions this method causes the
    database to roll back to the start of any pending transaction.  Closing a
    connection without committing the changes first will cause an implicit
    rollback to be performed.


.. _.cursor:

`.cursor`_\ ()
    Return a new Cursor_ Object using the connection.

    If the database does not provide a direct cursor concept, the module will
    have to emulate cursors using other means to the extent needed by this
    specification.  [4]_



.. _Cursor:

`Cursor Objects`_
=================

These objects represent a database cursor, which is used to manage the
context of a fetch operation. Cursors created from the same connection
are not isolated, *i.e.*, any changes done to the database by a cursor
are immediately visible by the other cursors.  Cursors created from
different connections can or can not be isolated, depending on how the
transaction support is implemented (see also the connection's
`.rollback`_\ () and `.commit`_\ () methods).

Cursor Objects should respond to the following methods and attributes.


`Cursor attributes`_
--------------------

.. _.description:

`.description`_
    This read-only attribute is a sequence of 7-item sequences.

    Each of these sequences contains information describing one result
    column:

    * ``name``
    * ``type_code``
    * ``display_size``
    * ``internal_size``
    * ``precision``
    * ``scale``
    * ``null_ok``

    The first two items (``name`` and ``type_code``) are mandatory,
    the other five are optional and are set to ``None`` if no
    meaningful values can be provided.

    This attribute will be ``None`` for operations that do not return
    rows or if the cursor has not had an operation invoked via the
    `.execute*()`_ method yet.

    The ``type_code`` can be interpreted by comparing it to the `Type
    Objects`_ specified in the section below.


.. _.rowcount:

`.rowcount`_
    This read-only attribute specifies the number of rows that the last
    `.execute*()`_ produced (for DQL statements like ``SELECT``) or affected
    (for DML statements like ``UPDATE`` or ``INSERT``). [9]_

    The attribute is -1 in case no `.execute*()`_ has been performed
    on the cursor or the rowcount of the last operation is cannot be
    determined by the interface. [7]_

    .. note::
        Future versions of the DB API specification could redefine the
        latter case to have the object return ``None`` instead of -1.


`Cursor methods`_
-----------------

.. _.callproc:
.. _.callproc():

`.callproc`_\ ( *procname* [, *parameters* ] )
    (This method is optional since not all databases provide stored
    procedures. [3]_)

    Call a stored database procedure with the given name. The sequence
    of parameters must contain one entry for each argument that the
    procedure expects. The result of the call is returned as modified
    copy of the input sequence. Input parameters are left untouched,
    output and input/output parameters replaced with possibly new
    values.

    The procedure may also provide a result set as output. This must
    then be made available through the standard `.fetch*()`_ methods.


.. .close:
.. _Cursor.close:
.. _Cursor.close():

`.close <#Cursor.close>`_\ ()
    Close the cursor now (rather than whenever ``__del__`` is called).

    The cursor will be unusable from this point forward; an Error_ (or
    subclass) exception will be raised if any operation is attempted
    with the cursor.


.. _.execute*:
.. _.execute*():

.. _.execute:
.. _.execute():

`.execute`_\ (*operation* [, *parameters*])
    Prepare and execute a database operation (query or command).

    Parameters may be provided as sequence or mapping and will be
    bound to variables in the operation.  Variables are specified in a
    database-specific notation (see the module's paramstyle_ attribute
    for details). [5]_

    A reference to the operation will be retained by the cursor.  If
    the same operation object is passed in again, then the cursor can
    optimize its behavior.  This is most effective for algorithms
    where the same operation is used, but different parameters are
    bound to it (many times).

    For maximum efficiency when reusing an operation, it is best to
    use the `.setinputsizes()`_ method to specify the parameter types
    and sizes ahead of time.  It is legal for a parameter to not match
    the predefined information; the implementation should compensate,
    possibly with a loss of efficiency.

    The parameters may also be specified as list of tuples to
    e.g. insert multiple rows in a single operation, but this kind of
    usage is deprecated: `.executemany()`_ should be used instead.

    Return values are not defined.


.. _.executemany:
.. _.executemany():

`.executemany`_\ ( *operation*, *seq_of_parameters* )
    Prepare a database operation (query or command) and then execute it
    against all parameter sequences or mappings found in the sequence
    *seq_of_parameters*.

    Modules are free to implement this method using multiple calls to
    the `.execute()`_ method or by using array operations to have the
    database process the sequence as a whole in one call.

