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`psycopg2.extensions` -- Extensions to the DB API
======================================================

.. sectionauthor:: Daniele Varrazzo <daniele.varrazzo@gmail.com>

.. module:: psycopg2.extensions

.. testsetup:: *

from psycopg2.extensions import AsIs, Binary, QuotedString, ISOLATION_LEVEL_AUTOCOMMIT

The module contains a few objects and function extending the minimum set of
functionalities defined by the |DBAPI|_.

Classes definitions
-------------------

Instances of these classes are usually returned by factory functions or
attributes. Their definitions are exposed here to allow subclassing,
introspection etc.

.. class:: connection(dsn, async=False)

Is the class usually returned by the `~psycopg2.connect()` function.
It is exposed by the `extensions` module in order to allow
subclassing to extend its behaviour: the subclass should be passed to the
`!connect()` function using the `connection_factory` parameter.
See also :ref:`subclassing-connection`.

For a complete description of the class, see `connection`.

.. versionchanged:: 2.7
*async_* can be used as alias for *async*.

.. class:: cursor(conn, name=None)

It is the class usually returned by the `connection.cursor()`
method. It is exposed by the `extensions` module in order to allow
subclassing to extend its behaviour: the subclass should be passed to the
`!cursor()` method using the `cursor_factory` parameter. See
also :ref:`subclassing-cursor`.

For a complete description of the class, see `cursor`.

.. class:: lobject(conn [, oid [, mode [, new_oid [, new_file ]]]])

Wrapper for a PostgreSQL large object. See :ref:`large-objects` for an
overview.

The class can be subclassed: see the `connection.lobject()` to know
how to specify a `!lobject` subclass.

.. versionadded:: 2.0.8

.. attribute:: oid

Database OID of the object.

.. attribute:: mode

The mode the database was open. See `connection.lobject()` for a
description of the available modes.

.. method:: read(bytes=-1)

Read a chunk of data from the current file position. If -1 (default)
read all the remaining data.

The result is an Unicode string (decoded according to
`connection.encoding`) if the file was open in ``t`` mode, a bytes
string for ``b`` mode.

.. versionchanged:: 2.4
added Unicode support.

.. method:: write(str)

Write a string to the large object. Return the number of bytes
written. Unicode strings are encoded in the `connection.encoding`
before writing.

.. versionchanged:: 2.4
added Unicode support.

.. method:: export(file_name)

Export the large object content to the file system.

The method uses the efficient |lo_export|_ libpq function.

.. |lo_export| replace:: `!lo_export()`
.. _lo_export: https://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-EXPORT

.. method:: seek(offset, whence=0)

Set the lobject current position.

.. versionchanged:: 2.6
added support for *offset* > 2GB.

.. method:: tell()

Return the lobject current position.

.. versionadded:: 2.2

.. versionchanged:: 2.6
added support for return value > 2GB.

.. method:: truncate(len=0)

Truncate the lobject to the given size.

The method will only be available if Psycopg has been built against
libpq from PostgreSQL 8.3 or later and can only be used with
PostgreSQL servers running these versions. It uses the |lo_truncate|_
libpq function.

.. |lo_truncate| replace:: `!lo_truncate()`
.. _lo_truncate: https://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-TRUNCATE

.. versionadded:: 2.2

.. versionchanged:: 2.6
added support for *len* > 2GB.

.. warning::

If Psycopg is built with |lo_truncate| support or with the 64 bits API
support (resp. from PostgreSQL versions 8.3 and 9.3) but at runtime an
older version of the dynamic library is found, the ``psycopg2`` module
will fail to import. See :ref:`the lo_truncate FAQ <faq-lo_truncate>`
about the problem.

.. method:: close()

Close the object.

.. attribute:: closed

Boolean attribute specifying if the object is closed.

.. method:: unlink()

Close the object and remove it from the database.

.. autoclass:: ConnectionInfo(connection)

.. versionadded:: 2.8

.. autoattribute:: dbname
.. autoattribute:: user
.. autoattribute:: password
.. autoattribute:: host
.. autoattribute:: port
.. autoattribute:: options
.. autoattribute:: dsn_parameters

Example::

>>> conn.info.dsn_parameters
{'dbname': 'test', 'user': 'postgres', 'port': '5432', 'sslmode': 'prefer'}

Requires libpq >= 9.3.

