PEP: 249 Title: Python Database API Specification v2.0 Author:
Marc-André Lemburg <mal@lemburg.com> Discussions-To: db-sig@python.org
Status: Final Type: Informational Content-Type: text/x-rst Created:
12-Apr-1999 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.

For more information on database interfacing with Python and available
packages see the Database Topic Guide.

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( 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

    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

    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

    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

    Exception raised for important warnings like data truncations while
    inserting, etc. It must be a subclass of the Python Exception
    class[3][4].

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 Exception
    class[5].

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

    Exception raised for errors that are related to the database. It
    must be a subclass of Error.

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

    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

    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

    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

    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

    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[6][7]:

    Exception
    |__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 Objects

Connection objects should respond to the following methods.

Connection methods

.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 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()

    This method is optional since not all databases provide transaction
    support.[8]

    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()

    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.[9]

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

    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

    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).[10]

    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.[11]

    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( procname [, parameters ] )

    (This method is optional since not all databases provide stored
    procedures.[12])

    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()

    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(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).[13]

    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( 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.

.fetchone()

    Fetch the next row of a query result set, returning a single
    sequence, or None when no more data is available.[14]

    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([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()

    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()

    (This method is optional since not all databases support multiple
    result sets.[15])

    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

    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(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(size [, column])

    Set a column buffer size for fetches of large columns (e.g. LONGs,
    BLOBs, 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 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(year, month, day)

    This function constructs an object holding a date value.

Time(hour, minute, second)

    This function constructs an object holding a time value.

Timestamp(year, month, day, hour, minute, second)

    This function constructs an object holding a time stamp value.

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 for details).

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(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(string)

    This function constructs an object capable of holding a binary
    (long) string value.

STRING type

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

BINARY type

    This type object is used to describe (long) binary columns in a
    database (e.g. LONG, RAW, BLOBs).

NUMBER type

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

DATETIME type

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

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 for Module Authors

-   Date/time objects can be implemented as Python datetime module
    objects (available since Python 2.3, with a C API since 2.4) or
    using the 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[16]:

        class Error(Exception):
            pass

        class Warning(Exception):
            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.

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, 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"

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"

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

    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

    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"

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"

Cursor.__iter__()

    Return self to make cursors compatible to the iteration protocol
    [17].

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

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"

Connection.autocommit

    Attribute to query and set the autocommit mode of the connection.

    Return True if the connection is operating in autocommit
    (non-transactional) mode. Return False if the connection is
    operating in manual commit (transactional) mode.

    Setting the attribute to True or False adjusts the connection's mode
    accordingly.

    Changing the setting from True to False (disabling autocommit) will
    have the database leave autocommit mode and start a new transaction.
    Changing from False to True (enabling autocommit) has database
    dependent semantics with respect to how pending transactions are
    handled.[18]

    Deprecation notice: Even though several database modules implement
    both the read and write nature of this attribute, setting the
    autocommit mode by writing to the attribute is deprecated, since
    this may result in I/O and related exceptions, making it difficult
    to implement in an async context.[19]

    Warning Message: "DB-API extension connection.autocommit 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

    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:

    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(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_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()

    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([ 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([ 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()

    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 Decimal object for use as loss-less
    monetary and decimal interchange format.

Footnotes

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 in
2001.

Many thanks to James Henstridge for leading the discussion which led to
the standardization of the two-phase commit API extensions in 2008.

Many thanks to Daniele Varrazzo for converting the specification from
text PEP format to ReST PEP format, which allows linking to various
parts in 2012.

Copyright

This document has been placed in the Public Domain.

[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$')

Also see regarding planned future additions to this list.

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

[3] In Python 2 and earlier versions of this PEP, StandardError was used
as the base class for all DB-API exceptions. Since StandardError was
removed in Python 3, database modules targeting Python 3 should use
Exception as base class instead. The PEP was updated to use Exception
throughout the text, to avoid confusion. The change should not affect
existing modules or uses of those modules, since all DB-API error
exception classes are still rooted at the Error or Warning classes.

[4] In a future revision of the DB-API, the base class for Warning will
likely change to the builtin Warning class. At the time of writing of
the DB-API 2.0 in 1999, the warning framework in Python did not yet
exist.

[5] In Python 2 and earlier versions of this PEP, StandardError was used
as the base class for all DB-API exceptions. Since StandardError was
removed in Python 3, database modules targeting Python 3 should use
Exception as base class instead. The PEP was updated to use Exception
throughout the text, to avoid confusion. The change should not affect
existing modules or uses of those modules, since all DB-API error
exception classes are still rooted at the Error or Warning classes.

[6] In Python 2 and earlier versions of this PEP, StandardError was used
as the base class for all DB-API exceptions. Since StandardError was
removed in Python 3, database modules targeting Python 3 should use
Exception as base class instead. The PEP was updated to use Exception
throughout the text, to avoid confusion. The change should not affect
existing modules or uses of those modules, since all DB-API error
exception classes are still rooted at the Error or Warning classes.

[7] In a future revision of the DB-API, the base class for Warning will
likely change to the builtin Warning class. At the time of writing of
the DB-API 2.0 in 1999, the warning framework in Python did not yet
exist.

[8] 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.

[9] 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.

[10] 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 UPDATEs 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.

[11] 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.

[12] 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.

[13] 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.

[14] 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.

[15] 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.

[16] In Python 2 and earlier versions of this PEP, StandardError was
used as the base class for all DB-API exceptions. Since StandardError
was removed in Python 3, database modules targeting Python 3 should use
Exception as base class instead. The PEP was updated to use Exception
throughout the text, to avoid confusion. The change should not affect
existing modules or uses of those modules, since all DB-API error
exception classes are still rooted at the Error or Warning classes.

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

[18] Many database modules implementing the autocommit attribute will
automatically commit any pending transaction and then enter autocommit
mode. It is generally recommended to explicitly .commit() or .rollback()
transactions prior to changing the autocommit setting, since this is
portable across database modules.

[19] In a future revision of the DB-API, we are going to introduce a new
method .setautocommit(value), which will allow setting the autocommit
mode, and make .autocommit a read-only attribute. Additionally, we are
considering to add a new standard keyword parameter autocommit to the
Connection constructor. Modules authors are encouraged to add these
changes in preparation for this change.