    Use of this method for an operation which produces one or more
    result sets constitutes undefined behavior, and the implementation
    is permitted (but not required) to raise an exception when it
    detects that a result set has been created by an invocation of the
    operation.

    The same comments as for `.execute()`_ also apply accordingly to
    this method.

    Return values are not defined.


.. _.fetch*:
.. _.fetch*():

.. _.fetchone:
.. _.fetchone():

`.fetchone`_\ ()
    Fetch the next row of a query result set, returning a single
    sequence, or ``None`` when no more data is available. [6]_

    An Error_ (or subclass) exception is raised if the previous call
    to `.execute*()`_ did not produce any result set or no call was
    issued yet.


.. _.fetchmany:
.. _.fetchmany():

`.fetchmany`_\ ([*size=cursor.arraysize*])
    Fetch the next set of rows of a query result, returning a sequence
    of sequences (e.g. a list of tuples). An empty sequence is
    returned when no more rows are available.

    The number of rows to fetch per call is specified by the
    parameter.  If it is not given, the cursor's arraysize determines
    the number of rows to be fetched. The method should try to fetch
    as many rows as indicated by the size parameter. If this is not
    possible due to the specified number of rows not being available,
    fewer rows may be returned.

    An Error_ (or subclass) exception is raised if the previous call
    to `.execute*()`_ did not produce any result set or no call was
    issued yet.

    Note there are performance considerations involved with the *size*
    parameter.  For optimal performance, it is usually best to use the
    `.arraysize`_ attribute.  If the size parameter is used, then it
    is best for it to retain the same value from one `.fetchmany()`_
    call to the next.


.. _.fetchall:
.. _.fetchall():

`.fetchall`_\ ()
    Fetch all (remaining) rows of a query result, returning them as a
    sequence of sequences (e.g. a list of tuples).  Note that the
    cursor's arraysize attribute can affect the performance of this
    operation.

    An Error_ (or subclass) exception is raised if the previous call
    to `.execute*()`_ did not produce any result set or no call was
    issued yet.


.. _.nextset:
.. _.nextset():

`.nextset`_\ ()
    (This method is optional since not all databases support multiple
    result sets. [3]_)

    This method will make the cursor skip to the next available set,
    discarding any remaining rows from the current set.

    If there are no more sets, the method returns ``None``. Otherwise,
    it returns a true value and subsequent calls to the `.fetch*()`_
    methods will return rows from the next result set.

    An Error_ (or subclass) exception is raised if the previous call
    to `.execute*()`_ did not produce any result set or no call was
    issued yet.


.. _.arraysize:

`.arraysize`_
    This read/write attribute specifies the number of rows to fetch at
    a time with `.fetchmany()`_. It defaults to 1 meaning to fetch a
    single row at a time.

    Implementations must observe this value with respect to the
    `.fetchmany()`_ method, but are free to interact with the database
    a single row at a time. It may also be used in the implementation
    of `.executemany()`_.


.. _.setinputsizes:
.. _.setinputsizes():

`.setinputsizes`_\ (*sizes*)
    This can be used before a call to `.execute*()`_ to predefine
    memory areas for the operation's parameters.

    *sizes* is specified as a sequence — one item for each input
    parameter.  The item should be a Type Object that corresponds to
    the input that will be used, or it should be an integer specifying
    the maximum length of a string parameter.  If the item is
    ``None``, then no predefined memory area will be reserved for that
    column (this is useful to avoid predefined areas for large
    inputs).

    This method would be used before the `.execute*()`_ method is
    invoked.

    Implementations are free to have this method do nothing and users
    are free to not use it.


.. _.setoutputsize:
.. _.setoutputsize():

`.setoutputsize`_\ (*size* [, *column*])
    Set a column buffer size for fetches of large columns
    (e.g. ``LONG``\s, ``BLOB``\s, etc.).  The column is specified as
    an index into the result sequence.  Not specifying the column will
    set the default size for all large columns in the cursor.

    This method would be used before the `.execute*()`_ method is
    invoked.

    Implementations are free to have this method do nothing and users
    are free to not use it.


