psycopg2.extensions – Extensions to the DB API

The module contains a few objects and function extending the minimum set of functionalities defined by the DB API 2.0.

class psycopg2.extensions.connection(dsn, async=False)

Is the class usually returned by the 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 Connection and cursor factories.

For a complete description of the class, see connection.

class psycopg2.extensions.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 Connection and cursor factories.

For a complete description of the class, see cursor.

class psycopg2.extensions.lobject(conn[, oid[, mode[, new_oid[, new_file]]]])

Wrapper for a PostgreSQL large object. See Access to PostgreSQL large objects for an overview.

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

New in version 2.0.8.

oid

Database OID of the object.

mode

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

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.

Changed in version 2.4: added Unicode support.

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.

Changed in version 2.4: added Unicode support.

export(file_name)

Export the large object content to the file system.

The method uses the efficient lo_export() libpq function.

seek(offset, whence=0)

Set the lobject current position.

tell()

Return the lobject current position.

truncate(len=0)

New in version 2.2.0.

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.

Warning

If Psycopg is built with lo_truncate() support (i.e. if the pg_config used during setup is version >= 8.3), but at runtime an older libpq is found, Psycopg will fail to import. See the lo_truncate FAQ about the problem.

close()

Close the object.

closed

Boolean attribute specifying if the object is closed.

Close the object and remove it from the database.

class psycopg2.extensions.Notify(pid, channel, payload='')

A notification received from the backend.

Notify instances are made available upon reception on the notifies member of the listening connection. The object can be also accessed as a 2 items tuple returning the members (pid,channel) for backward compatibility.

See Asynchronous notifications for details.

New in version 2.3.

channel

The name of the channel to which the notification was sent.

payload

The payload message of the notification.

Attaching a payload to a notification is only available since PostgreSQL 9.0: for notifications received from previous versions of the server this member is always the empty string.

pid

The ID of the backend process that sent the notification.

Note: if the sending session was handled by Psycopg, you can use get_backend_pid() to know its PID.

class psycopg2.extensions.Xid(format_id, gtrid, bqual)

A transaction identifier used for two-phase commit.

Usually returned by the connection methods xid() and tpc_recover(). Xid instances can be unpacked as a 3-item tuples containing the items (format_id,gtrid,bqual). The str() of the object returns the transaction ID used in the commands sent to the server.

See Two-Phase Commit protocol support for an introduction.

New in version 2.3.

from_string(s)

Create a Xid object from a string representation. Static method.

If s is a PostgreSQL transaction ID produced by a XA transaction, the returned object will have format_id, gtrid, bqual set to the values of the preparing XA id. Otherwise only the gtrid is populated with the unparsed string. The operation is the inverse of the one performed by str(xid).

bqual

Branch qualifier of the transaction.

In a XA transaction every resource participating to a transaction receives a distinct branch qualifier. None if the transaction doesn’t follow the XA standard.

database

Database the recovered transaction belongs to.

format_id

Format ID in a XA transaction.

A non-negative 32 bit integer. None if the transaction doesn’t follow the XA standard.

gtrid

Global transaction ID in a XA transaction.

If the transaction doesn’t follow the XA standard, it is the plain transaction ID used in the server commands.

owner

Name of the user who prepared a recovered transaction.

prepared

Timestamp (with timezone) in which a recovered transaction was prepared.

class psycopg2.extensions.Diagnostics(exception)

Details from a database error report.

The object is returned by the diag attribute of the Error object. All the information available from the PQresultErrorField() function are exposed as attributes by the object, e.g. the severity attribute returns the PG_DIAG_SEVERITY code. Please refer to the PostgreSQL documentation for the meaning of all the attributes.

New in version 2.5.

The attributes currently available are:

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.

psycopg2.extensions.set_wait_callback(f)

Register a callback function to block waiting for data.

The callback should have signature fun(conn) and is called to wait for data available whenever a blocking function from the libpq is called. Use set_wait_callback(None) to revert to the original behaviour (i.e. using blocking libpq functions).

