Connection – The connection object¶
- 
class pg.Connection¶
This object handles a connection to a PostgreSQL database. It embeds and hides all the parameters that define this connection, thus just leaving really significant parameters in function calls.
Note
Some methods give direct access to the connection socket.
Do not use them unless you really know what you are doing.
If you prefer disabling them,
set the -DNO_DIRECT option in the Python setup file.
These methods are specified by the tag [DA].
Note
Some other methods give access to large objects
(refer to PostgreSQL user manual for more information about these).
If you want to forbid access to these from the module,
set the -DNO_LARGE option in the Python setup file.
These methods are specified by the tag [LO].
query – execute a SQL command string¶
- 
Connection.query(command[, args])¶
- Execute a SQL command string - Parameters: - command (str) – SQL command
- args – optional positional arguments
 - Returns: - result values - Return type: - Query, None- Raises: - TypeError – bad argument type, or too many arguments
- TypeError – invalid connection
- ValueError – empty SQL query or lost connection
- pg.ProgrammingError – error in query
- pg.InternalError – error during query processing
 
This method simply sends a SQL query to the database. If the query is an
insert statement that inserted exactly one row into a table that has OIDs, the
return value is the OID of the newly inserted row. If the query is an update
or delete statement, or an insert statement that did not insert exactly one
row in a table with OIDs, then the number of rows affected is returned as a
string. If it is a statement that returns rows as a result (usually a select
statement, but maybe also an "insert/update ... returning" statement),
this method returns a Query that can be accessed via the
Query.getresult(), Query.dictresult() or
Query.namedresult() methods or simply printed.
Otherwise, it returns None.
The query may optionally contain positional parameters of the form $1,
$2, etc instead of literal data, and the values supplied as a tuple.
The values are substituted by the database in such a way that they don’t
need to be escaped, making this an effective way to pass arbitrary or
unknown data without worrying about SQL injection or syntax errors.
When the database could not process the query, a pg.ProgrammingError or
a pg.InternalError is raised. You can check the SQLSTATE error code
of this error by reading its sqlstate attribute.
Example:
name = input("Name? ")
phone = con.query("select phone from employees where name=$1",
    (name,)).getresult()
reset – reset the connection¶
- 
Connection.reset()¶
- Reset the - pgconnection- Return type: - None - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
This method resets the current database connection.
cancel – abandon processing of current SQL command¶
- 
Connection.cancel()¶
- Return type: - None - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
This method requests that the server abandon processing of the current SQL command.
close – close the database connection¶
- 
Connection.close()¶
- Close the - pgconnection- Return type: - None - Raises: - TypeError – too many (any) arguments 
This method closes the database connection. The connection will be closed in any case when the connection is deleted but this allows you to explicitly close it. It is mainly here to allow the DB-SIG API wrapper to implement a close function.
transaction – get the current transaction state¶
- 
Connection.transaction()¶
- Get the current in-transaction status of the server - Returns: - the current in-transaction status - Return type: - int - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
The status returned by this method can be TRANS_IDLE (currently idle),
TRANS_ACTIVE (a command is in progress), TRANS_INTRANS (idle,
in a valid transaction block), or TRANS_INERROR (idle, in a failed
transaction block).  TRANS_UNKNOWN is reported if the connection is
bad.  The status TRANS_ACTIVE is reported only when a query has been
sent to the server and not yet completed.
parameter – get a current server parameter setting¶
- 
Connection.parameter(name)¶
- Look up a current parameter setting of the server - Parameters: - name (str) – the name of the parameter to look up - Returns: - the current setting of the specified parameter - Return type: - str or None - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
Certain parameter values are reported by the server automatically at connection startup or whenever their values change. This method can be used to interrogate these settings. It returns the current value of a parameter if known, or None if the parameter is not known.
You can use this method to check the settings of important parameters such as server_version, server_encoding, client_encoding, application_name, is_superuser, session_authorization, DateStyle, IntervalStyle, TimeZone, integer_datetimes, and standard_conforming_strings.
Values that are not reported by this method can be requested using
DB.get_parameter().
New in version 4.0.
date_format – get the currently used date format¶
- 
Connection.date_format()¶
- Look up the date format currently being used by the database - Returns: - the current date format - Return type: - str - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
This method returns the current date format used by the server.  Note that
it is cheap to call this method, since there is no database query involved
and the setting is also cached internally.  You will need the date format
when you want to manually typecast dates and timestamps coming from the
database instead of using the built-in typecast functions.  The date format
returned by this method can be directly used with date formatting functions
such as datetime.strptime().  It is derived from the current setting
of the database parameter DateStyle.
New in version 5.0.
fileno – get the socket used to connect to the database¶
- 
Connection.fileno()¶
- Get the socket used to connect to the database - Returns: - the socket id of the database connection - Return type: - int - Raises: - TypeError – too many (any) arguments
- TypeError – invalid connection
 
This method returns the underlying socket id used to connect to the database. This is useful for use in select calls, etc.
getnotify – get the last notify from the server¶
- 
Connection.getnotify()¶
- Get the last notify from the server - Returns: - last notify from server - Return type: - tuple, None - Raises: - TypeError – too many parameters
- TypeError – invalid connection
 
