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.
All variables are set to None
at module initialization, specifying that
standard environment variables should be used.
connect – Open a PostgreSQL connection¶
- pg.connect([dbname][, host][, port][, opt][, user][, passwd][, nowait])¶
Open a
pg
connection- Parameters:
dbname – name of connected database (None =
defbase
)host (str or None) – name of the server host (None =
defhost
)port (int) – port used by the database server (-1 =
defport
)opt (str or None) – connection options (None =
defopt
)user (str or None) – PostgreSQL user (None =
defuser
)passwd (str or None) – password for user (None =
defpasswd
)nowait (bool) – whether the connection should happen asynchronously
- Returns:
If successful, the
Connection
handling the connection- Return type:
- Raises:
TypeError – bad argument type, or too many arguments
SyntaxError – duplicate argument definition
pg.InternalError – some error occurred during pg connection definition
Exception – (all exceptions relative to object allocation)
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. The opt
parameter can be used
to pass command-line options to the server. For a precise description
of the parameters, please refer to the PostgreSQL user manual.
See Connection.poll()
for a description of the nowait
parameter.
If you want to add additional parameters not specified here, you must
pass a connection string or a connection URI instead of the dbname
(as in con3
and con4
in the following example).
Changed in version 5.2: Support for asynchronous connections via the nowait
parameter.
Example:
import pg
con1 = pg.connect('testdb', 'myhost', 5432, None, 'bob', None)
con2 = pg.connect(dbname='testdb', host='myhost', user='bob')
con3 = pg.connect('host=myhost user=bob dbname=testdb connect_timeout=10')
con4 = pg.connect('postgresql://bob@myhost/testdb?connect_timeout=10')
get_pqlib_version – get the version of libpq¶
- pg.get_pqlib_version()¶
Get the version of libpq that is being used by PyGreSQL
- Returns:
the version of libpq
- Return type:
int
- Raises:
TypeError – too many arguments
The number is formed by converting the major, minor, and revision numbers of the libpq version into two-decimal-digit numbers and appending them together. For example, version 15.4 will be returned as 150400.
New in version 5.2.
get/set_defhost – default server host¶
- pg.get_defhost(host)¶
Get the default host
- Returns:
the current default host specification
- Return type:
str or None
- Raises:
TypeError – too many arguments
This method returns the current default host specification,
or None
if the environment variables should be used.
Environment variables won’t be looked up.
- pg.set_defhost(host)¶
Set the default host
- Parameters:
host (str or None) – the new default host specification
- Returns:
the previous default host specification
- Return type:
str or None
- Raises:
TypeError – bad argument type, or too many arguments
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/set_defport – default server port¶
- pg.get_defport()¶
Get the default port
- Returns:
the current default port specification
- Return type:
int
- Raises:
TypeError – too many arguments
This method returns the current default port specification,
or None
if the environment variables should be used.
Environment variables won’t be looked up.
- pg.set_defport(port)¶
Set the default port
- Parameters:
port (int) – the new default port
- Returns:
previous default port specification
- Return type:
int or None
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/set_defopt – default connection options¶
- pg.get_defopt()¶
Get the default connection options
- Returns:
the current default options specification
- Return type:
str or None
- Raises:
TypeError – too many arguments
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.
- pg.set_defopt(options)¶
Set the default connection options
- Parameters:
options (str or None) – the new default connection options
- Returns:
previous default options specification
- Return type:
str or None
- Raises:
TypeError – bad argument type, or too many arguments
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/set_defbase – default database name¶
- pg.get_defbase()¶
Get the default database name
- Returns:
the current default database name specification
- Return type:
str or None
- Raises:
TypeError – too many arguments
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.
- pg.set_defbase(base)¶
Set the default database name
- Parameters:
base (str or None) – the new default base name
- Returns:
the previous default database name specification
- Return type:
str or None
- Raises:
TypeError – bad argument type, or too many arguments
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.
get/set_defuser – default database user¶
- pg.get_defuser()¶
Get the default database user
- Returns:
the current default database user specification
- Return type:
str or None
- Raises:
TypeError – too many arguments
This method returns the current default database user specification, or
None
if the environment variables should be used. Environment variables
won’t be looked up.