.. _Type Objects:

`Type Objects and Constructors`_
================================

Many databases need to have the input in a particular format for
binding to an operation's input parameters.  For example, if an input
is destined for a ``DATE`` column, then it must be bound to the
database in a particular string format.  Similar problems exist for
"Row ID" columns or large binary items (e.g. blobs or ``RAW``
columns).  This presents problems for Python since the parameters to
the `.execute*()`_ method are untyped.  When the database module sees
a Python string object, it doesn't know if it should be bound as a
simple ``CHAR`` column, as a raw ``BINARY`` item, or as a ``DATE``.

To overcome this problem, a module must provide the constructors
defined below to create objects that can hold special values.  When
passed to the cursor methods, the module can then detect the proper
type of the input parameter and bind it accordingly.

A Cursor_ Object's description attribute returns information about
each of the result columns of a query.  The ``type_code`` must compare
equal to one of Type Objects defined below. Type Objects may be equal
to more than one type code (e.g. ``DATETIME`` could be equal to the
type codes for date, time and timestamp columns; see the
`Implementation Hints`_ below for details).

The module exports the following constructors and singletons:

.. _Date:

`Date`_\ (*year*, *month*, *day*)
    This function constructs an object holding a date value.


.. _Time:

`Time`_\ (*hour*, *minute*, *second*)
    This function constructs an object holding a time value.


.. _Timestamp:

`Timestamp`_\ (*year*, *month*, *day*, *hour*, *minute*, *second*)
    This function constructs an object holding a time stamp value.


.. _DateFromTicks:

`DateFromTicks`_\ (*ticks*)
    This function constructs an object holding a date value from the
    given ticks value (number of seconds since the epoch; see the
    documentation of `the standard Python time module
    <http://docs.python.org/library/time.html>`__ for details).

.. _TimeFromTicks:

`TimeFromTicks`_\ (*ticks*)
    This function constructs an object holding a time value from the
    given ticks value (number of seconds since the epoch; see the
    documentation of the standard Python time module for details).


.. _TimeStampFromTicks:

`TimestampFromTicks`_\ (*ticks*)
    This function constructs an object holding a time stamp value from
    the given ticks value (number of seconds since the epoch; see the
    documentation of the standard Python time module for details).


.. _Binary:

`Binary`_\ (*string*)
    This function constructs an object capable of holding a binary
    (long) string value.


.. _STRING:

`STRING`_ type
    This type object is used to describe columns in a database that
    are string-based (e.g. ``CHAR``).


.. _Binary type:

`BINARY`_ type
    This type object is used to describe (long) binary columns in a
    database (e.g. ``LONG``, ``RAW``, ``BLOB``\s).


.. _NUMBER:

`NUMBER`_ type
    This type object is used to describe numeric columns in a
    database.


.. _DATETIME:

`DATETIME`_ type
    This type object is used to describe date/time columns in a
    database.

.. _ROWID:

`ROWID`_ type
    This type object is used to describe the "Row ID" column in a
    database.


SQL ``NULL`` values are represented by the Python ``None`` singleton
on input and output.

.. Note::
    Usage of Unix ticks for database interfacing can cause troubles
    because of the limited date range they cover.



.. _Implementation Hints:

`Implementation Hints for Module Authors`_
==========================================

* Date/time objects can be implemented as `Python datetime module
  <http://docs.python.org/library/datetime.html>`__ objects (available
  since Python 2.3, with a C API since 2.4) or using the `mxDateTime
  <http://www.egenix.com/products/python/mxBase/mxDateTime/>`_ package
  (available for all Python versions since 1.5.2). They both provide
  all necessary constructors and methods at Python and C level.

* Here is a sample implementation of the Unix ticks based constructors
  for date/time delegating work to the generic constructors::

        import time

        def DateFromTicks(ticks):
            return Date(*time.localtime(ticks)[:3])

        def TimeFromTicks(ticks):
            return Time(*time.localtime(ticks)[3:6])

        def TimestampFromTicks(ticks):
            return Timestamp(*time.localtime(ticks)[:6])

* The preferred object type for Binary objects are the buffer types
  available in standard Python starting with version 1.5.2.  Please
  see the Python documentation for details. For information about the
  C interface have a look at ``Include/bufferobject.h`` and
  ``Objects/bufferobject.c`` in the Python source distribution.

* This Python class allows implementing the above type objects even
  though the description type code field yields multiple values for on
  type object::

        class DBAPITypeObject:
            def __init__(self,*values):
                self.values = values
            def __cmp__(self,other):
                if other in self.values:
                    return 0
                if other < self.values:
                    return 1
                else:
                    return -1

  The resulting type object compares equal to all values passed to the
  constructor.