.. autoattribute:: status
.. autoattribute:: transaction_status
.. automethod:: parameter_status(name)

.. autoattribute:: protocol_version

Currently Psycopg supports only protocol 3, which allows connection
to PostgreSQL server from version 7.4. Psycopg versions previous than
2.3 support both protocols 2 and 3.

.. autoattribute:: server_version

The number is formed by converting the major, minor, and revision
numbers into two-decimal-digit numbers and appending them together.
After PostgreSQL 10 the minor version was dropped, so the second group
of digits is always ``00``. For example, version 9.3.5 will be
returned as ``90305``, version 10.2 as ``100002``.

.. autoattribute:: error_message
.. autoattribute:: socket
.. autoattribute:: backend_pid
.. autoattribute:: needs_password
.. autoattribute:: used_password
.. autoattribute:: ssl_in_use
.. automethod:: ssl_attribute(name)
.. autoattribute:: ssl_attribute_names

.. class:: Column(\*args, \*\*kwargs)

Description of one result column, exposed as items of the
`cursor.description` sequence.

.. versionadded:: 2.8

in previous version the `!description` attribute was a sequence of
simple tuples or namedtuples.

.. attribute:: name

The name of the column returned.

.. attribute:: type_code

The PostgreSQL OID of the column. You can use the |pg_type|_ system
table to get more informations about the type. This is the value used
by Psycopg to decide what Python type use to represent the value. See
also :ref:`type-casting-from-sql-to-python`.

.. attribute:: display_size

Supposed to be the actual length of the column in bytes. Obtaining
this value is computationally intensive, so it is always `!None`.

.. versionchanged:: 2.8
It was previously possible to obtain this value using a compiler
flag at builtin.

.. attribute:: internal_size

The size in bytes of the column associated to this column on the
server. Set to a negative value for variable-size types See also
PQfsize_.

.. attribute:: precision

Total number of significant digits in columns of type |NUMERIC|_.
`!None` for other types.

.. attribute:: scale

Count of decimal digits in the fractional part in columns of type
|NUMERIC|. `!None` for other types.

.. attribute:: null_ok

Always `!None` as not easy to retrieve from the libpq.

.. attribute:: table_oid

The oid of the table from which the column was fetched (matching
:sql:`pg_class.oid`). `!None` if the column is not a simple reference
to a table column. See also PQftable_.

.. versionadded:: 2.8

.. attribute:: table_column

The number of the column (within its table) making up the result
(matching :sql:`pg_attribute.attnum`, so it will start from 1).
`!None` if the column is not a simple reference to a table column. See
also PQftablecol_.

.. versionadded:: 2.8

.. |pg_type| replace:: :sql:`pg_type`
.. _pg_type: https://www.postgresql.org/docs/current/static/catalog-pg-type.html
.. _PQgetlength: https://www.postgresql.org/docs/current/static/libpq-exec.html#LIBPQ-PQGETLENGTH
.. _PQfsize: https://www.postgresql.org/docs/current/static/libpq-exec.html#LIBPQ-PQFSIZE
.. _PQftable: https://www.postgresql.org/docs/current/static/libpq-exec.html#LIBPQ-PQFTABLE
.. _PQftablecol: https://www.postgresql.org/docs/current/static/libpq-exec.html#LIBPQ-PQFTABLECOL
.. _NUMERIC: https://www.postgresql.org/docs/current/static/datatype-numeric.html#DATATYPE-NUMERIC-DECIMAL
.. |NUMERIC| replace:: :sql:`NUMERIC`

.. autoclass:: Notify(pid, channel, payload='')
:members: pid, channel, payload

.. versionadded:: 2.3

.. autoclass:: Xid(format_id, gtrid, bqual)
:members: format_id, gtrid, bqual, prepared, owner, database

.. versionadded:: 2.3

.. automethod:: from_string(s)

.. autoclass:: Diagnostics(exception)

.. versionadded:: 2.5

The attributes currently available are:

.. attribute::
column_name
constraint_name
context
datatype_name
internal_position
internal_query
message_detail
message_hint
message_primary
schema_name
severity
severity_nonlocalized
source_file
source_function
source_line
sqlstate
statement_position
table_name

A string with the error field if available; `!None` if not available.
The attribute value is available only if the error sent by the server:
not all the fields are available for all the errors and for all the
server versions.

