================================
PyGreSQL Programming Information
================================

------------------------------------------
The classic PyGreSQL interface (pg module)
------------------------------------------

.. meta::
   :description: The classic PyGreSQL interface (pg module)
   :keywords: PyGreSQL, pg, PostGreSQL, Python

.. sectnum::
.. contents:: Contents


Introduction
============
You may either choose to use the
`"classic" PyGreSQL interface <pg.html>`_
provided by the `pg` module or else the
`DB-API 2.0 compliant interface <pgdb.html>`_
provided by the `pgdb` module.

The following documentation covers only the older `pg` API.

The `pg` module handles three types of objects,

- the `pgobject`, which handles the connection
  and all the requests to the database,
- the `pglarge` object, which handles
  all the accesses to PostgreSQL large objects,
- the `pgqueryobject` that handles query results

and it provides a convenient wrapper class `DB` for the `pgobject`.

If you want to see a simple example of the use of some of these functions,
see http://ontario.bikerides.ca where you can find a link at the bottom to the
actual Python code for the page.


Module functions and constants
==============================
The `pg` module defines a few functions that allow to connect
to a database and to define "default variables" that override
the environment variables used by PostgreSQL.

These "default variables" were designed to allow you to handle general
connection parameters without heavy code in your programs. You can prompt the
user for a value, put it in the default variable, and forget it, without
having to modify your environment. The support for default variables can be
disabled by setting the -DNO_DEF_VAR option in the Python setup file. Methods
relative to this are specified by the tag [DV].

All variables are set to `None` at module initialization, specifying that
standard environment variables should be used.

connect - opens a pg connection
-------------------------------
Syntax::

  connect([dbname], [host], [port], [opt], [tty], [user], [passwd])

Parameters:
  :dbname: name of connected database (string/None)
  :host:   name of the server host (string/None)
  :port:   port used by the database server (integer/-1)
  :opt:    connection options (string/None)
  :tty:    debug terminal (string/None)
  :user:   PostgreSQL user (string/None)
  :passwd: password for user (string/None)

Return type:
  :pgobject: If successful, the `pgobject` handling the connection

Exceptions raised:
  :TypeError: bad argument type, or too many arguments
  :SyntaxError: duplicate argument definition
  :pg.InternalError: some error occurred during pg connection definition

  (plus all exceptions relative to object allocation)

Description:
  This function opens a connection to a specified database on a given
  PostgreSQL server. You can use keywords here, as described in the
  Python tutorial. The names of the keywords are the name of the
  parameters given in the syntax line. For a precise description
  of the parameters, please refer to the PostgreSQL user manual.

Examples::

  import pg

  con1 = pg.connect('testdb', 'myhost', 5432, None, None, 'bob', None)
  con2 = pg.connect(dbname='testdb', host='localhost', user='bob')

get_defhost, set_defhost - default server host [DV]
---------------------------------------------------
Syntax::

  get_defhost()

Parameters:
  None

Return type:
  :string, None: default host specification

Exceptions raised:
  :TypeError: too many arguments

Description:
  This method returns the current default host specification,
  or `None` if the environment variables should be used.
  Environment variables won't be looked up.

Syntax::

  set_defhost(host)

Parameters:
  :host: new default host (string/None)

Return type:
  :string, None: previous default host specification

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  This methods sets the default host value for new connections.
  If `None` is supplied as parameter, environment variables will
  be used in future connections. It returns the previous setting
  for default host.

get_defport, set_defport - default server port [DV]
---------------------------------------------------
Syntax::

  get_defport()

Parameters:
  None

Return type:
  :integer, None: default port specification

Exceptions raised:
  :TypeError: too many arguments

Description:
  This method returns the current default port specification,
  or `None` if the environment variables should be used.
  Environment variables won't be looked up.

Syntax::

  set_defport(port)

Parameters:
  :port: new default port (integer/-1)

Return type:
  :integer, None: previous default port specification

Description:
  This methods sets the default port value for new connections. If -1 is
  supplied as parameter, environment variables will be used in future
  connections. It returns the previous setting for default port.

get_defopt, set_defopt - default connection options [DV]
--------------------------------------------------------
Syntax::

  get_defopt()

Parameters:
  None

Return type:
  :string, None: default options specification

Exceptions raised:
  :TypeError: too many arguments

Description:
  This method returns the current default connection options  specification,
  or `None` if the environment variables should be used. Environment variables
  won't be looked up.

