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, do not set the direct_access 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 large_objects 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 parameter values

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 as an integer. If the query is an update or delete statement, or an insert statement that did not insert exactly one row, or on a table without 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. Otherwise, it returns None.

You can use the Query object as an iterator that yields all results as tuples, or call Query.getresult() to get the result as a list of tuples. Alternatively, you can call Query.dictresult() or Query.dictiter() if you want to get the rows as dictionaries, or Query.namedresult() or Query.namediter() if you want to get the rows as named tuples. You can also simply print the Query object to show the query results on the console.

The SQL command may optionally contain positional parameters of the form $1, $2, etc instead of literal data, in which case the values must be supplied separately 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.

If you don’t pass any parameters, the command string can also include multiple SQL commands (separated by semicolons). You will only get the return value for the last command in this case.

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()

send_query - executes a SQL command string asynchronously

Connection.send_query(command[, args])

Submits a command to the server without waiting for the result(s).

Parameters
  • command (str) – SQL command

  • args – optional parameter values

Returns

a query object, as described below

Return type

Query

Raises
  • TypeError – bad argument type, or too many arguments

  • TypeError – invalid connection

  • ValueError – empty SQL query or lost connection

  • pg.ProgrammingError – error in query

This method is much the same as Connection.query(), except that it returns without waiting for the query to complete. The database connection cannot be used for other operations until the query completes, but the application can do other things, including executing queries using other database connections. The application can call select() using the fileno` obtained by the connection#s Connection.fileno() method to determine when the query has results to return.

This method always returns a Query object. This object differs from the Query object returned by Connection.query() in a few way. Most importantly, when Connection.send_query() is used, the application must call one of the result-returning methods such as Query.getresult() or Query.dictresult() until it either raises an exception or returns None.

Otherwise, the database connection will be left in an unusable state.

In cases when Connection.query() would return something other than a Query object, that result will be returned by calling one of the result-returning methods on the Query object returned by Connection.send_query(). There’s one important difference in these result codes: if Connection.query() returns None, the result-returning methods will return an empty string (‘’). It’s still necessary to call a result-returning method until it returns None.

Query.listfields(), Query.fieldname(), Query.fieldnum(), and Query.ntuples() only work after a call to a result-returning method with a non-None return value. Query.ntuples() returns only the number of rows returned by the previous result-returning method.

If multiple semi-colon-delimited statements are passed to Connection.query(), only the results of the last statement are returned in the Query object. With Connection.send_query(), all results are returned. Each result set will be returned by a separate call to Query.getresult() or other result-returning methods.

New in version 5.2.

Examples:

name = input("Name? ")
query = con.send_query("select phone from employees where name=$1",
                      (name,))
phone = query.getresult()
query.getresult()  # to close the query

# Run two queries in one round trip:
# (Note that you cannot use a union here
# when the result sets have different row types.)
query = con.send_query("select a,b,c from x where d=e;
                      "select e,f from y where g")
result_x = query.dictresult()
result_y = query.dictresult()
query.dictresult()  # to close the query

# Using select() to wait for the query to be ready:
query = con.send_query("select pg_sleep(20)")
r, w, e = select([con.fileno(), other, sockets], [], [])
if con.fileno() in r:
    results = query.getresult()
    query.getresult()  # to close the query

# Concurrent queries on separate connections:
con1 = connect()
con2 = connect()
s = con1.query("begin; set transaction isolation level repeatable read;"
               "select pg_export_snapshot();").single()
con2.query("begin; set transaction isolation level repeatable read;"
           "set transaction snapshot '%s'" % (s,))
q1 = con1.send_query("select a,b,c from x where d=e")
q2 = con2.send_query("select e,f from y where g")
r1 = q1.getresult()
q1.getresult()
r2 = q2.getresult()
q2.getresult()
con1.query("commit")
con2.query("commit")

query_prepared – execute a prepared statement

Connection.query_prepared(name[, args])

Execute a prepared statement

Parameters
  • name (str) – name of the prepared statement

  • args – optional parameter values

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

  • pg.OperationalError – prepared statement does not exist

This method works exactly like Connection.query() except that instead of passing the command itself, you pass the name of a prepared statement. An empty name corresponds to the unnamed statement. You must have previously created the corresponding named or unnamed statement with Connection.prepare(), or an pg.OperationalError will be raised.

New in version 5.1.

prepare – create a prepared statement

Connection.prepare(name, command)

Create a prepared statement

Parameters
  • name (str) – name of the prepared statement

  • command (str) – SQL command

Return type

None

Raises
  • TypeError – bad argument types, or wrong number of arguments

  • TypeError – invalid connection

  • pg.ProgrammingError – error in query or duplicate query

This method creates a prepared statement with the specified name for the given command for later execution with the Connection.query_prepared() method. The name can be empty to create an unnamed statement, in which case any pre-existing unnamed statement is automatically replaced; otherwise a pg.ProgrammingError is raised if the statement name is already defined in the current database session.

The SQL command may optionally contain positional parameters of the form $1, $2, etc instead of literal data. The corresponding values must then later be passed to the Connection.query_prepared() method separately as a tuple.