* Here is a snippet of Python code that implements the exception
  hierarchy defined above::

        import exceptions

        class Error(exceptions.StandardError):
            pass

        class Warning(exceptions.StandardError):
            pass

        class InterfaceError(Error):
            pass

        class DatabaseError(Error):
            pass

        class InternalError(DatabaseError):
            pass

        class OperationalError(DatabaseError):
            pass

        class ProgrammingError(DatabaseError):
            pass

        class IntegrityError(DatabaseError):
            pass

        class DataError(DatabaseError):
            pass

        class NotSupportedError(DatabaseError):
            pass

  In C you can use the ``PyErr_NewException(fullname, base, NULL)``
  API to create the exception objects.


`Optional DB API Extensions`_
=============================

During the lifetime of DB API 2.0, module authors have often extended
their implementations beyond what is required by this DB API
specification. To enhance compatibility and to provide a clean upgrade
path to possible future versions of the specification, this section
defines a set of common extensions to the core DB API 2.0
specification.

As with all DB API optional features, the database module authors are
free to not implement these additional attributes and methods (using
them will then result in an ``AttributeError``) or to raise a
NotSupportedError_ in case the availability can only be checked at
run-time.

It has been proposed to make usage of these extensions optionally
visible to the programmer by issuing Python warnings through the
Python warning framework. To make this feature useful, the warning
messages must be standardized in order to be able to mask them. These
standard messages are referred to below as *Warning Message*.


.. _.rownumber:

Cursor\ `.rownumber`_
    This read-only attribute should provide the current 0-based index
    of the cursor in the result set or ``None`` if the index cannot be
    determined.

    The index can be seen as index of the cursor in a sequence (the
    result set). The next fetch operation will fetch the row indexed
    by `.rownumber`_ in that sequence.

    *Warning Message:* "DB-API extension cursor.rownumber used"


.. _Connection.Error:
.. _Connection.ProgrammingError:

`Connection.Error`_, `Connection.ProgrammingError`_, etc.
    All exception classes defined by the DB API standard should be
    exposed on the Connection_ objects as attributes (in addition to
    being available at module scope).

    These attributes simplify error handling in multi-connection
    environments.

    *Warning Message:* "DB-API extension connection.<exception> used"


.. _.connection:

Cursor\ `.connection`_
    This read-only attribute return a reference to the Connection_
    object on which the cursor was created.

    The attribute simplifies writing polymorph code in
    multi-connection environments.

    *Warning Message:* "DB-API extension cursor.connection used"


.. _.scroll:
.. _.scroll():

Cursor\ `.scroll`_\ (*value* [, *mode='relative'* ])
    Scroll the cursor in the result set to a new position according to
    *mode*.

    If mode is ``relative`` (default), value is taken as offset to the
    current position in the result set, if set to ``absolute``, value
    states an absolute target position.

    An ``IndexError`` should be raised in case a scroll operation
    would leave the result set. In this case, the cursor position is
    left undefined (ideal would be to not move the cursor at all).

    .. Note::
        This method should use native scrollable cursors, if available,
        or revert to an emulation for forward-only scrollable
        cursors. The method may raise NotSupportedError_ to signal
        that a specific operation is not supported by the database
        (e.g. backward scrolling).

    *Warning Message:* "DB-API extension cursor.scroll() used"


.. _Cursor.messages:

`Cursor.messages`_
    This is a Python list object to which the interface appends tuples
    (exception class, exception value) for all messages which the
    interfaces receives from the underlying database for this cursor.

    The list is cleared by all standard cursor methods calls (prior to
    executing the call) except for the `.fetch*()`_ calls
    automatically to avoid excessive memory usage and can also be
    cleared by executing ``del cursor.messages[:]``.

    All error and warning messages generated by the database are
    placed into this list, so checking the list allows the user to
    verify correct operation of the method calls.

    The aim of this attribute is to eliminate the need for a Warning
    exception which often causes problems (some warnings really only
    have informational character).

    *Warning Message:* "DB-API extension cursor.messages used"


.. _Connection.messages:

`Connection.messages`_
    Same as Cursor.messages_ except that the messages in the list are
    connection oriented.

    The list is cleared automatically by all standard connection
    methods calls (prior to executing the call) to avoid excessive
    memory usage and can also be cleared by executing ``del
    connection.messages[:]``.