Syntax::

  set_defopt(options)

Parameters:
  :options: new default connection options (string/None)

Return type:
  :string, None: previous default options specification

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  This methods sets the default connection options value for new connections.
  If `None` is supplied as parameter, environment variables will be used in
  future connections. It returns the previous setting for default options.

get_deftty, set_deftty - default debug tty [DV]
-----------------------------------------------
Syntax::

  get_deftty()

Parameters:
  None

Return type:
  :string, None: default debug terminal specification

Exceptions raised:
  :TypeError: too many arguments

Description:
  This method returns the current default debug terminal specification, or
  `None` if the environment variables should be used. Environment variables
  won't be looked up.

Syntax::

  set_deftty(terminal)

Parameters:
  :terminal: new default debug terminal (string/None)

Return type:
  :string, None: previous default debug terminal specification

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  This methods sets the default debug terminal value for new connections. If
  `None` is supplied as parameter, environment variables will be used in future
  connections. It returns the previous setting for default terminal.

get_defbase, set_defbase - default database name [DV]
-----------------------------------------------------
Syntax::

  get_defbase()

Parameters:
  None

Return type:
  :string, None: default database name specification

Exceptions raised:
  :TypeError: too many arguments

Description:
  This method returns the current default database name specification, or
  `None` if the environment variables should be used. Environment variables
  won't be looked up.

Syntax::

  set_defbase(base)

Parameters:
  :base: new default base name (string/None)

Return type:
  :string, None: previous default database name specification

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  This method sets the default database name value for new connections. If
  `None` is supplied as parameter, environment variables will be used in
  future connections. It returns the previous setting for default host.

escape_string - escape a string for use within SQL
--------------------------------------------------
Syntax::

  escape_string(string)

Parameters:
  :string: the string that is to be escaped

Return type:
  :str: the escaped string

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  This function escapes a string for use within an SQL command.
  This is useful when inserting data values as literal constants
  in SQL commands. Certain characters (such as quotes and backslashes)
  must be escaped to prevent them from being interpreted specially
  by the SQL parser. `escape_string` performs this operation.

.. caution:: It is especially important to do proper escaping when
  handling strings that were received from an untrustworthy source.
  Otherwise there is a security risk: you are vulnerable to "SQL injection"
  attacks wherein unwanted SQL commands are fed to your database.

Example::

  name = raw_input("Name? ")
  phone = con.query("select phone from employees"
    " where name='%s'" % escape_string(name)).getresult()

escape_bytea - escape binary data for use within SQL as type `bytea`
--------------------------------------------------------------------
Syntax::

  escape_bytea(datastring)

Parameters:
  :datastring: string containing the binary data that is to be escaped

Return type:
  :str: the escaped string

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  Escapes binary data for use within an SQL command with the type `bytea`.
  As with `escape_string`, this is only used when inserting data directly
  into an SQL command string.

Example::

  picture = file('garfield.gif', 'rb').read()
  con.query("update pictures set img='%s' where name='Garfield'"
    % escape_bytea(picture))

unescape_bytea -- unescape `bytea` data that has been retrieved as text
-----------------------------------------------------------------------
Syntax::

  unescape_bytea(string)

Parameters:
  :datastring: the `bytea` data string that has been retrieved as text

Return type:
  :str: string containing the binary data

Exceptions raised:
  :TypeError: bad argument type, or too many arguments

Description:
  Converts an escaped string representation of binary data into binary
  data - the reverse of `escape_bytea`. This is needed when retrieving
  `bytea` data with the `getresult()` or `dictresult()` method.

Example::

  picture = unescape_bytea(con.query(
    "select img from pictures where name='Garfield'").getresult[0][0])
  file('garfield.gif', 'wb').write(picture)

Module constants
----------------
Some constants are defined in the module dictionary.
They are intended to be used as parameters for methods calls.
You should refer to the libpq description in the PostgreSQL user manual
for more information about them. These constants are:

:version, __version__: constants that give the current version.
:INV_READ, INV_WRITE: large objects access modes,
  used by `(pgobject.)locreate` and `(pglarge.)open`
:SEEK_SET, SEEK_CUR, SEEK_END: positional flags,
  used by `(pglarge.)seek`


Connection objects: pgobject
============================
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.