The function is an hook to allow coroutine-based libraries (such as Eventlet or gevent) to switch when Psycopg is blocked, allowing other coroutines to run concurrently.

See wait_select() for an example of a wait callback implementation.

New in version 2.2.0.

psycopg2.extensions.get_wait_callback()

Return the currently registered wait callback.

Return None if no callback is currently registered.

New in version 2.2.0.

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 Adapting new Python types to SQL syntax for a detailed description. The following objects deal with Python objects adaptation:

psycopg2.extensions.adapt(obj)

Return the SQL representation of obj as an ISQLQuote. Raise a 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.

psycopg2.extensions.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 psycopg2.extensions.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.

_wrapped

The wrapped object passes to the constructor

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.

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 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 psycopg2.extensions.AsIs(object)

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

getquoted()

Return the str() conversion of the wrapped object.

>>> AsIs(42).getquoted()
'42'
class psycopg2.extensions.QuotedString(str)

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

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 psycopg2.extensions.Binary(str)

Adapter conform to the ISQLQuote protocol for binary objects.

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'"

Changed in version 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 psycopg2.extensions.Boolean
class psycopg2.extensions.Float
class psycopg2.extensions.SQL_IN

Specialized adapters for builtin objects.

class psycopg2.extensions.DateFromPy
class psycopg2.extensions.TimeFromPy
class psycopg2.extensions.TimestampFromPy
class psycopg2.extensions.IntervalFromPy

Specialized adapters for Python datetime objects.

class psycopg2.extensions.DateFromMx
class psycopg2.extensions.TimeFromMx
class psycopg2.extensions.TimestampFromMx
class psycopg2.extensions.IntervalFromMx

Specialized adapters for mx.DateTime objects.

psycopg2.extensions.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 Type casting of SQL types into Python objects for details.

psycopg2.extensions.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.

Parameters:
  • oids – tuple of OIDs of the PostgreSQL type to convert.
  • name – the name of the new type adapter.
  • 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 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 NULL, value will be None. The adapter should return the converted object.

See Type casting of SQL types into Python objects for an usage example.

psycopg2.extensions.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.

Parameters:
  • 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).
  • name – the name of the new type adapter.
  • 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.

New in version 2.4.3.

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 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))
psycopg2.extensions.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.

psycopg2.extensions.string_types

The global register of type casters.

psycopg2.extensions.encodings

Mapping from PostgreSQL encoding names to Python codec names. Used by Psycopg when adapting or casting unicode strings. See Unicode handling.

Additional exceptions

The module exports a few exceptions in addition to the standard ones defined by the DB API 2.0.

exception psycopg2.extensions.QueryCanceledError

(subclasses OperationalError)

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

New in version 2.0.7.

exception psycopg2.extensions.TransactionRollbackError

(subclasses OperationalError)

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

New in version 2.0.7.

Isolation level constants

Psycopg2 connection objects hold informations about the PostgreSQL transaction isolation level. The current transaction level can be read from the isolation_level attribute. The default isolation level is READ COMMITTED. A different isolation level con be set through the set_isolation_level() method. The level can be set to one of the following constants:

psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT

No transaction is started when command are issued and no commit() or rollback() is required. Some PostgreSQL command such as CREATE DATABASE or VACUUM can’t run into a transaction: to run such command use:

>>> conn.set_isolation_level(ISOLATION_LEVEL_AUTOCOMMIT)

See also Transactions control.

psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED

The 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 READ COMMITTED.

psycopg2.extensions.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 execute() command on a cursor and at each new execute() after a commit() or a rollback(). The transaction runs in the PostgreSQL READ COMMITTED isolation level: a 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.

See also

Read Committed Isolation Level in PostgreSQL documentation.

psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ

As in ISOLATION_LEVEL_READ_COMMITTED, a new transaction is started at the first execute() command. Transactions run at a 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.