    *Warning Message:* "DB-API extension connection.messages used"


.. _.next:
.. _.next():

Cursor\ `.next`_\ ()
    Return the next row from the currently executing SQL statement
    using the same semantics as `.fetchone()`_.  A ``StopIteration``
    exception is raised when the result set is exhausted for Python
    versions 2.2 and later.  Previous versions don't have the
    ``StopIteration`` exception and so the method should raise an
    ``IndexError`` instead.

    *Warning Message:* "DB-API extension cursor.next() used"


.. _.__iter__:
.. _.__iter__():

Cursor\ `.__iter__`_\ ()
    Return self to make cursors compatible to the iteration protocol
    [8]_.

    *Warning Message:* "DB-API extension cursor.__iter__() used"


.. _.lastrowid:

Cursor\ `.lastrowid`_
    This read-only attribute provides the rowid of the last modified
    row (most databases return a rowid only when a single ``INSERT``
    operation is performed). If the operation does not set a rowid or
    if the database does not support rowids, this attribute should be
    set to ``None``.

    The semantics of ``.lastrowid`` are undefined in case the last
    executed statement modified more than one row, e.g. when using
    ``INSERT`` with ``.executemany()``.

    *Warning Message:* "DB-API extension cursor.lastrowid used"


`Optional Error Handling Extensions`_
=====================================

The core DB API specification only introduces a set of exceptions
which can be raised to report errors to the user. In some cases,
exceptions may be too disruptive for the flow of a program or even
render execution impossible.

For these cases and in order to simplify error handling when dealing
with databases, database module authors may choose to implement user
definable error handlers. This section describes a standard way of
defining these error handlers.

.. _Connection.errorhandler:
.. _Cursor.errorhandler:

`Connection.errorhandler`_, `Cursor.errorhandler`_
    Read/write attribute which references an error handler to call in
    case an error condition is met.

    The handler must be a Python callable taking the following arguments:

    .. parsed-literal::

        errorhandler(*connection*, *cursor*, *errorclass*, *errorvalue*)

    where connection is a reference to the connection on which the
    cursor operates, cursor a reference to the cursor (or ``None`` in
    case the error does not apply to a cursor), *errorclass* is an
    error class which to instantiate using *errorvalue* as
    construction argument.

    The standard error handler should add the error information to the
    appropriate ``.messages`` attribute (`Connection.messages`_ or
    `Cursor.messages`_) and raise the exception defined by the given
    *errorclass* and *errorvalue* parameters.

    If no ``.errorhandler`` is set (the attribute is ``None``), the
    standard error handling scheme as outlined above, should be
    applied.

    *Warning Message:* "DB-API extension .errorhandler used"

Cursors should inherit the ``.errorhandler`` setting from their
connection objects at cursor creation time.


`Optional Two-Phase Commit Extensions`_
=======================================

Many databases have support for two-phase commit (TPC) which allows
managing transactions across multiple database connections and other
resources.

If a database backend provides support for two-phase commit and the
database module author wishes to expose this support, the following
API should be implemented. NotSupportedError_ should be raised, if the
database backend support for two-phase commit can only be checked at
run-time.

`TPC Transaction IDs`_
----------------------

As many databases follow the XA specification, transaction IDs are
formed from three components:

* a format ID
* a global transaction ID
* a branch qualifier

For a particular global transaction, the first two components should
be the same for all resources.  Each resource in the global
transaction should be assigned a different branch qualifier.

The various components must satisfy the following criteria:

* format ID: a non-negative 32-bit integer.

* global transaction ID and branch qualifier: byte strings no
  longer than 64 characters.

Transaction IDs are created with the `.xid()`_ Connection method:


.. _.xid:
.. _.xid():

`.xid`_\ (*format_id*, *global_transaction_id*, *branch_qualifier*)
    Returns a transaction ID object suitable for passing to the
    `.tpc_*()`_ methods of this connection.

    If the database connection does not support TPC, a
    NotSupportedError_ is raised.

    The type of the object returned by `.xid()`_ is not defined, but
    it must provide sequence behaviour, allowing access to the three
    components.  A conforming database module could choose to
    represent transaction IDs with tuples rather than a custom object.