This method tries to get a notify from the server (from the SQL statement
NOTIFY). If the server returns no notify, the methods returns None.
Otherwise, it returns a tuple (triplet) (relname, pid, extra), where
relname is the name of the notify, pid is the process id of the
connection that triggered the notify, and extra is a payload string
that has been sent with the notification. Remember to do a listen query
first, otherwise Connection.getnotify() will always return None.
Changed in version 4.1: Support for payload strings was added in version 4.1.
inserttable – insert a list into a table¶
- 
Connection.inserttable(table, values)¶
- Insert a Python list into a database table - Parameters: - table (str) – the table name
- values (list) – list of rows values
 - Return type: - None - Raises: - TypeError – invalid connection, bad argument type, or too many arguments
- MemoryError – insert buffer could not be allocated
- ValueError – unsupported values
 
This method allows to quickly insert large blocks of data in a table: It inserts the whole values list into the given table. Internally, it uses the COPY command of the PostgreSQL database. The list is a list of tuples/lists that define the values for each inserted row. The rows values may contain string, integer, long or double (real) values.
Warning
This method doesn’t type check the fields according to the table definition; it just look whether or not it knows how to handle such types.
get/set_notice_receiver – custom notice receiver¶
- 
Connection.get_notice_receiver()¶
- Get the current notice receiver - Returns: - the current notice receiver callable - Return type: - callable, None - Raises: - TypeError – too many (any) arguments 
This method gets the custom notice receiver callback function that has
been set with Connection.set_notice_receiver(), or None if no
custom notice receiver has ever been set on the connection.
New in version 4.1.
- 
Connection.set_notice_receiver(func)¶
- Set a custom notice receiver - Parameters: - func – the custom notice receiver callback function - Return type: - None - Raises: - TypeError – the specified notice receiver is not callable 
This method allows setting a custom notice receiver callback function.
When a notice or warning message is received from the server,
or generated internally by libpq, and the message level is below
the one set with client_min_messages, the specified notice receiver
function will be called. This function must take one parameter,
the Notice object, which provides the following read-only
attributes:
Notice.pgcnx¶
the connection
Notice.message¶
the full message with a trailing newline
Notice.severity¶
the level of the message, e.g. ‘NOTICE’ or ‘WARNING’
Notice.primary¶
the primary human-readable error message
Notice.detail¶
an optional secondary error message
Notice.hint¶
an optional suggestion what to do about the problem
New in version 4.1.
putline – write a line to the server socket [DA]¶
- 
Connection.putline(line)¶
- Write a line to the server socket - Parameters: - line (str) – line to be written - Return type: - None - Raises: - TypeError – invalid connection, bad parameter type, or too many parameters 
This method allows to directly write a string to the server socket.
getline – get a line from server socket [DA]¶
- 
Connection.getline()¶
- Get a line from server socket - Returns: - the line read - Return type: - str - Raises: - TypeError – invalid connection
- TypeError – too many parameters
- MemoryError – buffer overflow
 
This method allows to directly read a string from the server socket.
endcopy – synchronize client and server [DA]¶
- 
Connection.endcopy()¶
- Synchronize client and server - Return type: - None - Raises: - TypeError – invalid connection
- TypeError – too many parameters
 
The use of direct access methods may desynchronize client and server. This method ensure that client and server will be synchronized.
locreate – create a large object in the database [LO]¶
- 
Connection.locreate(mode)¶
- Create a large object in the database - Parameters: - mode (int) – large object create mode - Returns: - object handling the PostGreSQL large object - Return type: - Raises: - TypeError – invalid connection, bad parameter type, or too many parameters
- pg.OperationalError – creation error
 
This method creates a large object in the database. The mode can be defined
by OR-ing the constants defined in the pg module (INV_READ,
INV_WRITE and INV_ARCHIVE). Please refer to PostgreSQL
user manual for a description of the mode values.
getlo – build a large object from given oid [LO]¶
- 
Connection.getlo(oid)¶
- Create a large object in the database - Parameters: - oid (int) – OID of the existing large object - Returns: - object handling the PostGreSQL large object - Return type: - Raises: - TypeError – invalid connection, bad parameter type, or too many parameters
- ValueError – bad OID value (0 is invalid_oid)
 
This method allows to reuse a formerly created large object through the
LargeObject interface, providing the user have its OID.
loimport – import a file to a large object [LO]¶
- 
Connection.loimport(name)¶
- Import a file to a large object - Parameters: - name (str) – the name of the file to be imported - Returns: - object handling the PostGreSQL large object - Return type: - Raises: - TypeError – invalid connection, bad argument type, or too many arguments
- pg.OperationalError – error during file import
 
This methods allows to create large objects in a very simple way. You just give the name of a file containing the data to be used.
Object attributes¶
Every Connection defines a set of read-only attributes that describe
the connection and its status. These attributes are:
- 
Connection.host¶
- the host name of the server (str) 
- 
Connection.port¶
- the port of the server (int) 
- 
Connection.db¶
- the selected database (str) 
- 
Connection.options¶
- the connection options (str) 
- 
Connection.user¶
- user name on the database system (str) 
- 
Connection.protocol_version¶
- the frontend/backend protocol being used (int) 
New in version 4.0.
- 
Connection.server_version¶
- the backend version (int, e.g. 90305 for 9.3.5) 
New in version 4.0.
- 
Connection.status¶
- the status of the connection (int: 1 = OK, 0 = bad) 
- 
Connection.error¶
- the last warning/error message from the server (str) 
