766 lines
30 KiB
Plaintext
766 lines
30 KiB
Plaintext
PEP: 249
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Title: Python Database API Specification v2.0
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Version: $Revision$
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Author: db-sig@python.org (Python Database SIG)
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Editor: mal@lemburg.com (Marc-Andre Lemburg)
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Status: Draft
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Type: Informational
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Replaces: 248
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Introduction
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This API has been defined to encourage similarity between the
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Python modules that are used to access databases. By doing this,
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we hope to achieve a consistency leading to more easily understood
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modules, code that is generally more portable across databases,
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and a broader reach of database connectivity from Python.
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The interface specification consists of several sections:
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* Module Interface
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* Connection Objects
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* Cursor Objects
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* DBI Helper Objects
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* Type Objects and Constructors
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* Implementation Hints
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* Major Changes from 1.0 to 2.0
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Comments and questions about this specification may be directed
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to the SIG for Database Interfacing with Python
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(db-sig@python.org).
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For more information on database interfacing with Python and
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available packages see the Database Topic
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Guide at http://www.python.org/topics/database/.
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This document describes the Python Database API Specification
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2.0. The previous version 1.0 version is still available as
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reference, in PEP 248. Package writers are encouraged to use
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this version of the specification as basis for new interfaces.
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Module Interface
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Access to the database is made available through connection
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objects. The module must provide the following constructor for
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these:
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connect(parameters...)
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Constructor for creating a connection to the database.
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Returns a Connection Object. It takes a number of
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parameters which are database dependent. [1]
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These module globals must be defined:
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apilevel
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String constant stating the supported DB API level.
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Currently only the strings '1.0' and '2.0' are allowed.
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If not given, a DB-API 1.0 level interface should be
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assumed.
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threadsafety
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Integer constant stating the level of thread safety the
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interface supports. Possible values are:
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0 Threads may not share the module.
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1 Threads may share the module, but not connections.
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2 Threads may share the module and connections.
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3 Threads may share the module, connections and
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cursors.
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Sharing in the above context means that two threads may
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use a resource without wrapping it using a mutex semaphore
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to implement resource locking. Note that you cannot always
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make external resources thread safe by managing access
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using a mutex: the resource may rely on global variables
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or other external sources that are beyond your control.
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paramstyle
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String constant stating the type of parameter marker
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formatting expected by the interface. Possible values are
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[2]:
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'qmark' Question mark style,
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e.g. '...WHERE name=?'
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'numeric' Numeric, positional style,
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e.g. '...WHERE name=:1'
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'named' Named style,
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e.g. '...WHERE name=:name'
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'format' ANSI C printf format codes,
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e.g. '...WHERE name=%s'
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'pyformat' Python extended format codes,
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e.g. '...WHERE name=%(name)s'
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The module should make all error information available through
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these exceptions or subclasses thereof:
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Warning
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Exception raised for important warnings like data
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truncations while inserting, etc. It must be a subclass of
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the Python StandardError (defined in the module
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exceptions).
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Error
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Exception that is the base class of all other error
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exceptions. You can use this to catch all errors with one
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single 'except' statement. Warnings are not considered
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errors and thus should not use this class as base. It must
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be a subclass of the Python StandardError (defined in the
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module exceptions).
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InterfaceError
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Exception raised for errors that are related to the
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database interface rather than the database itself. It
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must be a subclass of Error.
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DatabaseError
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Exception raised for errors that are related to the
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database. It must be a subclass of Error.
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DataError
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Exception raised for errors that are due to problems with
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the processed data like division by zero, numeric value
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out of range, etc. It must be a subclass of DatabaseError.
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OperationalError
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Exception raised for errors that are related to the
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database's operation and not necessarily under the control
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of the programmer, e.g. an unexpected disconnect occurs,
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the data source name is not found, a transaction could not
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be processed, a memory allocation error occurred during
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processing, etc. It must be a subclass of DatabaseError.
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IntegrityError
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Exception raised when the relational integrity of the
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database is affected, e.g. a foreign key check fails. It
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must be a subclass of DatabaseError.