- pg.set_defuser(user)¶
Set the default database user
- Parameters:
user – the new default database user
- Returns:
the previous default database user specification
- Return type:
str or None
- Raises:
TypeError – bad argument type, or too many arguments
This method sets the default database user name 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/set_defpasswd – default database password¶
- pg.get_defpasswd()¶
Get the default database password
- Returns:
the current default database password specification
- Return type:
str or None
- Raises:
TypeError – too many arguments
This method returns the current default database password specification, or
None
if the environment variables should be used. Environment variables
won’t be looked up.
- pg.set_defpasswd(passwd)¶
Set the default database password
- Parameters:
passwd – the new default database password
- Returns:
the previous default database password specification
- Return type:
str or None
- Raises:
TypeError – bad argument type, or too many arguments
This method sets the default database password 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¶
- pg.escape_string(string)¶
Escape a string for use within SQL
- Parameters:
string (str) – the string that is to be escaped
- Returns:
the escaped string
- Return type:
str
- Raises:
TypeError – bad argument type, or too many arguments
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.
Note that there is also a Connection
method with the same name
which takes connection properties into account.
Note
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 = input("Name? ")
phone = con.query("select phone from employees"
f" where name='{escape_string(name)}'").singlescalar()
escape_bytea – escape binary data for use within SQL¶
- pg.escape_bytea(datastring)¶
escape binary data for use within SQL as type
bytea
- Parameters:
datastring (bytes/str) – the binary data that is to be escaped
- Returns:
the escaped string
- Return type:
bytes/str
- Raises:
TypeError – bad argument type, or too many arguments
Escapes binary data for use within an SQL command with the type bytea
.
The return value will have the same type as the given datastring.
As with escape_string()
, this is only used when inserting data directly
into an SQL command string.
Note that there is also a Connection
method with the same name
which takes connection properties into account.
Example:
picture = open('garfield.gif', 'rb').read()
con.query(f"update pictures set img='{escape_bytea(picture)}'"
" where name='Garfield'")
unescape_bytea – unescape data that has been retrieved as text¶
- pg.unescape_bytea(string)¶
Unescape
bytea
data that has been retrieved as text- Parameters:
string (str) – the
bytea
string that has been retrieved as text- Returns:
byte string containing the binary data
- Return type:
bytes
- Raises:
TypeError – bad argument type, or too many arguments
Converts an escaped string representation of binary data stored as bytea
into the raw byte string representing the binary data – this is the reverse
of escape_bytea()
. Since the Query
results will already
return unescaped byte strings, you normally don’t have to use this method.
Note that there is also a DB
method with the same name
which does exactly the same.
get/set_decimal – decimal type to be used for numeric values¶
- pg.get_decimal()¶
Get the decimal type to be used for numeric values
- Returns:
the Python class used for PostgreSQL numeric values
- Return type:
class
This function returns the Python class that is used by PyGreSQL to hold
PostgreSQL numeric values. The default class is decimal.Decimal
.
- pg.set_decimal(cls)¶
Set a decimal type to be used for numeric values
- Parameters:
cls (class) – the Python class to be used for PostgreSQL numeric values
This function can be used to specify the Python class that shall
be used by PyGreSQL to hold PostgreSQL numeric values.
The default class is decimal.Decimal
.
get/set_decimal_point – decimal mark used for monetary values¶
- pg.get_decimal_point()¶
Get the decimal mark used for monetary values
- Returns:
string with one character representing the decimal mark
- Return type:
str
This function returns the decimal mark used by PyGreSQL to interpret
PostgreSQL monetary values when converting them to decimal numbers.
The default setting is '.'
as a decimal point. This setting is not
adapted automatically to the locale used by PostgreSQL, but you can use
set_decimal()
to set a different decimal mark manually. A return
value of None
means monetary values are not interpreted as decimal
numbers, but returned as strings including the formatting and currency.