.. versionadded:: 2.8
The `!severity_nonlocalized` attribute.

.. _sql-adaptation-objects:

SQL adaptation protocol objects
-------------------------------

Psycopg provides a flexible system to adapt Python objects to the SQL syntax
(inspired to the :pep:`246`), allowing serialization in PostgreSQL. See
:ref:`adapting-new-types` for a detailed description. The following objects
deal with Python objects adaptation:

.. function:: adapt(obj)

Return the SQL representation of *obj* as an `ISQLQuote`. Raise a
`~psycopg2.ProgrammingError` if how to adapt the object is unknown.
In order to allow new objects to be adapted, register a new adapter for it
using the `register_adapter()` function.

The function is the entry point of the adaptation mechanism: it can be
used to write adapters for complex objects by recursively calling
`!adapt()` on its components.

.. function:: register_adapter(class, adapter)

*adapter* should be a function taking a single argument (the object
to adapt) and returning an object conforming to the `ISQLQuote`
protocol (e.g. exposing a `!getquoted()` method). The `AsIs` is
often useful for this task.

Once an object is registered, it can be safely used in SQL queries and by
the `adapt()` function.

.. class:: ISQLQuote(wrapped_object)

Represents the SQL adaptation protocol. Objects conforming this protocol
should implement a `getquoted()` and optionally a `prepare()` method.

Adapters may subclass `!ISQLQuote`, but is not necessary: it is
enough to expose a `!getquoted()` method to be conforming.

.. attribute:: _wrapped

The wrapped object passes to the constructor

.. method:: getquoted()

Subclasses or other conforming objects should return a valid SQL
string representing the wrapped object. In Python 3 the SQL must be
returned in a `!bytes` object. The `!ISQLQuote` implementation does
nothing.

.. method:: prepare(conn)

Prepare the adapter for a connection. The method is optional: if
implemented, it will be invoked before `!getquoted()` with the
connection to adapt for as argument.

A conform object can implement this method if the SQL
representation depends on any server parameter, such as the server
version or the :envvar:`standard_conforming_string` setting. Container
objects may store the connection and use it to recursively prepare
contained objects: see the implementation for
`psycopg2.extensions.SQL_IN` for a simple example.

.. class:: AsIs(object)

Adapter conform to the `ISQLQuote` protocol useful for objects
whose string representation is already valid as SQL representation.

.. method:: getquoted()

Return the `str()` conversion of the wrapped object.

>>> AsIs(42).getquoted()
'42'

.. class:: QuotedString(str)

Adapter conform to the `ISQLQuote` protocol for string-like
objects.

.. method:: getquoted()

Return the string enclosed in single quotes. Any single quote appearing
in the string is escaped by doubling it according to SQL string
constants syntax. Backslashes are escaped too.

>>> QuotedString(r"O'Reilly").getquoted()
"'O''Reilly'"

.. class:: Binary(str)

Adapter conform to the `ISQLQuote` protocol for binary objects.

.. method:: getquoted()

Return the string enclosed in single quotes. It performs the same
escaping of the `QuotedString` adapter, plus it knows how to
escape non-printable chars.

>>> Binary("\x00\x08\x0F").getquoted()
"'\\\\000\\\\010\\\\017'"

.. versionchanged:: 2.0.14
previously the adapter was not exposed by the `extensions`
module. In older versions it can be imported from the implementation
module `!psycopg2._psycopg`.

.. class:: Boolean
Float
SQL_IN

Specialized adapters for builtin objects.

.. class:: DateFromPy
TimeFromPy
TimestampFromPy
IntervalFromPy

Specialized adapters for Python datetime objects.

.. class:: DateFromMx
TimeFromMx
TimestampFromMx
IntervalFromMx

Specialized adapters for `mx.DateTime`_ objects.

.. data:: adapters

Dictionary of the currently registered object adapters. Use
`register_adapter()` to add an adapter for a new type.

Database types casting functions
--------------------------------

These functions are used to manipulate type casters to convert from PostgreSQL
types to Python objects. See :ref:`type-casting-from-sql-to-python` for
details.

.. function:: new_type(oids, name, adapter)

Create a new type caster to convert from a PostgreSQL type to a Python
object. The object created must be registered using
`register_type()` to be used.