`TPC Connection Methods`_
-------------------------

.. _.tpc_*:
.. _.tpc_*():

.. _.tpc_begin:
.. _.tpc_begin():

`.tpc_begin`_\ (*xid*)
    Begins a TPC transaction with the given transaction ID *xid*.

    This method should be called outside of a transaction (*i.e.*
    nothing may have executed since the last `.commit()`_ or
    `.rollback()`_).

    Furthermore, it is an error to call `.commit()`_ or `.rollback()`_
    within the TPC transaction. A ProgrammingError_ is raised, if the
    application calls `.commit()`_ or `.rollback()`_ during an active
    TPC transaction.

    If the database connection does not support TPC, a
    NotSupportedError_ is raised.


.. _.tpc_prepare:
.. _.tpc_prepare():

`.tpc_prepare`_\ ()
    Performs the first phase of a transaction started with
    `.tpc_begin()`_.  A ProgrammingError_ should be raised if this
    method outside of a TPC transaction.

    After calling `.tpc_prepare()`_, no statements can be executed
    until `.tpc_commit()`_ or `.tpc_rollback()`_ have been called.


.. _.tpc_commit:
.. _.tpc_commit():

`.tpc_commit`_\ ([ *xid* ])
    When called with no arguments, `.tpc_commit()`_ commits a TPC
    transaction previously prepared with `.tpc_prepare()`_.

    If `.tpc_commit()`_ is called prior to `.tpc_prepare()`_, a single
    phase commit is performed.  A transaction manager may choose to do
    this if only a single resource is participating in the global
    transaction.

    When called with a transaction ID *xid*, the database commits the
    given transaction.  If an invalid transaction ID is provided, a
    ProgrammingError_ will be raised.  This form should be called
    outside of a transaction, and is intended for use in recovery.

    On return, the TPC transaction is ended.


.. _.tpc_rollback:
.. _.tpc_rollback():

`.tpc_rollback`_\ ([ *xid* ])
    When called with no arguments, `.tpc_rollback()`_ rolls back a TPC
    transaction.  It may be called before or after `.tpc_prepare()`_.

    When called with a transaction ID *xid*, it rolls back the given
    transaction.  If an invalid transaction ID is provided, a
    ProgrammingError_ is raised.  This form should be called outside
    of a transaction, and is intended for use in recovery.

    On return, the TPC transaction is ended.

.. _.tpc_recover:
.. _.tpc_recover():

`.tpc_recover`_\ ()
    Returns a list of pending transaction IDs suitable for use with
    ``.tpc_commit(xid)`` or ``.tpc_rollback(xid)``.

    If the database does not support transaction recovery, it may
    return an empty list or raise NotSupportedError_.



`Frequently Asked Questions`_
=============================

The database SIG often sees reoccurring questions about the DB API
specification. This section covers some of the issues people sometimes
have with the specification.

**Question:**

How can I construct a dictionary out of the tuples returned by
`.fetch*()`_:

**Answer:**

There are several existing tools available which provide helpers for
this task. Most of them use the approach of using the column names
defined in the cursor attribute `.description`_ as basis for the keys
in the row dictionary.

Note that the reason for not extending the DB API specification to
also support dictionary return values for the `.fetch*()`_ methods is
that this approach has several drawbacks:

* Some databases don't support case-sensitive column names or
  auto-convert them to all lowercase or all uppercase characters.

* Columns in the result set which are generated by the query (e.g.
  using SQL functions) don't map to table column names and databases
  usually generate names for these columns in a very database specific
  way.

As a result, accessing the columns through dictionary keys varies
between databases and makes writing portable code impossible.



`Major Changes from Version 1.0 to Version 2.0`_
================================================

The Python Database API 2.0 introduces a few major changes compared to
the 1.0 version. Because some of these changes will cause existing DB
API 1.0 based scripts to break, the major version number was adjusted
to reflect this change.

These are the most important changes from 1.0 to 2.0:

* The need for a separate dbi module was dropped and the functionality
  merged into the module interface itself.

* New constructors and `Type Objects`_ were added for date/time
  values, the ``RAW`` Type Object was renamed to ``BINARY``. The
  resulting set should cover all basic data types commonly found in
  modern SQL databases.

* New constants (apilevel_, threadsafety_, paramstyle_) and methods
  (`.executemany()`_, `.nextset()`_) were added to provide better
  database bindings.

* The semantics of `.callproc()`_ needed to call stored procedures are
  now clearly defined.