New in version 5.1.

describe_prepared – describe a prepared statement

Connection.describe_prepared(name)

Describe a prepared statement

Parameters

name (str) – name of the prepared statement

Return type

Query

Raises
  • TypeError – bad argument type, or too many arguments

  • TypeError – invalid connection

  • pg.OperationalError – prepared statement does not exist

This method returns a Query object describing the prepared statement with the given name. You can also pass an empty name in order to describe the unnamed statement. Information on the fields of the corresponding query can be obtained through the Query.listfields(), Query.fieldname() and Query.fieldnum() methods.

New in version 5.1.

reset – reset the connection

Connection.reset()

Reset the pg connection

Return type

None

Raises
  • TypeError – too many (any) arguments

  • TypeError – invalid connection

This method resets the current database connection.

poll - completes an asynchronous connection

Connection.poll()

Complete an asynchronous pg connection and get its state

Returns

state of the connection

Return type

int

Raises
  • TypeError – too many (any) arguments

  • TypeError – invalid connection

  • pg.InternalError – some error occurred during pg connection

The database connection can be performed without any blocking calls. This allows the application mainline to perform other operations or perhaps connect to multiple databases concurrently. Once the connection is established, it’s no different from a connection made using blocking calls.

The required steps are to pass the parameter nowait=True to the pg.connect() call, then call Connection.poll() until it either returns :const’:POLLING_OK or raises an exception. To avoid blocking in Connection.poll(), use select() or poll() to wait for the connection to be readable or writable, depending on the return code of the previous call to Connection.poll(). The initial state of the connection is POLLING_WRITING. The possible states are defined as constants in the pg module (POLLING_OK, POLLING_FAILED, POLLING_READING and POLLING_WRITING).

New in version 5.2.

Example:

con = pg.connect('testdb', nowait=True)
fileno = con.fileno()
rd = []
wt = [fileno]
rc = pg.POLLING_WRITING
while rc not in (pg.POLLING_OK, pg.POLLING_FAILED):
    ra, wa, xa = select(rd, wt, [], timeout)
    if not ra and not wa:
        timedout()
    rc = con.poll()
    if rc == pg.POLLING_READING:
        rd = [fileno]
        wt = []
    else:
        rd = []
        wt = [fileno]

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 pg connection

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.

set_non_blocking - set the non-blocking status of the connection

pg.set_non_blocking(nb)

Set the non-blocking mode of the connection

Parameters

nb (bool) – True to put the connection into non-blocking mode. False to put it into blocking mode.

Raises
  • TypeError – too many parameters

  • TypeError – invalid connection

Puts the socket connection into non-blocking mode or into blocking mode. This affects copy commands and large object operations, but not queries.

New in version 5.2.

is_non_blocking - report the blocking status of the connection

pg.is_non_blocking()

get the non-blocking mode of the connection

Returns

True if the connection is in non-blocking mode. False if it is in blocking mode.

Return type

bool

Raises
  • TypeError – too many parameters

  • TypeError – invalid connection

Returns True if the connection is in non-blocking mode, False otherwise.

New in version 5.2.

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[, columns])

Insert a Python list into a database table

Parameters
  • table (str) – the table name

  • values (list) – list of rows values

  • columns (list) – list of column names

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. columns is an optional sequence of column names to be passed on to the COPY command.

Warning

This method doesn’t type check the fields according to the table definition; it just looks whether or not it knows how to handle such types.

get/set_cast_hook – fallback typecast function

Connection.get_cast_hook()

Get the function that handles all external typecasting

Returns

the current external typecast function

Return type

callable, None

Raises

TypeError – too many (any) arguments

This returns the callback function used by PyGreSQL to provide plug-in Python typecast functions for the connection.

New in version 5.0.

Connection.set_cast_hook(func)

Set a function that will handle all external typecasting

Parameters

func – the function to be used as a callback

Return type

None

Raises

TypeError – the specified notice receiver is not callable

This methods allows setting a custom fallback function for providing Python typecast functions for the connection to supplement the C extension module. If you set this function to None, then only the typecast functions implemented in the C extension module are enabled. You normally would not want to change this. Instead, you can use get_typecast() and set_typecast() to add or change the plug-in Python typecast functions.

New in version 5.0.

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

LargeObject

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, and INV_WRITE). 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

LargeObject

Raises
  • TypeError – invalid connection, bad parameter type, or too many parameters

  • ValueError – bad OID value (0 is invalid_oid)

This method allows reusing a previously created large object through the LargeObject interface, provided the user has 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

LargeObject

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)

Connection.socket

the file descriptor number of the connection socket to the server (int)

New in version 5.1.

Connection.backend_pid

the PID of the backend process handling this connection (int)

New in version 5.1.

Connection.ssl_in_use

this is True if the connection uses SSL, False if not

New in version 5.1: (needs PostgreSQL >= 9.5)

Connection.ssl_attributes

SSL-related information about the connection (dict)

New in version 5.1: (needs PostgreSQL >= 9.5)