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psycopg2/doc/src/extensions.rst
Daniele Varrazzo c008f9d1ce Added documentation about the lo64 support
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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|_.
.. 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`.
.. 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: http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-EXPORT
.. method:: seek(offset, whence=0)
Set the lobject current position.
.. versionchanged:: 2.6.0
added support for *offset* > 2GB.
.. method:: tell()
Return the lobject current position.
.. versionadded:: 2.2.0
.. versionchanged:: 2.6.0
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: http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-TRUNCATE
.. versionadded:: 2.2.0
.. versionchanged:: 2.6.0
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:: 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
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.
.. autofunction:: set_wait_callback(f)
.. versionadded:: 2.2.0
.. autofunction:: get_wait_callback()
.. versionadded:: 2.2.0
.. _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)
Register a new adapter for the objects of class *class*.
*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 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])
Register a type caster created using `new_type()`.
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`__ names to `Python codec`__ names.
Used by Psycopg when adapting or casting unicode strings. See
:ref:`unicode-handling`.
.. __: http://www.postgresql.org/docs/current/static/multibyte.html
.. __: http://docs.python.org/library/codecs.html#standard-encodings
.. index::
single: Exceptions; Additional
Additional exceptions
---------------------
The module exports a few exceptions in addition to the :ref:`standard ones
<dbapi-exceptions>` defined by the |DBAPI|_.
.. 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
.. index::
pair: Isolation level; Constants
.. _isolation-level-constants:
Isolation level constants
-------------------------
Psycopg2 `connection` objects hold informations about the PostgreSQL
`transaction isolation level`_. The current transaction level can be read
from the `~connection.isolation_level` attribute. The default isolation
level is :sql:`READ COMMITTED`. A different isolation level con be set
through the `~connection.set_isolation_level()` method. The level can be
set to one of the following constants:
.. data:: ISOLATION_LEVEL_AUTOCOMMIT
No transaction is started when command are issued 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)
See also :ref:`transactions-control`.
.. data:: ISOLATION_LEVEL_READ_UNCOMMITTED
The :sql:`READ UNCOMMITTED` isolation level is defined in the SQL standard
but not available in the |MVCC| model of PostgreSQL: it is replaced by the
stricter :sql:`READ COMMITTED`.
.. data:: ISOLATION_LEVEL_READ_COMMITTED
This is usually the the default PostgreSQL value, but a different default
may be set in the database configuration.
A new transaction is started at the first `~cursor.execute()` command on a
cursor and at each new `!execute()` after a `~connection.commit()` or a
`~connection.rollback()`. The transaction runs in the PostgreSQL
:sql:`READ COMMITTED` isolation level: a :sql:`SELECT` query sees only
data committed before the query began; it never sees either uncommitted
data or changes committed during query execution by concurrent
transactions.
.. seealso:: `Read Committed Isolation Level`__ in PostgreSQL
documentation.
.. __: http://www.postgresql.org/docs/current/static/transaction-iso.html#XACT-READ-COMMITTED
.. data:: ISOLATION_LEVEL_REPEATABLE_READ
As in `!ISOLATION_LEVEL_READ_COMMITTED`, a new transaction is started at
the first `~cursor.execute()` command. Transactions run at a
:sql:`REPEATABLE READ` isolation level: all the queries in a transaction
see a snapshot as of the start of the transaction, not as of the start of
the current query within the transaction. However applications using this
level must be prepared to retry transactions due to serialization
failures.
While this level provides a guarantee that each transaction sees a
completely stable view of the database, this view will not necessarily
always be consistent with some serial (one at a time) execution of
concurrent transactions of the same level.
.. versionchanged:: 2.4.2
The value was an alias for `!ISOLATION_LEVEL_SERIALIZABLE` before. The
two levels are distinct since PostgreSQL 9.1
.. seealso:: `Repeatable Read Isolation Level`__ in PostgreSQL
documentation.
.. __: http://www.postgresql.org/docs/current/static/transaction-iso.html#XACT-REPEATABLE-READ
.. data:: ISOLATION_LEVEL_SERIALIZABLE
As in `!ISOLATION_LEVEL_READ_COMMITTED`, a new transaction is started at
the first `~cursor.execute()` command. Transactions run at a
:sql:`SERIALIZABLE` isolation level. This is the strictest transactions
isolation level, equivalent to having the transactions executed serially
rather than concurrently. However applications using this level must be
prepared to retry transactions due to serialization failures.
Starting from PostgreSQL 9.1, this mode monitors for conditions which
could make execution of a concurrent set of serializable transactions
behave in a manner inconsistent with all possible serial (one at a time)
executions of those transaction. In previous version the behaviour was the
same of the :sql:`REPEATABLE READ` isolation level.
.. seealso:: `Serializable Isolation Level`__ in PostgreSQL documentation.