.. caution:: 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 - executes a SQL command string
-------------------------------------
Syntax::

  query(command)

Parameters:
  :command: SQL command (string)

Return type:
  :pgqueryobject, None: result values

Exceptions raised:
  :TypeError: bad argument type, or too many arguments
  :ValueError: empty SQL query or lost connection
  :pg.ProgrammingError: error in query
  :pg.InternalError': error during query processing

Description:
  This method simply sends a SQL query to the database. If the query is
  an insert statement, the return value is the OID of the newly
  inserted row. If it is otherwise a query that does not return a result
  (i.e., is not a some kind of SELECT statement), it returns `None`.
  Otherwise, it returns a `pgqueryobject` that can be accessed via the
  `getresult()` or `dictresult()` method or simply printed.

reset - resets the connection
-----------------------------
Syntax::

  reset()

Parameters:
  None

Return type:
  None

Exceptions raised:
  :TypeError: too many (any) arguments

Description:
  This method resets the current database connection.

cancel - abandon processing of current SQL command
--------------------------------------------------
Syntax::

  cancel()

Parameters:
  None

Return type:
  None

Exceptions raised:
  :TypeError: too many (any) arguments

Description:
  This method requests that the server abandon processing
  of the current SQL command.

close - close the database connection
-------------------------------------
Syntax::

  close()

Parameters:
  None

Return type:
  None

Exceptions raised:
  :TypeError: too many (any) arguments

Description:
  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.

fileno - returns the socket used to connect to the database
-----------------------------------------------------------
Syntax::

  fileno()

Parameters:
  None

Exceptions raised:
  :TypeError: too many (any) arguments

Description:
  This method returns the underlying socket id used to connect
  to the database. This is useful for use in select calls, etc.

getnotify - gets the last notify from the server
------------------------------------------------
Syntax::

  getnotify()

Parameters:
  None

Return type:
  :tuple, None: last notify from server

Exceptions raised:
  :TypeError: too many parameters
  :TypeError: invalid connection

Description:
  This methods try 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 (couple) `(relname, pid)`, where `relname`
  is the name of the notify and `pid` the process id of the connection that
  triggered the notify. Remember to do a listen query first otherwise
  getnotify() will always return `None`.

inserttable - insert a list into a table
----------------------------------------
Syntax::

  inserttable(table, values)

Parameters:
  :table: the table name (string)
  :values: list of rows values (list)

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection, bad argument type, or too many arguments
  :MemoryError: insert buffer could not be allocated
  :ValueError: unsupported values

Description:
  This method allow 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.

.. caution:: *Be very careful*:
  This method doesn't typecheck the fields according to the table definition;
  it just look whether or not it knows how to handle such types.

putline - writes a line to the server socket [DA]
-------------------------------------------------
Syntax::

  putline(line)

Parameters:
  :line: line to be written (string)

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection, bad parameter type, or too many parameters

Description:
  This method allows to directly write a string to the server socket.

getline - gets a line from server socket [DA]
---------------------------------------------
Syntax::

  getline()

Parameters:
  None

Return type:
  :string: the line read

Exceptions raised:
  :TypeError: invalid connection
  :TypeError: too many parameters
  :MemoryError: buffer overflow

Description:
  This method allows to directly read a string from the server socket.

endcopy - synchronizes client and server [DA]
---------------------------------------------
Syntax::

  endcopy()

Parameters:
  None

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection
  :TypeError: too many parameters

Description:
  The use of direct access methods may desynchonize client and server.
  This method ensure that client and server will be synchronized.

locreate - create a large object in the database [LO]
-----------------------------------------------------
Syntax::

  locreate(mode)

Parameters:
  :mode: large object create mode

Return type:
  :pglarge: object handling the PostGreSQL large object

Exceptions raised:

  :TypeError: invalid connection, bad parameter type, or too many parameters
  :pg.OperationalError: creation error

Description:
  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]
------------------------------------------------
Syntax::

  getlo(oid)

Parameters:
  :oid: OID of the existing large object (integer)

Return type:
  :pglarge: object handling the PostGreSQL large object

Exceptions raised:
  :TypeError:  invalid connection, bad parameter type, or too many parameters
  :ValueError: bad OID value (0 is invalid_oid)

Description:
  This method allows to reuse a formerly created large object through the
  `pglarge` interface, providing the user have its OID.

loimport - import a file to a large object [LO]
-----------------------------------------------
Syntax::

  loimport(name)

Parameters:
  :name: the name of the file to be imported (string)

Return type:
  :pglarge: object handling the PostGreSQL large object

Exceptions raised:
  :TypeError: invalid connection, bad argument type, or too many arguments
  :pg.OperationalError: error during file import

Description:
  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 use.