* The definition of the `.execute()`_ return value changed.
  Previously, the return value was based on the SQL statement type
  (which was hard to implement right) — it is undefined now; use the
  more flexible `.rowcount`_ attribute instead. Modules are free to
  return the old style return values, but these are no longer mandated
  by the specification and should be considered database interface
  dependent.

* Class based exceptions_ were incorporated into the specification.
  Module implementors are free to extend the exception layout defined
  in this specification by subclassing the defined exception classes.


Post-publishing additions to the DB API 2.0 specification:

* Additional optional DB API extensions to the set of core
  functionality were specified.


`Open Issues`_
==============

Although the version 2.0 specification clarifies a lot of questions
that were left open in the 1.0 version, there are still some remaining
issues which should be addressed in future versions:

* Define a useful return value for `.nextset()`_ for the case where a
  new result set is available.

* Integrate the `decimal module
  <http://docs.python.org/library/decimal.html>`__ ``Decimal`` object
  for use as loss-less monetary and decimal interchange format.



`Footnotes`_
============

.. [1] As a guideline the connection constructor parameters should be
    implemented as keyword parameters for more intuitive use and
    follow this order of parameters:

    ============= ====================================
    Parameter     Meaning
    ============= ====================================
    ``dsn``       Data source name as string
    ``user``      User name as string (optional)
    ``password``  Password as string (optional)
    ``host``      Hostname (optional)
    ``database``  Database name (optional)
    ============= ====================================

    E.g. a connect could look like this::

        connect(dsn='myhost:MYDB', user='guido', password='234$')

.. [2] Module implementors should prefer ``numeric``, ``named`` or
    ``pyformat`` over the other formats because these offer more
    clarity and flexibility.


.. [3] If the database does not support the functionality required by
    the method, the interface should throw an exception in case the
    method is used.

    The preferred approach is to not implement the method and thus
    have Python generate an ``AttributeError`` in case the method is requested. This
    allows the programmer to check for database capabilities using the
    standard ``hasattr()`` function.

    For some dynamically configured interfaces it may not be
    appropriate to require dynamically making the method
    available. These interfaces should then raise a
    ``NotSupportedError`` to indicate the non-ability to perform the
    roll back when the method is invoked.

.. [4] A database interface may choose to support named cursors by
    allowing a string argument to the method. This feature is not part
    of the specification, since it complicates semantics of the
    `.fetch*()`_ methods.

.. [5] The module will use the ``__getitem__`` method of the
    parameters object to map either positions (integers) or names
    (strings) to parameter values.  This allows for both sequences and
    mappings to be used as input.

    The term *bound* refers to the process of binding an input value
    to a database execution buffer. In practical terms, this means
    that the input value is directly used as a value in the operation.
    The client should not be required to "escape" the value so that it
    can be used — the value should be equal to the actual database
    value.

.. [6] Note that the interface may implement row fetching using arrays
    and other optimizations. It is not guaranteed that a call to this
    method will only move the associated cursor forward by one row.

.. [7] The ``rowcount`` attribute may be coded in a way that updates
    its value dynamically. This can be useful for databases that
    return usable ``rowcount`` values only after the first call to a
    `.fetch*()`_ method.

.. [8] Implementation Note: Python C extensions will have to implement
    the ``tp_iter`` slot on the cursor object instead of the
    ``.__iter__()`` method.

.. [9] The term *number of affected rows* generally refers to the
    number of rows deleted, updated or inserted by the last statement
    run on the database cursor. Most databases will return the total
    number of rows that were found by the corresponding ``WHERE``
    clause of the statement. Some databases use a different
    interpretation for ``UPDATE``\s and only return the number of rows
    that were changed by the ``UPDATE``, even though the ``WHERE``
    clause of the statement may have found more matching rows.
    Database module authors should try to implement the more common
    interpretation of returning the total number of rows found by the
    ``WHERE`` clause, or clearly document a different interpretation
    of the ``.rowcount`` attribute.


`Acknowledgements`_
===================

Many thanks go to Andrew Kuchling who converted the Python Database
API Specification 2.0 from the original HTML format into the PEP
format.

Many thanks to James Henstridge for leading the discussion which led
to the standardization of the two-phase commit API extensions.

Many thanks to Daniele Varrazzo for converting the specification from
text PEP format to ReST PEP format, which allows linking to various
parts.

`Copyright`_
============

This document has been placed in the Public Domain.