:param oids: tuple of OIDs of the PostgreSQL type to convert.
:param name: the name of the new type adapter.
:param adapter: the adaptation function.

The object OID can be read from the `cursor.description` attribute
or by querying from the PostgreSQL catalog.

*adapter* should have signature :samp:`fun({value}, {cur})` where
*value* is the string representation returned by PostgreSQL and
*cur* is the cursor from which data are read. In case of
:sql:`NULL`, *value* will be `!None`. The adapter should return the
converted object.

See :ref:`type-casting-from-sql-to-python` for an usage example.

.. function:: new_array_type(oids, name, base_caster)

Create a new type caster to convert from a PostgreSQL array type to a list
of Python object. The object created must be registered using
`register_type()` to be used.

:param oids: tuple of OIDs of the PostgreSQL type to convert. It should
probably contain the oid of the array type (e.g. the ``typarray``
field in the ``pg_type`` table).
:param name: the name of the new type adapter.
:param base_caster: a Psycopg typecaster, e.g. created using the
`new_type()` function. The caster should be able to parse a single
item of the desired type.

.. versionadded:: 2.4.3

.. _cast-array-unknown:

.. note::

The function can be used to create a generic array typecaster,
returning a list of strings: just use `psycopg2.STRING` as base
typecaster. For instance, if you want to receive an array of
:sql:`macaddr` from the database, each address represented by string,
you can use::

# select typarray from pg_type where typname = 'macaddr' -> 1040
psycopg2.extensions.register_type(
psycopg2.extensions.new_array_type(
(1040,), 'MACADDR[]', psycopg2.STRING))

.. function:: register_type(obj [, scope])

If *scope* is specified, it should be a `connection` or a
`cursor`: the type caster will be effective only limited to the
specified object. Otherwise it will be globally registered.

.. data:: string_types

The global register of type casters.

.. index::
single: Encoding; Mapping

.. data:: encodings

Mapping from `PostgreSQL encoding`__ to `Python encoding`__ names.
Used by Psycopg when adapting or casting unicode strings. See
:ref:`unicode-handling`.

.. __: https://www.postgresql.org/docs/current/static/multibyte.html
.. __: https://docs.python.org/library/codecs.html#standard-encodings

.. index::
single: Exceptions; Additional

.. _extension-exceptions:

Additional exceptions
---------------------

The module exports a few exceptions in addition to the :ref:`standard ones
<dbapi-exceptions>` defined by the |DBAPI|_.

.. note::
From psycopg 2.8 these error classes are also exposed by the
`psycopg2.errors` module.

.. exception:: QueryCanceledError

(subclasses `~psycopg2.OperationalError`)

Error related to SQL query cancellation. It can be trapped specifically to
detect a timeout.

.. versionadded:: 2.0.7

.. exception:: TransactionRollbackError

(subclasses `~psycopg2.OperationalError`)

Error causing transaction rollback (deadlocks, serialization failures,
etc). It can be trapped specifically to detect a deadlock.

.. versionadded:: 2.0.7

.. _coroutines-functions:

Coroutines support functions
----------------------------

These functions are used to set and retrieve the callback function for
:ref:`cooperation with coroutine libraries <green-support>`.

.. versionadded:: 2.2

.. autofunction:: set_wait_callback(f)

.. autofunction:: get_wait_callback()

Other functions
---------------

.. function:: libpq_version()

Return the version number of the ``libpq`` dynamic library loaded as an
integer, in the same format of `~connection.server_version`.

Raise `~psycopg2.NotSupportedError` if the ``psycopg2`` module was
compiled with a ``libpq`` version lesser than 9.1 (which can be detected
by the `~psycopg2.__libpq_version__` constant).

.. versionadded:: 2.7

.. seealso:: libpq docs for `PQlibVersion()`__.

.. __: https://www.postgresql.org/docs/current/static/libpq-misc.html#LIBPQ-PQLIBVERSION

.. function:: make_dsn(dsn=None, \*\*kwargs)

Create a valid connection string from arguments.

Put together the arguments in *kwargs* into a connection string. If *dsn*
is specified too, merge the arguments coming from both the sources. If the
same argument name is specified in both the sources, the *kwargs* value
overrides the *dsn* value.