New in version 4.1.1.
- pg.set_decimal_point(string)¶
Specify which decimal mark is used for interpreting monetary values
- Parameters:
string (str) – string with one character representing the decimal mark
This function can be used to specify the decimal mark used by PyGreSQL
to interpret PostgreSQL monetary values. The default value is ‘.’ as
a decimal point. This value is not adapted automatically to the locale
used by PostgreSQL, so if you are dealing with a database set to a
locale that uses a ','
instead of '.'
as the decimal point,
then you need to call set_decimal(',')
to have PyGreSQL interpret
monetary values correctly. If you don’t want money values to be converted
to decimal numbers, then you can call set_decimal(None)
, which will
cause PyGreSQL to return monetary values as strings including their
formatting and currency.
New in version 4.1.1.
get/set_bool – whether boolean values are returned as bool objects¶
- pg.get_bool()¶
Check whether boolean values are returned as bool objects
- Returns:
whether or not bool objects will be returned
- Return type:
bool
This function checks whether PyGreSQL returns PostgreSQL boolean
values converted to Python bool objects, or as 'f'
and 't'
strings which are the values used internally by PostgreSQL. By default,
conversion to bool objects is activated, but you can disable this with
the set_bool()
function.
New in version 4.2.
- pg.set_bool(on)¶
Set whether boolean values are returned as bool objects
- Parameters:
on – whether or not bool objects shall be returned
This function can be used to specify whether PyGreSQL shall return
PostgreSQL boolean values converted to Python bool objects, or as
'f'
and 't'
strings which are the values used internally by
PostgreSQL. By default, conversion to bool objects is activated,
but you can disable this by calling set_bool(True)
.
New in version 4.2.
Changed in version 5.0: Boolean values had been returned as string by default in earlier versions.
get/set_array – whether arrays are returned as list objects¶
- pg.get_array()¶
Check whether arrays are returned as list objects
- Returns:
whether or not list objects will be returned
- Return type:
bool
This function checks whether PyGreSQL returns PostgreSQL arrays converted
to Python list objects, or simply as text in the internal special output
syntax of PostgreSQL. By default, conversion to list objects is activated,
but you can disable this with the set_array()
function.
New in version 5.0.
- pg.set_array(on)¶
Set whether arrays are returned as list objects
- Parameters:
on – whether or not list objects shall be returned
This function can be used to specify whether PyGreSQL shall return PostgreSQL
arrays converted to Python list objects, or simply as text in the internal
special output syntax of PostgreSQL. By default, conversion to list objects
is activated, but you can disable this by calling set_array(False)
.
New in version 5.0.
Changed in version 5.0: Arrays had been always returned as text strings in earlier versions.
get/set_bytea_escaped – whether bytea data is returned escaped¶
- pg.get_bytea_escaped()¶
Check whether bytea values are returned as escaped strings
- Returns:
whether or not bytea objects will be returned escaped
- Return type:
bool
This function checks whether PyGreSQL returns PostgreSQL bytea
values in
escaped form or in unescaped from as byte strings. By default, bytea values
will be returned unescaped as byte strings, but you can change this with the
set_bytea_escaped()
function.
New in version 5.0.
- pg.set_bytea_escaped(on)¶
Set whether bytea values are returned as escaped strings
- Parameters:
on – whether or not bytea objects shall be returned escaped
This function can be used to specify whether PyGreSQL shall return
PostgreSQL bytea
values in escaped form or in unescaped from as byte
strings. By default, bytea values will be returned unescaped as byte
strings, but you can change this by calling set_bytea_escaped(True)
.
New in version 5.0.
Changed in version 5.0: Bytea data had been returned in escaped form by default in earlier versions.
get/set_jsondecode – decoding JSON format¶
- pg.get_jsondecode()¶
Get the function that deserializes JSON formatted strings
This returns the function used by PyGreSQL to construct Python objects from JSON formatted strings.
