python-peps/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
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.
    
    The interface specification consists of several sections:
    
        * Module Interface
        * Connection Objects
        * Cursor Objects
        * Type Objects and Constructors
        * Implementation Hints for Module Authors
        * Optional DB API Extensions
        * Optional Error Handling Extensions
        * Optional Two-Phase Commit Extensions
        * Frequently Asked Questions
        * Major Changes from Version 1.0 to Version 2.0
        * Open Issues
        * Footnotes
        * Acknowledgments
    
    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 at 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
        
    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]
        
    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:

                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]:

                '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'

    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 StandardError (defined in the module
            exceptions).
            
        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

            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:

        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 Objects

    Connection Objects should respond to the following 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. [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()
          
            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 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:

        .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').
            
            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.
            
        .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()
          
            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). [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(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. [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([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. [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
          
            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

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

        BINARY

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

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

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

        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".

    Cursor Attribute .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 Attributes .Error, .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 Attributes .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 Method .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 NotSupportedErrors 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 Attribute .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 Attribute .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 Method .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 Method .__iter__()

        Return self to make cursors compatible to the iteration
        protocol [8].

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

    Cursor Attribute .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 defineable error handlers. This section describes a
    standard way of defining these error handlers.

    Cursor/Connection Attribute .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, threadlevel, 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

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

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.

Copyright

    This document has been placed in the Public Domain.



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