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PEP 248 -- Python Database API Specification v1.0

PEP:248
Title:Python Database API Specification v1.0
Author:mal at lemburg.com (Marc-André Lemburg)
Discussions-To:db-sig at 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Superseded-By:249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.

Source: https://github.com/python/peps/blob/master/pep-0248.txt