Object attributes
-----------------
Every `pgobject` defines a set of read-only attributes that describe the
connection and its status. These attributes are:

  :host:    the host name of the server (string)
  :port:    the port of the server (integer)
  :db:      the selected database (string)
  :options: the connection options (string)
  :tty:     the connection debug terminal (string)
  :user:    user name on the database system (string)
  :status:  the status of the connection (integer: 1 - OK, 0 - bad)
  :error:   the last warning/error message from the server (string)


The DB wrapper class
====================
The `pgobject` methods are wrapped in the class `DB`.
The preferred way to use this module is as follows::

  import pg

  db = pg.DB(...) # see below

  for r in db.query( # just for example
      """SELECT foo,bar
         FROM foo_bar_table
         WHERE foo !~ bar"""
      ).dictresult():

      print '%(foo)s %(bar)s' % r

The following describes the methods and variables of this class.

Initialization
--------------
The DB class is initialized with the same arguments as the connect
function described in section 2. It also initializes a few
internal variables. The statement ``db = DB()`` will open the
local database with the name of the user just like connect() does.

pkey - return the primary key of a table
----------------------------------------
Syntax::

  pkey(table)

Parameters:
  :table: name of table

Return type:
  :string: Name of the field which is the primary key of the table

Description:
  This method returns the primary key of a table. Note that this raises
  an exception if the table does not have a primary key.

get_databases - get list of databases in the system
---------------------------------------------------
Syntax::

  get_databases()

Parameters:
  None

Return type:
  :list: all databases in the system

Description:
  Although you can do this with a simple select, it is added here for
  convenience.

get_relations - get list of relations in connected database
-----------------------------------------------------------
Syntax::

  get_relations(kinds)

Parameters:
  :kinds: a string or sequence of type letters

Description:
  The type letters are `r` = ordinary table, `i` = index, `S` = sequence,
  `v` = view, `c` = composite type, `s` = special, `t` = TOAST table.
  If `kinds` is None or an empty string, all relations are returned (this is
  also the default). Although you can do this with a simple select, it is
  added here for convenience.

get_tables - get list of tables in connected database
-----------------------------------------------------
Syntax::

  get_tables()

Parameters:
  None

Returns:
  :list: all tables in connected database

Description:
  Although you can do this with a simple select, it is added here for
  convenience.

get_attnames - get the attribute names of a table
-------------------------------------------------
Syntax::

  get_attnames(table)

Parameters:
  :table: name of table

Returns:
  :dictionary:  The keys are the attribute names,
    the values are the type names of the attributes.

Description:
  Given the name of a table, digs out the set of attribute names.

get - get a row from a database table or view
---------------------------------------------
Syntax::

 get(table, arg, [keyname])

Parameters:
  :table:   name of table or view
  :arg:     either a dictionary or the value to be looked up
  :keyname: name of field to use as key (optional)

Return type:
  :dictionary: The keys are the attribute names,
    the values are the row values.

Description:
  This method is the basic mechanism to get a single row. It assumes
  that the key specifies a unique row. If keyname is not specified
  then the primary key for the table is used. If `arg` is a dictionary
  then the value for the key is taken from it and it is modified to
  include the new values, replacing existing values where necessary.
  The OID is also put into the dictionary, but in order to allow the
  caller to work with multiple tables, it is munged as `oid(schema.table)`.

insert - insert a row into a database table
-------------------------------------------
Syntax::

  insert(table, a)

Parameters:
  :table: name of table
  :a:     a dictionary of values

Return type:
  :integer: the OID of the newly inserted row

Description:
  This method inserts values into the table specified filling in the
  values from the dictionary. It then reloads the dictionary with the
  values from the database. This causes the dictionary to be updated
  with values that are modified by rules, triggers, etc.