Changed in version 2.4.2: The value was an alias for ISOLATION_LEVEL_SERIALIZABLE before. The two levels are distinct since PostgreSQL 9.1

See also

Repeatable Read Isolation Level in PostgreSQL documentation.

psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE

As in ISOLATION_LEVEL_READ_COMMITTED, a new transaction is started at the first execute() command. Transactions run at a 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 REPEATABLE READ isolation level.

See also

Serializable Isolation Level in PostgreSQL documentation.

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.

psycopg2.extensions.TRANSACTION_STATUS_IDLE

The session is idle and there is no current transaction.

psycopg2.extensions.TRANSACTION_STATUS_ACTIVE

A command is currently in progress.

psycopg2.extensions.TRANSACTION_STATUS_INTRANS

The session is idle in a valid transaction block.

psycopg2.extensions.TRANSACTION_STATUS_INERROR

The session is idle in a failed transaction block.

psycopg2.extensions.TRANSACTION_STATUS_UNKNOWN

Reported if the connection with the server is bad.

Connection status constants

These values represent the possible status of a connection: the current value can be read from the 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.

psycopg2.extensions.STATUS_READY

Connection established. No transaction in progress.

psycopg2.extensions.STATUS_BEGIN

Connection established. A transaction is currently in progress.

psycopg2.extensions.STATUS_IN_TRANSACTION

An alias for STATUS_BEGIN

psycopg2.extensions.STATUS_PREPARED

The connection has been prepared for the second phase in a two-phase commit transaction. The connection can’t be used to send commands to the database until the transaction is finished with tpc_commit() or tpc_rollback().

New in version 2.3.

Poll constants

New in version 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 Asynchronous support and Support for coroutine libraries.

psycopg2.extensions.POLL_OK

The data being read is available, or the file descriptor is ready for writing: reading or writing will not block.

psycopg2.extensions.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()], [], [])
psycopg2.extensions.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()], [])
psycopg2.extensions.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 Unicode handling for details.

psycopg2.extensions.BOOLEAN
psycopg2.extensions.DATE
psycopg2.extensions.DECIMAL
psycopg2.extensions.FLOAT
psycopg2.extensions.INTEGER
psycopg2.extensions.INTERVAL
psycopg2.extensions.LONGINTEGER
psycopg2.extensions.TIME
psycopg2.extensions.UNICODE

Typecasters for basic types. Note that a few other ones (BINARY, DATETIME, NUMBER, ROWID, STRING) are exposed by the psycopg2 module for DB API 2.0 compliance.

psycopg2.extensions.BINARYARRAY
psycopg2.extensions.BOOLEANARRAY
psycopg2.extensions.DATEARRAY
psycopg2.extensions.DATETIMEARRAY
psycopg2.extensions.DECIMALARRAY
psycopg2.extensions.FLOATARRAY
psycopg2.extensions.INTEGERARRAY
psycopg2.extensions.INTERVALARRAY
psycopg2.extensions.LONGINTEGERARRAY
psycopg2.extensions.ROWIDARRAY
psycopg2.extensions.STRINGARRAY
psycopg2.extensions.TIMEARRAY
psycopg2.extensions.UNICODEARRAY

Typecasters to convert arrays of sql types into Python lists.

psycopg2.extensions.PYDATE
psycopg2.extensions.PYDATETIME
psycopg2.extensions.PYINTERVAL
psycopg2.extensions.PYTIME
psycopg2.extensions.PYDATEARRAY
psycopg2.extensions.PYDATETIMEARRAY
psycopg2.extensions.PYINTERVALARRAY
psycopg2.extensions.PYTIMEARRAY

Typecasters to convert time-related data types to Python datetime objects.

psycopg2.extensions.MXDATE
psycopg2.extensions.MXDATETIME
psycopg2.extensions.MXINTERVAL
psycopg2.extensions.MXTIME
psycopg2.extensions.MXDATEARRAY
psycopg2.extensions.MXDATETIMEARRAY
psycopg2.extensions.MXINTERVALARRAY
psycopg2.extensions.MXTIMEARRAY

Typecasters to convert time-related data types to mx.DateTime objects. Only available if Psycopg was compiled with mx support.

Changed in version 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.