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InternalError
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Exception raised when the database encounters an internal
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error, e.g. the cursor is not valid anymore, the
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transaction is out of sync, etc. It must be a subclass of
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DatabaseError.
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ProgrammingError
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Exception raised for programming errors, e.g. table not
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found or already exists, syntax error in the SQL
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statement, wrong number of parameters specified, etc. It
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must be a subclass of DatabaseError.
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NotSupportedError
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Exception raised in case a method or database API was used
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which is not supported by the database, e.g. requesting a
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.rollback() on a connection that does not support
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transaction or has transactions turned off. It must be a
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subclass of DatabaseError.
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This is the exception inheritance layout:
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StandardError
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|__Warning
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|__Error
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|__InterfaceError
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|__DatabaseError
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|__DataError
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|__OperationalError
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|__IntegrityError
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|__InternalError
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|__ProgrammingError
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|__NotSupportedError
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Note: The values of these exceptions are not defined. They should
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give the user a fairly good idea of what went wrong, though.
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Connection Objects
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Connection Objects should respond to the following methods:
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close()
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Close the connection now (rather than whenever __del__ is
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called). The connection will be unusable from this point
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forward; an Error (or subclass) exception will be raised
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if any operation is attempted with the connection. The
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same applies to all cursor objects trying to use the
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connection. Note that closing a connection without
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committing the changes first will cause an implicit
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rollback to be performed.
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commit()
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Commit any pending transaction to the database. Note that
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if the database supports an auto-commit feature, this must
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be initially off. An interface method may be provided to
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turn it back on.
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Database modules that do not support transactions should
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implement this method with void functionality.
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rollback()
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This method is optional since not all databases provide
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transaction support. [3]
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In case a database does provide transactions this method
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causes the the database to roll back to the start of any
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pending transaction. Closing a connection without
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committing the changes first will cause an implicit
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rollback to be performed.
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cursor()
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Return a new Cursor Object using the connection. If the
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database does not provide a direct cursor concept, the
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module will have to emulate cursors using other means to
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the extent needed by this specification. [4]
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Cursor Objects
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These objects represent a database cursor, which is used to
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manage the context of a fetch operation. Cursors created from
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the same connection are not isolated, i.e., any changes
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done to the database by a cursor are immediately visible by the
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other cursors. Cursors created from different connections can
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or can not be isolated, depending on how the transaction support
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is implemented (see also the connection's rollback() and commit()
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methods.)
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Cursor Objects should respond to the following methods and
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attributes:
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description
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This read-only attribute is a sequence of 7-item
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sequences. Each of these sequences contains information
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describing one result column: (name, type_code,
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display_size, internal_size, precision, scale,
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null_ok). The first two items (name and type_code) are
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mandatory, the other five are optional and must be set to
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None if meaningfull values are not provided.
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This attribute will be None for operations that
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do not return rows or if the cursor has not had an
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operation invoked via the executeXXX() method yet.
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The type_code can be interpreted by comparing it to the
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Type Objects specified in the section below.
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rowcount
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This read-only attribute specifies the number of rows that
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the last executeXXX() produced (for DQL statements like
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'select') or affected (for DML statements like 'update' or
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'insert').
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The attribute is -1 in case no executeXXX() has been
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performed on the cursor or the rowcount of the last
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operation is not determinable by the interface. [7]
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callproc(procname[,parameters])
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(This method is optional since not all databases provide
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stored procedures. [3])
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Call a stored database procedure with the given name. The
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sequence of parameters must contain one entry for each
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argument that the procedure expects. The result of the
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call is returned as modified copy of the input
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sequence. Input parameters are left untouched, output and
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input/output parameters replaced with possibly new values.
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The procedure may also provide a result set as
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output. This must then be made available through the
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standard fetchXXX() methods.
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close()
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Close the cursor now (rather than whenever __del__ is
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called). The cursor will be unusable from this point
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forward; an Error (or subclass) exception will be raised
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if any operation is attempted with the cursor.
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execute(operation[,parameters])
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Prepare and execute a database operation (query or
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command). Parameters may be provided as sequence or
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mapping and will be bound to variables in the operation.
