The psycopg2 module content

The module interface respects the standard defined in the DB API 2.0.

psycopg2.connect(dsn=None, connection_factory=None, cursor_factory=None, async=False, \*\*kwargs)

Create a new database session and return a new connection object.

The connection parameters can be specified as a libpq connection string using the dsn parameter:

conn = psycopg2.connect("dbname=test user=postgres password=secret")

or using a set of keyword arguments:

conn = psycopg2.connect(dbname="test", user="postgres", password="secret")

or using a mix of both: if the same parameter name is specified in both sources, the kwargs value will have precedence over the dsn value. Note that either the dsn or at least one connection-related keyword argument is required.

The basic connection parameters are:

  • dbname – the database name (database is a deprecated alias)

  • user – user name used to authenticate

  • password – password used to authenticate

  • host – database host address (defaults to UNIX socket if not provided)

  • port – connection port number (defaults to 5432 if not provided)

Any other connection parameter supported by the client library/server can be passed either in the connection string or as a keyword. The PostgreSQL documentation contains the complete list of the supported parameters. Also note that the same parameters can be passed to the client library using environment variables.

Using the connection_factory parameter a different class or connections factory can be specified. It should be a callable object taking a dsn string argument. See Connection and cursor factories for details. If a cursor_factory is specified, the connection’s cursor_factory is set to it. If you only need customized cursors you can use this parameter instead of subclassing a connection.

Using async=True an asynchronous connection will be created: see Asynchronous support to know about advantages and limitations. async_ is a valid alias for the Python version where async is a keyword.

Changed in version 2.4.3: any keyword argument is passed to the connection. Previously only the basic parameters (plus sslmode) were supported as keywords.

Changed in version 2.5: added the cursor_factory parameter.

Changed in version 2.7: both dsn and keyword arguments can be specified.

Changed in version 2.7: added async_ alias.

See also

DB API extension

The non-connection-related keyword parameters are Psycopg extensions to the DB API 2.0.

psycopg2.apilevel

String constant stating the supported DB API level. For psycopg2 is 2.0.

psycopg2.threadsafety

Integer constant stating the level of thread safety the interface supports. For psycopg2 is 2, i.e. threads can share the module and the connection. See Thread and process safety for details.

psycopg2.paramstyle

String constant stating the type of parameter marker formatting expected by the interface. For psycopg2 is pyformat. See also Passing parameters to SQL queries.

psycopg2.__libpq_version__

Integer constant reporting the version of the libpq library this psycopg2 module was compiled with (in the same format of server_version). If this value is greater or equal than 90100 then you may query the version of the actually loaded library using the libpq_version() function.

Exceptions

In compliance with the DB API 2.0, the module makes informations about errors available through the following exceptions:

exception psycopg2.Warning

Exception raised for important warnings like data truncations while inserting, etc. It is a subclass of the Python StandardError (Exception on Python 3).

exception psycopg2.Error

Exception that is the base class of all other error exceptions. You can use this to catch all errors with one single except statement. Warnings are not considered errors and thus not use this class as base. It is a subclass of the Python StandardError (Exception on Python 3).

pgerror

String representing the error message returned by the backend, None if not available.

pgcode

String representing the error code returned by the backend, None if not available. The errorcodes module contains symbolic constants representing PostgreSQL error codes.

>>> try:
...     cur.execute("SELECT * FROM barf")
... except psycopg2.Error as e:
...     pass

>>> e.pgcode
'42P01'
>>> print e.pgerror
ERROR:  relation "barf" does not exist
LINE 1: SELECT * FROM barf
                      ^
cursor

The cursor the exception was raised from; None if not applicable.

diag

A Diagnostics object containing further information about the error.

>>> try:
...     cur.execute("SELECT * FROM barf")
... except psycopg2.Error, e:
...     pass

>>> e.diag.severity
'ERROR'
>>> e.diag.message_primary
'relation "barf" does not exist'

New in version 2.5.

