[DB-SIG] Extensions to DB API 2.0
Fri, 23 Nov 2001 12:29:47 +0100
I would like to integrate the proposed standard extensions to=20
DB API 2.0 into the DB API PEP in the next few days. If you have
more comments to make, please speak up now.
PEP: Addition to PEP 249
Title: Optional Database API Extensions
Version: $Revision: 1.2$
Author: firstname.lastname@example.org (Marc-Andr=E9 Lemburg)
Type: Standards Track
This pre-PEP should provide a discussion basis for an "Optional
Extensions" section in the DB API 2.0 standard PEP 249.
Feel free to add/change text as required.
Even though the DB API 2.0 standard already provides a multitude
of APIs, there are some common situations where a database package
writer will want to add features which are specific to the
database, but which could also be implemented for other databases
This pre-PEP will collect these common extensions for subsequent
inclusion in the DB API 2.0 standard. Note that all extensions
will be marked "optional", meaning that database writers are free
to implement them, but are not required to do so in order to have
a DB API 2.0 compatible package.
Proposed Common Extensions
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), to raise
a NotSupportedError in case the availability can only be checked
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.
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 -1 if the index cannot
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 are attributes (in addition
to being available at module scope).
These attributes simplify error handling in multi-connection
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
Warning Message: "DB-API extension cursor.connection used"
Cursor Method .scroll(value[,mode=3D'relative'])
Scroll the cursor in the result set to a new position according
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
The list is cleared by all standard cursor methods calls (prior
to executing the call) except for the .fetchXXX() 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
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
Warning Message: "DB-API extension cursor.next() used"
Cursor Method .__iter__()
Return self to make cursors compatible to the iteration protocol.
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"
Proposed Error Handling Extension
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.=20
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 error
value as construction arguments.
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
Proposed Informational Extensions
A very common question on the DB-SIG mailing list is how to
construct a dictionary out of the tuples returned by .fetchxxx().
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 .fetchxxx() methods
is that this approach has several drawbacks:=20
* 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
As a result, accessing the columns through dictionary keys varies
between databases and makes writing portable code impossible.
This PEP only affects database modules/packages which adhere to
the Python DB API 2.0 standard. Some of these extensions may
become manditory features in future DB API standard versions.
This document has been placed in the public domain.
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