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Variables are specified in a database-specific notation
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(see the module's paramstyle attribute for details). [5]
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A reference to the operation will be retained by the
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cursor. If the same operation object is passed in again,
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then the cursor can optimize its behavior. This is most
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effective for algorithms where the same operation is used,
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but different parameters are bound to it (many times).
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For maximum efficiency when reusing an operation, it is
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best to use the setinputsizes() method to specify the
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parameter types and sizes ahead of time. It is legal for
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a parameter to not match the predefined information; the
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implementation should compensate, possibly with a loss of
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efficiency.
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The parameters may also be specified as list of tuples to
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e.g. insert multiple rows in a single operation, but this
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kind of usage is depreciated: executemany() should be used
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instead.
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Return values are not defined.
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executemany(operation,seq_of_parameters)
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Prepare a database operation (query or command) and then
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execute it against all parameter sequences or mappings
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found in the sequence seq_of_parameters.
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Modules are free to implement this method using multiple
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calls to the execute() method or by using array operations
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to have the database process the sequence as a whole in
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one call.
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The same comments as for execute() also apply accordingly
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to this method.
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Return values are not defined.
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fetchone()
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Fetch the next row of a query result set, returning a
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single sequence, or None when no more data is
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available. [6]
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An Error (or subclass) exception is raised if the previous
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call to executeXXX() did not produce any result set or no
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call was issued yet.
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fetchmany([size=cursor.arraysize])
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Fetch the next set of rows of a query result, returning a
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sequence of sequences (e.g. a list of tuples). An empty
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sequence is returned when no more rows are available.
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The number of rows to fetch per call is specified by the
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parameter. If it is not given, the cursor's arraysize
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determines the number of rows to be fetched. The method
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should try to fetch as many rows as indicated by the size
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parameter. If this is not possible due to the specified
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number of rows not being available, fewer rows may be
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returned.
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An Error (or subclass) exception is raised if the previous
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call to executeXXX() did not produce any result set or no
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call was issued yet.
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Note there are performance considerations involved with
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the size parameter. For optimal performance, it is
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usually best to use the arraysize attribute. If the size
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parameter is used, then it is best for it to retain the
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same value from one fetchmany() call to the next.
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fetchall()
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Fetch all (remaining) rows of a query result, returning
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them as a sequence of sequences (e.g. a list of tuples).
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Note that the cursor's arraysize attribute can affect the
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performance of this operation.
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An Error (or subclass) exception is raised if the previous
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call to executeXXX() did not produce any result set or no
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call was issued yet.
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nextset()
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(This method is optional since not all databases support
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multiple result sets. [3])
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This method will make the cursor skip to the next
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available set, discarding any remaining rows from the
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current set.
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If there are no more sets, the method returns
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None. Otherwise, it returns a true value and subsequent
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calls to the fetch methods will return rows from the next
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result set.
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An Error (or subclass) exception is raised if the previous
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call to executeXXX() did not produce any result set or no
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call was issued yet.
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arraysize
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This read/write attribute specifies the number of rows to
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fetch at a time with fetchmany(). It defaults to 1 meaning
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to fetch a single row at a time.
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Implementations must observe this value with respect to
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the fetchmany() method, but are free to interact with the
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database a single row at a time. It may also be used in
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the implementation of executemany().
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setinputsizes(sizes)
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This can be used before a call to executeXXX() to
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predefine memory areas for the operation's parameters.
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sizes is specified as a sequence -- one item for each
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input parameter. The item should be a Type Object that
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corresponds to the input that will be used, or it should
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be an integer specifying the maximum length of a string
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parameter. If the item is None, then no predefined memory
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area will be reserved for that column (this is useful to
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avoid predefined areas for large inputs).
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This method would be used before the executeXXX() method
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is invoked.
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Implementations are free to have this method do nothing
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and users are free to not use it.
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setoutputsize(size[,column])
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Set a column buffer size for fetches of large columns
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(e.g. LONGs, BLOBs, etc.). The column is specified as an
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index into the result sequence. Not specifying the column
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will set the default size for all large columns in the
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cursor.