  Due to the way that this function works you will find inserts taking
  longer and longer as your table gets bigger. To overcome this problem
  simply add an index onto the OID of any table that you think may get
  large over time. You may also consider using the inserttable() method
  described in section 3.

update - update a row in a database table
-----------------------------------------
Syntax::

  update(table, a)

Parameters:
  :table: name of table
  :a:     a dictionary of values

Return type:
  :dictionary: the new row

Description:
  Similar to insert but updates an existing row. The update is based
  on the OID value as munged by get. The array returned is the
  one sent modified to reflect any changes caused by the update due
  to triggers, rules, defaults, etc.

clear - clears row values in memory
-----------------------------------
Syntax::

 clear(table, [a])

Parameters:
  :table: name of table
  :a:     a dictionary of values

Return type:
  :dictionary: an empty row

Description:
  This method clears all the attributes to values determined by the types.
  Numeric types are set to 0, Booleans are set to 'f', dates are set
  to 'now()' and everything else is set to the empty string.
  If the array argument is present, it is used as the array and any entries
  matching attribute names are cleared with everything else left unchanged.

delete - delete a row from a database table
-------------------------------------------
Syntax::

  delete(table, [a])

Parameters:
  :table: name of table
  :a:     a dictionary of values

Returns:
  None

Description:
  This method deletes the row from a table. It deletes based on the OID
  as munged as described above.

escape_string - escape a string for use within SQL
--------------------------------------------------
Syntax::

  escape_string(string)

Parameters:
  :string: the string that is to be escaped

Return type:
  :str: the escaped string

Description:
  See the module function with the same name.

escape_bytea - escape binary data for use within SQL as type `bytea`
--------------------------------------------------------------------
Syntax::

  escape_bytea(datastring)

Parameters:
  :datastring: string containing the binary data that is to be escaped

Return type:
  :str: the escaped string

Description:
  See the module function with the same name.

unescape_bytea -- unescape `bytea` data that has been retrieved as text
-----------------------------------------------------------------------
Syntax::

  unescape_bytea(string)

Parameters:
  :datastring: the `bytea` data string that has been retrieved as text

Return type:
  :str: string containing the binary data

Description:
  See the module function with the same name.


pgqueryobject methods
=====================

getresult - get query values as list of tuples
-----------------------------------------------
Syntax::

  getresult()

Parameters:
  None

Return type:
  :list: result values as a list of tuples

Exceptions raised:
  :TypeError: too many parameters
  :pg.InternalError: invalid previous result

Description:
  This method returns the list of the values returned by the query.
  More information about this result may be accessed using listfields(),
  fieldname() and fieldnum() methods.

dictresult - get query values as list of dictionaries
-----------------------------------------------------
Syntax::

  dictresult()

Parameters:
  None

Return type:
  :list: result values as a list of dictionaries

Exceptions raised:
  :TypeError: too many parameters
  :pg.InternalError: invalid previous result

Description:
  This method returns the list of the values returned by the query
  with each tuple returned as a dictionary with the field names
  used as the dictionary index.


listfields - lists fields names of previous query result
--------------------------------------------------------
Syntax::

  listfields()

Parameters:
  None

Return type:
  :list: field names

Exceptions raised:
  :TypeError: too many parameters
  :pg.InternalError: invalid previous result, or lost connection

Description:
  This method returns the list of names of the fields defined for the
  query result. The fields are in the same order as the result values.

fieldname, fieldnum - field name/number conversion
--------------------------------------------------
Syntax::

  fieldname(i)

Parameters:
  :i: field number (integer)

Return type:
  :string: field name

Exceptions raised:
  :TypeError: invalid connection, bad parameter type, or too many parameters
  :ValueError: invalid field number
  :pg.InternalError: invalid previous result, or lost connection

Description:
  This method allows to find a field name from its rank number. It can be
  useful for displaying a result. The fields are in the same order as the
  result values.

Syntax::

  fieldnum(name)

Parameters:
  :name: field name (string)

Return type:
  :integer: field number

Exceptions raised:
  :TypeError: invalid connection, bad parameter type, or too many parameters
  :ValueError: unknown field name
  :pg.InternalError: invalid previous result, or lost connection

Description:
  This method returns a field number from its name. It can be used to
  build a function that converts result list strings to their correct
  type, using a hardcoded table definition. The number returned is the
  field rank in the result values list.

ntuples - return number of tuples in query object
-------------------------------------------------
Syntax::

  ntuples()

Parameters:
  None

Return type:
  :integer: number of tuples in `pgqueryobject`

Exceptions raised:
  :TypeError: Too many arguments.

Description:
  This method returns the number of tuples found in a query.