.. __: http://www.postgresql.org/docs/current/static/transaction-iso.html#XACT-SERIALIZABLE
.. index::
pair: Transaction status; Constants
.. _transaction-status-constants:
Transaction status constants
----------------------------
These values represent the possible status of a transaction: the current value
can be read using the `connection.get_transaction_status()` method.
.. data:: TRANSACTION_STATUS_IDLE
The session is idle and there is no current transaction.
.. data:: TRANSACTION_STATUS_ACTIVE
A command is currently in progress.
.. data:: TRANSACTION_STATUS_INTRANS
The session is idle in a valid transaction block.
.. data:: TRANSACTION_STATUS_INERROR
The session is idle in a failed transaction block.
.. data:: TRANSACTION_STATUS_UNKNOWN
Reported if the connection with the server is bad.
.. index::
pair: Connection status; Constants
.. _connection-status-constants:
Connection status constants
---------------------------
These values represent the possible status of a connection: the current value
can be read from the `~connection.status` attribute.
It is possible to find the connection in other status than the one shown below.
Those are the only states in which a working connection is expected to be found
during the execution of regular Python client code: other states are for
internal usage and Python code should not rely on them.
.. data:: STATUS_READY
Connection established. No transaction in progress.
.. data:: STATUS_BEGIN
Connection established. A transaction is currently in progress.
.. data:: STATUS_IN_TRANSACTION
An alias for `STATUS_BEGIN`
.. data:: STATUS_PREPARED
The connection has been prepared for the second phase in a :ref:`two-phase
commit <tpc>` transaction. The connection can't be used to send commands
to the database until the transaction is finished with
`~connection.tpc_commit()` or `~connection.tpc_rollback()`.
.. versionadded:: 2.3
.. index::
pair: Poll status; Constants
.. _poll-constants:
Poll constants
--------------
.. versionadded:: 2.2.0
These values can be returned by `connection.poll()` during asynchronous
connection and communication. They match the values in the libpq enum
`!PostgresPollingStatusType`. See :ref:`async-support` and
:ref:`green-support`.
.. data:: POLL_OK
The data being read is available, or the file descriptor is ready for
writing: reading or writing will not block.
.. data:: POLL_READ
Some data is being read from the backend, but it is not available yet on
the client and reading would block. Upon receiving this value, the client
should wait for the connection file descriptor to be ready *for reading*.
For example::
select.select([conn.fileno()], [], [])
.. data:: POLL_WRITE
Some data is being sent to the backend but the connection file descriptor
can't currently accept new data. Upon receiving this value, the client
should wait for the connection file descriptor to be ready *for writing*.
For example::
select.select([], [conn.fileno()], [])
.. data:: POLL_ERROR
There was a problem during connection polling. This value should actually
never be returned: in case of poll error usually an exception containing
the relevant details is raised.
Additional database types
-------------------------
The `!extensions` module includes typecasters for many standard
PostgreSQL types. These objects allow the conversion of returned data into
Python objects. All the typecasters are automatically registered, except
`UNICODE` and `UNICODEARRAY`: you can register them using
`register_type()` in order to receive Unicode objects instead of strings
from the database. See :ref:`unicode-handling` for details.
.. data:: BOOLEAN
DATE
DECIMAL
FLOAT
INTEGER
INTERVAL
LONGINTEGER
TIME
UNICODE
Typecasters for basic types. Note that a few other ones (`~psycopg2.BINARY`,
`~psycopg2.DATETIME`, `~psycopg2.NUMBER`, `~psycopg2.ROWID`,
`~psycopg2.STRING`) are exposed by the `psycopg2` module for |DBAPI|_
compliance.
.. data:: BINARYARRAY
BOOLEANARRAY
DATEARRAY
DATETIMEARRAY
DECIMALARRAY
FLOATARRAY
INTEGERARRAY
INTERVALARRAY
LONGINTEGERARRAY
ROWIDARRAY
STRINGARRAY
TIMEARRAY
UNICODEARRAY
Typecasters to convert arrays of sql types into Python lists.
.. data:: PYDATE
PYDATETIME
PYINTERVAL
PYTIME
PYDATEARRAY
PYDATETIMEARRAY
PYINTERVALARRAY
PYTIMEARRAY
Typecasters to convert time-related data types to Python `!datetime`
objects.
.. data:: MXDATE
MXDATETIME
MXINTERVAL
MXTIME
MXDATEARRAY
MXDATETIMEARRAY
MXINTERVALARRAY
MXTIMEARRAY
Typecasters to convert time-related data types to `mx.DateTime`_ objects.
Only available if Psycopg was compiled with `!mx` support.
.. versionchanged:: 2.2.0
previously the `DECIMAL` typecaster and the specific time-related
typecasters (`!PY*` and `!MX*`) were not exposed by the `extensions`
module. In older versions they can be imported from the implementation
module `!psycopg2._psycopg`.
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