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This method would be used before the executeXXX() method
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is invoked.
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Implementations are free to have this method do nothing
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and users are free to not use it.
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Type Objects and Constructors
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Many databases need to have the input in a particular format for
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binding to an operation's input parameters. For example, if an
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input is destined for a DATE column, then it must be bound to the
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database in a particular string format. Similar problems exist
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for "Row ID" columns or large binary items (e.g. blobs or RAW
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columns). This presents problems for Python since the parameters
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to the executeXXX() method are untyped. When the database module
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sees a Python string object, it doesn't know if it should be bound
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as a simple CHAR column, as a raw BINARY item, or as a DATE.
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To overcome this problem, a module must provide the constructors
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defined below to create objects that can hold special values.
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When passed to the cursor methods, the module can then detect the
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proper type of the input parameter and bind it accordingly.
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A Cursor Object's description attribute returns information about
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each of the result columns of a query. The type_code must compare
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equal to one of Type Objects defined below. Type Objects may be
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equal to more than one type code (e.g. DATETIME could be equal to
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the type codes for date, time and timestamp columns; see the
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Implementation Hints below for details).
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The module exports the following constructors and singletons:
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Date(year,month,day)
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This function constructs an object holding a date value.
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Time(hour,minute,second)
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This function constructs an object holding a time value.
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Timestamp(year,month,day,hour,minute,second)
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This function constructs an object holding a time stamp
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value.
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DateFromTicks(ticks)
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This function constructs an object holding a date value
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from the given ticks value (number of seconds since the
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epoch; see the documentation of the standard Python time
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module for details).
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TimeFromTicks(ticks)
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This function constructs an object holding a time value
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from the given ticks value (number of seconds since the
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epoch; see the documentation of the standard Python time
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module for details).
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TimestampFromTicks(ticks)
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This function constructs an object holding a time stamp
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value from the given ticks value (number of seconds since
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the epoch; see the documentation of the standard Python
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time module for details).
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Binary(string)
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This function constructs an object capable of holding a
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binary (long) string value.
|
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STRING
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This type object is used to describe columns in a database
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that are string-based (e.g. CHAR).
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BINARY
|
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This type object is used to describe (long) binary columns
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in a database (e.g. LONG, RAW, BLOBs).
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NUMBER
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This type object is used to describe numeric columns in a
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database.
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DATETIME
|
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This type object is used to describe date/time columns in
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a database.
|
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|
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ROWID
|
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This type object is used to describe the "Row ID" column
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in a database.
|
||
|
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SQL NULL values are represented by the Python None singleton on
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input and output.
|
||
|
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Note: Usage of Unix ticks for database interfacing can cause
|
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troubles because of the limited date range they cover.
|
||
|
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Implementation Hints
|
||
|
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* The preferred object types for the date/time objects are those
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defined in the mxDateTime package. It provides all necessary
|
||
constructors and methods both at Python and C level.
|
||
|
||
* The preferred object type for Binary objects are the
|
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buffer types available in standard Python starting with
|
||
version 1.5.2. Please see the Python documentation for
|
||
details. For information about the the C interface have a
|
||
look at Include/bufferobject.h and
|
||
Objects/bufferobject.c in the Python source
|
||
distribution.
|
||
|
||
* 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 apply(Date,time.localtime(ticks)[:3])
|
||
|
||
def TimeFromTicks(ticks):
|
||
return apply(Time,time.localtime(ticks)[3:6])
|
||
|
||
def TimestampFromTicks(ticks):
|
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return apply(Timestamp,time.localtime(ticks)[:6])
|
||
|
||
* 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.
|
||
|
||
|
||
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.
|
||
|
||
|
||
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:
|
||
|
||
* Define a useful return value for .nextset() for the case where
|
||
a new result set is available.
|
||
|
||
* Create a fixed point numeric type 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 .fetchXXX() 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 .fetchXXX() 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.
|
||
|
||
Copyright
|
||
|
||
This document has been placed in the Public Domain.
|
||
|
||
|
||
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
End:
|