Large objects: pglarge
======================
This object handles all the request concerning a PostgreSQL large object. It
embeds and hides all the "recurrent" variables (object OID and connection),
exactly in the same way `pgobjects` do, thus only keeping significant
parameters in function calls. It keeps a reference to the `pgobject` used for
its creation, sending requests though with its parameters. Any modification but
dereferencing the `pgobject` will thus affect the `pglarge` object.
Dereferencing the initial `pgobject` is not a problem since Python won't
deallocate it before the `pglarge` object dereference it.
All functions return a generic error message on call error, whatever the
exact error was. The `error` attribute of the object allow to get the exact
error message.

See also the PostgreSQL programmer's guide for more information about the
large object interface.

open - opens a large object
---------------------------
Syntax::

  open(mode)

Parameters:
  :mode: open mode definition (integer)

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection, bad parameter type, or too many parameters
  :IOError: already opened object, or open error

Description:
  This method opens a large object for reading/writing, in the same way than
  the Unix open() function. The mode value can be obtained by OR-ing the
  constants defined in the pgmodule (INV_READ, INV_WRITE).

close - closes a large object
-----------------------------
Syntax::

  close()

Parameters:
  None

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection
  :TypeError: too many parameters
  :IOError: object is not opened, or close error

Description:
  This method closes a previously opened large object, in the same way than
  the Unix close() function.

read, write, tell, seek, unlink - file like large object handling
-----------------------------------------------------------------
Syntax::

  read(size)

Parameters:
  :size: maximal size of the buffer to be read

Return type:
  :sized string: the read buffer

Exceptions raised:
  :TypeError: invalid connection, invalid object,
    bad parameter type, or too many parameters
  :ValueError: if `size` is negative
  :IOError: object is not opened, or read error

Description:
  This function allows to read data from a large object, starting at current
  position.

Syntax::

  write(string)

Parameters:
  (sized) string - buffer to be written

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection, bad parameter type, or too many parameters
  :IOError: object is not opened, or write error

Description:
  This function allows to write data to a large object, starting at current
  position.

Syntax::

  seek(offset, whence)

Parameters:
  :offset: position offset
  :whence: positional parameter

Return type:
  :integer: new position in object

Exceptions raised:
  :TypeError: binvalid connection or invalid object,
    bad parameter type, or too many parameters
  :IOError: object is not opened, or seek error

Description:
  This method allows to move the position cursor in the large object. The
  whence parameter can be obtained by OR-ing the constants defined in the
  `pg` module (`SEEK_SET`, `SEEK_CUR`, `SEEK_END`).

Syntax::

  tell()

Parameters:
  None

Return type:
  :integer: current position in large object

Exceptions raised:
  :TypeError: invalid connection or invalid object
  :TypeError: too many parameters
  :IOError: object is not opened, or seek error

Description:
  This method allows to get the current position in the large object.

Syntax::

  unlink()

Parameter:
  None

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection or invalid object
  :TypeError: too many parameters
  :IOError: object is not closed, or unlink error

Description:
  This methods unlinks (deletes) the PostgreSQL large object.

size - gives the large object size
----------------------------------

Syntax::

  size()

Parameters:
  None

Return type:
  :integer: the large object size

Exceptions raised:
  :TypeError: invalid connection or invalid object
  :TypeError: too many parameters
  :IOError: object is not opened, or seek/tell error

Description:
  This (composite) method allows to get the size of a large object. It was
  implemented because this function is very useful for a web interfaced
  database. Currently, the large object needs to be opened first.

export - saves a large object to a file
---------------------------------------
Syntax::

  export(name)

Parameters:
  :name: file to be created

Return type:
  None

Exceptions raised:
  :TypeError: invalid connection or invalid object,
    bad parameter type, or too many parameters
  :IOError:   object is not closed, or export error

Description:
  This methods allows to dump the content of a large object in a very simple
  way. The exported file is created on the host of the program, not the
  server host.

Object attributes
-----------------
`pglarge` objects define a read-only set of attributes that allow to get
some information about it. These attributes are:

  :oid:   the OID associated with the object
  :pgcnx: the `pgobject` associated with the object
  :error: the last warning/error message of the connection

.. caution:: *Be careful*:
  In multithreaded environments, `error` may be modified by another thread
  using the same pgobject. Remember these object are shared, not duplicated.
  You should provide some locking to be able if you want to check this.
  The `oid` attribute is very interesting because it allow you reuse the OID
  later, creating the `pglarge` object with a `pgobject` getlo() method call.