- pg.set_jsondecode(func)¶
Set a function that will deserialize JSON formatted strings
- Parameters:
func – the function to be used for deserializing JSON strings
You can use this if you do not want to deserialize JSON strings coming
in from the database, or if want to use a different function than the
standard function json.loads()
or if you want to use it with parameters
different from the default ones. If you set this function to None, then
the automatic deserialization of JSON strings will be deactivated.
New in version 5.0.
Changed in version 5.0: JSON data had been always returned as text strings in earlier versions.
get/set_datestyle – assume a fixed date style¶
- pg.get_datestyle()¶
Get the assumed date style for typecasting
This returns the PostgreSQL date style that is silently assumed when
typecasting dates or None if no fixed date style is assumed, in which case
the date style is requested from the database when necessary (this is the
default). Note that this method will not get the date style that is
currently set in the session or in the database. You can get the current
setting with the methods DB.get_parameter()
and
Connection.parameter()
. You can also get the date format corresponding
to the current date style by calling Connection.date_format()
.
New in version 5.0.
- pg.set_datestyle(datestyle)¶
Set a fixed date style that shall be assumed when typecasting
- Parameters:
datestyle (str) – the date style that shall be assumed, or None if no fixed dat style shall be assumed
PyGreSQL is able to automatically pick up the right date style for typecasting
date values from the database, even if you change it for the current session
with a SET DateStyle
command. This is happens very effectively without
an additional database request being involved. If you still want to have
PyGreSQL always assume a fixed date style instead, then you can set one with
this function. Note that calling this function will not alter the date
style of the database or the current session. You can do that by calling
the method DB.set_parameter()
instead.
New in version 5.0.
get/set_typecast – custom typecasting¶
PyGreSQL uses typecast functions to cast the raw data coming from the database to Python objects suitable for the particular database type. These functions take a single string argument that represents the data to be casted and must return the casted value.
PyGreSQL provides through its C extension module basic typecast functions for the common database types, but if you want to add more typecast functions, you can set these using the following functions.
- pg.get_typecast(typ)¶
Get the global cast function for the given database type
- Parameters:
typ (str) – PostgreSQL type name
- Returns:
the typecast function for the specified type
- Return type:
function or None
New in version 5.0.
- pg.set_typecast(typ, cast)¶
Set a global typecast function for the given database type(s)
- Parameters:
typ (str or int) – PostgreSQL type name or list of type names
cast – the typecast function to be set for the specified type(s)
The typecast function must take one string object as argument and return a Python object into which the PostgreSQL type shall be casted. If the function takes another parameter named connection, then the current database connection will also be passed to the typecast function. This may sometimes be necessary to look up certain database settings.
New in version 5.0.
Note that database connections cache types and their cast functions using
connection specific DbTypes
objects. You can also get, set and
reset typecast functions on the connection level using the methods
DbTypes.get_typecast()
, DbTypes.set_typecast()
and
DbTypes.reset_typecast()
of the DB.dbtypes
object. This will
not affect other connections or future connections. In order to be sure
a global change is picked up by a running connection, you must reopen it or
call DbTypes.reset_typecast()
on the DB.dbtypes
object.
Also note that the typecasting for all of the basic types happens already in the C extension module. The typecast functions that can be set with the above methods are only called for the types that are not already supported by the C extension module.
cast_array/record – fast parsers for arrays and records¶
PostgreSQL returns arrays and records (composite types) using a special output syntax with several quirks that cannot easily and quickly be parsed in Python. Therefore the C extension module provides two fast parsers that allow quickly turning these text representations into Python objects: Arrays will be converted to Python lists, and records to Python tuples. These fast parsers are used automatically by PyGreSQL in order to return arrays and records from database queries as lists and tuples, so you normally don’t need to call them directly. You may only need them for typecasting arrays of data types that are not supported by default in PostgreSQL.