The input arguments are validated: the output should always be a valid
connection string (as far as `parse_dsn()` is concerned). If not raise
`~psycopg2.ProgrammingError`.

Example::

>>> from psycopg2.extensions import make_dsn
>>> make_dsn('dbname=foo host=example.com', password="s3cr3t")
'host=example.com password=s3cr3t dbname=foo'

.. versionadded:: 2.7

.. function:: parse_dsn(dsn)

Parse connection string into a dictionary of keywords and values.

Parsing is delegated to the libpq: different versions of the client
library may support different formats or parameters (for example,
`connection URIs`__ are only supported from libpq 9.2). Raise
`~psycopg2.ProgrammingError` if the *dsn* is not valid.

.. __: https://www.postgresql.org/docs/current/static/libpq-connect.html#LIBPQ-CONNSTRING

Example::

>>> from psycopg2.extensions import parse_dsn
>>> parse_dsn('dbname=test user=postgres password=secret')
{'password': 'secret', 'user': 'postgres', 'dbname': 'test'}
>>> parse_dsn("postgresql://someone@example.com/somedb?connect_timeout=10")
{'host': 'example.com', 'user': 'someone', 'dbname': 'somedb', 'connect_timeout': '10'}

.. versionadded:: 2.7

.. seealso:: libpq docs for `PQconninfoParse()`__.

.. __: https://www.postgresql.org/docs/current/static/libpq-connect.html#LIBPQ-PQCONNINFOPARSE

.. function:: quote_ident(str, scope)

Return quoted identifier according to PostgreSQL quoting rules.

The *scope* must be a `connection` or a `cursor`, the underlying
connection encoding is used for any necessary character conversion.

.. versionadded:: 2.7

.. seealso:: libpq docs for `PQescapeIdentifier()`__

.. __: https://www.postgresql.org/docs/current/static/libpq-exec.html#LIBPQ-PQESCAPEIDENTIFIER

.. method:: encrypt_password(password, user, scope=None, algorithm=None)

Return the encrypted form of a PostgreSQL password.

:param password: the cleartext password to encrypt
:param user: the name of the user to use the password for
:param scope: the scope to encrypt the password into; if *algorithm* is
``md5`` it can be `!None`
:type scope: `connection` or `cursor`
:param algorithm: the password encryption algorithm to use

The *algorithm* ``md5`` is always supported. Other algorithms are only
supported if the client libpq version is at least 10 and may require a
compatible server version: check the `PostgreSQL encryption
documentation`__ to know the algorithms supported by your server.

.. __: https://www.postgresql.org/docs/current/static/encryption-options.html

Using `!None` as *algorithm* will result in querying the server to know the
current server password encryption setting, which is a blocking operation:
query the server separately and specify a value for *algorithm* if you
want to maintain a non-blocking behaviour.

.. versionadded:: 2.8

.. seealso:: PostgreSQL docs for the `password_encryption`__ setting, libpq `PQencryptPasswordConn()`__, `PQencryptPassword()`__ functions.

.. __: https://www.postgresql.org/docs/current/static/runtime-config-connection.html#GUC-PASSWORD-ENCRYPTION
.. __: https://www.postgresql.org/docs/current/static/libpq-misc.html#LIBPQ-PQENCRYPTPASSWORDCONN
.. __: https://www.postgresql.org/docs/current/static/libpq-misc.html#LIBPQ-PQENCRYPTPASSWORD

.. index::
pair: Isolation level; Constants

.. _isolation-level-constants:

Isolation level constants
-------------------------

Psycopg2 `connection` objects hold informations about the PostgreSQL
`transaction isolation level`_. By default Psycopg doesn't change the default
configuration of the server (`ISOLATION_LEVEL_DEFAULT`); the default for
PostgreSQL servers is typically :sql:`READ COMMITTED`, but this may be changed
in the server configuration files. A different isolation level can be set
through the `~connection.set_isolation_level()` or `~connection.set_session()`
methods. The level can be set to one of the following constants:

.. data:: ISOLATION_LEVEL_AUTOCOMMIT

No transaction is started when commands are executed and no
`~connection.commit()` or `~connection.rollback()` is required.
Some PostgreSQL command such as :sql:`CREATE DATABASE` or :sql:`VACUUM`
can't run into a transaction: to run such command use::

>>> conn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)