python-peps/pep-0248.txt

PEP: 248
Title: Python Database API Specification v1.0
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:
Superseded-By: 249

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.
 
    This interface specification consists of several items:

        * Module Interface
        * Connection Objects
        * Cursor Objects
        * DBI Helper Objects
 
    Comments and questions about this specification may be directed to
    the SIG on Tabular Databases in Python
    (http://www.python.org/sigs/db-sig).

    This specification document was last updated on: April 9, 1996.
    It will be known as Version 1.0 of this specification.


Module Interface

    The database interface modules should typically be named with
    something terminated by 'db'.  Existing examples are: 'oracledb',
    'informixdb', and 'pg95db'.  These modules should export several
    names:
 
        modulename(connection_string)

            Constructor for creating a connection to the database.
            Returns a Connection Object.
 
        error
        
            Exception raised for errors from the database module.


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 exception will be raised if any operation is
            attempted with the connection.
 
        commit()

            Commit any pending transaction to the database.
 
        rollback()

            Roll the database back to the start of any pending
            transaction.
 
        cursor()

            Return a new Cursor Object.  An exception may be thrown if
            the database does not support a cursor concept.
 
        callproc([params])

            (Note: this method is not well-defined yet.)  Call a
            stored database procedure with the given (optional)
            parameters.  Returns the result of the stored procedure.
 
        (all Cursor Object attributes and methods)

            For databases that do not have cursors and for simple
            applications that do not require the complexity of a
            cursor, a Connection Object should respond to each of the
            attributes and methods of the Cursor Object.  Databases
            that have cursor can implement this by using an implicit,
            internal cursor.

 
Cursor Objects

    These objects represent a database cursor, which is used to manage
    the context of a fetch operation.
 
    Cursor Objects should respond to the following methods and
    attributes:
 
        arraysize

            This read/write attribute specifies the number of rows to
            fetch at a time with fetchmany().  This value is also used
            when inserting multiple rows at a time (passing a
            tuple/list of tuples/lists as the params value to
            execute()).  This attribute will default to a single row.
 
            Note that the arraysize is optional and is merely provided
            for higher performance database interactions.
            Implementations should observe it with respect to the
            fetchmany() method, but are free to interact with the
            database a single row at a time.
 
        description

            This read-only attribute is a tuple of 7-tuples.  Each
            7-tuple contains information describing each result
            column: (name, type_code, display_size, internal_size,
            precision, scale, null_ok). 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' is one of the 'dbi' values specified in
            the section below.
 
            Note: this is a bit in flux. Generally, the first two
            items of the 7-tuple will always be present; the others
            may be database specific.
 
        close()

            Close the cursor now (rather than whenever __del__ is
            called).  The cursor will be unusable from this point
            forward; an exception will be raised if any operation is
            attempted with the cursor.
 
        execute(operation [,params])

            Execute (prepare) a database operation (query or command).
            Parameters may be provided (as a sequence
            (e.g. tuple/list)) and will be bound to variables in the
            operation.  Variables are specified in a database-specific
            notation that is based on the index in the parameter tuple
            (position-based rather than name-based).
 
            The parameters may also be specified as a sequence of
            sequences (e.g. a list of tuples) to insert multiple rows
            in a single operation.
 
            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.
 
            Using SQL terminology, these are the possible result
            values from the execute() method:

                If the statement is DDL (e.g. CREATE TABLE), then 1 is
                returned.

                If the statement is DML (e.g. UPDATE or INSERT), then the
                number of rows affected is returned (0 or a positive
                integer).

                If the statement is DQL (e.g. SELECT), None is returned,
                indicating that the statement is not really complete until
                you use one of the  'fetch' methods.

        fetchone()

            Fetch the next row of a query result, returning a single
            tuple.

        fetchmany([size])

            Fetch the next set of rows of a query result, returning as
            a list of tuples. An empty list is returned when no more
            rows are available. The number of rows to fetch is
            specified by the parameter.  If it is None, then the
            cursor's arraysize determines the number of rows to be
            fetched.
 
            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 rows of a query result, returning as a list of
            tuples.  Note that the cursor's arraysize attribute can
            affect the performance of this operation.
 
        setinputsizes(sizes)

            (Note: this method is not well-defined yet.)  This can be
            used before a call to 'execute()' to predefine memory
            areas for the operation's parameters.  sizes is specified
            as a tuple -- 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.
 
            Note that this method is optional and is merely provided
            for higher performance database interaction.
            Implementations are free to do nothing and users are free
            to not use it.
 
        setoutputsize(size [,col])

            (Note: this method is not well-defined yet.)

            Set a column buffer size for fetches of large columns
            (e.g. LONG).  The column is specified as an index into the
            result tuple.  Using a column of None will set the default
            size for all large columns in the cursor.
 
            This method would be used before the 'execute()' method is
            invoked.
 
            Note that this method is optional and is merely provided
            for higher performance database interaction.
            Implementations are free to do nothing and users are free
            to not use it.
 

DBI Helper Objects

    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, the 'dbi' module was created.  This
    module specifies some basic database interface types for working
    with databases.  There are two classes: 'dbiDate' and 'dbiRaw'.
    These are simple container classes that wrap up a value.  When
    passed to the database modules, the module can then detect that
    the input parameter is intended as a DATE or a RAW.  For symmetry,
    the database modules will return DATE and RAW columns as instances
    of these classes.
 
    A Cursor Object's 'description' attribute returns information
    about each of the result columns of a query.  The 'type_code is
    defined to be one of five types exported by this module: 'STRING',
    'RAW', 'NUMBER', 'DATE', or 'ROWID'.
 
    The module exports the following names:
 
        dbiDate(value)

            This function constructs a 'dbiDate' instance that holds a
            date value.  The value should be specified as an integer
            number of seconds since the "epoch" (e.g. time.time()).
 
        dbiRaw(value)

            This function constructs a 'dbiRaw' instance that holds a
            raw (binary) value.  The value should be specified as a
            Python string.

        STRING

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

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

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

            This object is used to describe date columns in a
            database.
 
        ROWID

            This object is used to describe the "Row ID" column in a
            database.

Acknowledgements

    Many thanks go to Andrew Kuchling who converted the Python
    Database API Specification 1.0 from the original HTML format into
    the PEP format.


Copyright

    This document has been placed in the Public Domain.



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