DB API extension

The pgerror, pgcode, cursor, and diag attributes are Psycopg extensions.

exception psycopg2.InterfaceError

Exception raised for errors that are related to the database interface rather than the database itself. It is a subclass of Error.

exception psycopg2.DatabaseError

Exception raised for errors that are related to the database. It is a subclass of Error.

exception psycopg2.DataError

Exception raised for errors that are due to problems with the processed data like division by zero, numeric value out of range, etc. It is a subclass of DatabaseError.

exception psycopg2.OperationalError

Exception raised for errors that are related to the database’s operation and not necessarily under the control of the programmer, e.g. an unexpected disconnect occurs, the data source name is not found, a transaction could not be processed, a memory allocation error occurred during processing, etc. It is a subclass of DatabaseError.

exception psycopg2.IntegrityError

Exception raised when the relational integrity of the database is affected, e.g. a foreign key check fails. It is a subclass of DatabaseError.

exception psycopg2.InternalError

Exception raised when the database encounters an internal error, e.g. the cursor is not valid anymore, the transaction is out of sync, etc. It is a subclass of DatabaseError.

exception psycopg2.ProgrammingError

Exception raised for programming errors, e.g. table not found or already exists, syntax error in the SQL statement, wrong number of parameters specified, etc. It is a subclass of DatabaseError.

exception psycopg2.NotSupportedError

Exception raised in case a method or database API was used which is not supported by the database, e.g. requesting a rollback() on a connection that does not support transaction or has transactions turned off. It is a subclass of DatabaseError.

DB API extension

Psycopg actually raises a different exception for each SQLSTATE error returned by the database: the classes are available in the psycopg2.errors module. Every exception class is a subclass of one of the exception classes defined here though, so they don’t need to be trapped specifically: trapping Error or DatabaseError is usually what needed to write a generic error handler; trapping a specific error such as NotNullViolation can be useful to write specific exception handlers.

This is the exception inheritance layout:

StandardError
|__ Warning
|__ Error
    |__ InterfaceError
    |__ DatabaseError
        |__ DataError
        |__ OperationalError
        |__ IntegrityError
        |__ InternalError
        |__ ProgrammingError
        |__ NotSupportedError

Type Objects and Constructors

Note

This section is mostly copied verbatim from the DB API 2.0 specification. While these objects are exposed in compliance to the DB API, Psycopg offers very accurate tools to convert data between Python and PostgreSQL formats. See Adapting new Python types to SQL syntax and Type casting of SQL types into Python 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, a module must provide the constructors defined below to create objects that can hold special values. When passed to the cursor methods, the module can then detect the proper type of the input parameter and bind it accordingly.

A Cursor Object’s description attribute returns information about each of the result columns of a query. The type_code must compare equal to one of Type Objects defined below. Type Objects may be equal to more than one type code (e.g. DATETIME could be equal to the type codes for date, time and timestamp columns; see the Implementation Hints below for details).

The module exports the following constructors and singletons:

psycopg2.Date(year, month, day)

This function constructs an object holding a date value.

psycopg2.Time(hour, minute, second)

This function constructs an object holding a time value.

psycopg2.Timestamp(year, month, day, hour, minute, second)

This function constructs an object holding a time stamp value.

psycopg2.DateFromTicks(ticks)

This function constructs an object holding a date value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).

psycopg2.TimeFromTicks(ticks)

This function constructs an object holding a time value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).

psycopg2.TimestampFromTicks(ticks)

This function constructs an object holding a time stamp value from the given ticks value (number of seconds since the epoch; see the documentation of the standard Python time module for details).

psycopg2.Binary(string)

This function constructs an object capable of holding a binary (long) string value.

Note

All the adapters returned by the module level factories (Binary, Date, Time, Timestamp and the *FromTicks variants) expose the wrapped object (a regular Python object such as datetime) in an adapted attribute.

psycopg2.STRING

This type object is used to describe columns in a database that are string-based (e.g. CHAR).

psycopg2.BINARY

This type object is used to describe (long) binary columns in a database (e.g. LONG, RAW, BLOBs).

psycopg2.NUMBER

This type object is used to describe numeric columns in a database.

psycopg2.DATETIME

This type object is used to describe date/time columns in a database.

psycopg2.ROWID

This type object is used to describe the “Row ID” column in a database.