- pg.cast_array(string[, cast][, delim])¶
Cast a string representing a PostgreSQL array to a Python list
- Parameters:
string (str) – the string with the text representation of the array
cast (callable or None) – a typecast function for the elements of the array
delim (bytes) – delimiter character between adjacent elements
- Returns:
a list representing the PostgreSQL array in Python
- Return type:
list
- Raises:
TypeError – invalid argument types
ValueError – error in the syntax of the given array
This function takes a string containing the text representation of a
PostgreSQL array (which may look like '{{1,2}{3,4}}'
for a two-dimensional
array), a typecast function cast that is called for every element, and
an optional delimiter character delim (usually a comma), and returns a
Python list representing the array (which may be nested like
[[1, 2], [3, 4]]
in this example). The cast function must take a single
argument which will be the text representation of the element and must output
the corresponding Python object that shall be put into the list. If you don’t
pass a cast function or set it to None, then unprocessed text strings will
be returned as elements of the array. If you don’t pass a delimiter character,
then a comma will be used by default.
New in version 5.0.
- pg.cast_record(string[, cast][, delim])¶
Cast a string representing a PostgreSQL record to a Python tuple
- Parameters:
string (str) – the string with the text representation of the record
cast (callable, list or tuple of callables, or None) – typecast function(s) for the elements of the record
delim (bytes) – delimiter character between adjacent elements
- Returns:
a tuple representing the PostgreSQL record in Python
- Return type:
tuple
- Raises:
TypeError – invalid argument types
ValueError – error in the syntax of the given array
This function takes a string containing the text representation of a
PostgreSQL record (which may look like '(1,a,2,b)'
for a record composed
of four fields), a typecast function cast that is called for every element,
or a list or tuple of such functions corresponding to the individual fields
of the record, and an optional delimiter character delim (usually a comma),
and returns a Python tuple representing the record (which may be inhomogeneous
like (1, 'a', 2, 'b')
in this example). The cast function(s) must take a
single argument which will be the text representation of the element and must
output the corresponding Python object that shall be put into the tuple. If
you don’t pass cast function(s) or pass None instead, then unprocessed text
strings will be returned as elements of the tuple. If you don’t pass a
delimiter character, then a comma will be used by default.
New in version 5.0.
Note that besides using parentheses instead of braces, there are other subtle differences in escaping special characters and NULL values between the syntax used for arrays and the one used for composite types, which these functions take into account.
Type helpers¶
The module provides the following type helper functions. You can wrap
parameters with these functions when passing them to DB.query()
or DB.query_formatted()
in order to give PyGreSQL a hint about the
type of the parameters, if it cannot be derived from the context.
- pg.Bytea(bytes)¶
A wrapper for holding a bytea value
New in version 5.0.
- pg.HStore(dict)¶
A wrapper for holding an hstore dictionary
New in version 5.0.
- pg.Json(obj)¶
A wrapper for holding an object serializable to JSON
New in version 5.0.
The following additional type helper is only meaningful when used with
DB.query_formatted()
. It marks a parameter as text that shall be
literally included into the SQL. This is useful for passing table names
for instance.
- pg.Literal(sql)¶
A wrapper for holding a literal SQL string
New in version 5.0.
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:
- pg.version¶
- pg.__version__¶
constants that give the current version
- pg.INV_READ¶
- pg.INV_WRITE¶
large objects access modes, used by
Connection.locreate()
andLargeObject.open()
- pg.POLLING_OK¶
- pg.POLLING_FAILED¶
- pg.POLLING_READING¶
- pg.POLLING_WRITING¶
polling states, returned by
Connection.poll()
- pg.SEEK_SET¶
- pg.SEEK_CUR¶
- pg.SEEK_END¶
positional flags, used by
LargeObject.seek()
- pg.TRANS_IDLE¶
- pg.TRANS_ACTIVE¶
- pg.TRANS_INTRANS¶
- pg.TRANS_INERROR¶
- pg.TRANS_UNKNOWN¶
transaction states, used by
Connection.transaction()