<!-- $PostgreSQL: pgsql/doc/src/sgml/plpython.sgml,v 1.31 2006/09/02 12:30:01 momjian Exp $ -->

Functions in PL/Python are declared via the standard <xref

syntax:

<programlisting>

CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-list</replaceable>)

RETURNS <replaceable>return-type</replaceable>

AS $$

# PL/Python function body

$$ LANGUAGE plpythonu;

</programlisting>

</para>

<para>

The body of a function is simply a Python script. When the function

is called, all unnamed arguments are passed as elements to the array

<varname>args[]</varname> and named arguments as ordinary variables to the

Python script. The result is returned from the Python code in the usual way,

with <literal>return</literal> or <literal>yield</literal> (in case of

a resultset statement).

</para>

<para>

For example, a function to return the greater of two integers can be

defined as:

CREATE FUNCTION pymax (a integer, b integer)

RETURNS integer

AS $$

if a &gt; b:

return a

return b

$$ LANGUAGE plpythonu;

is transformed into a Python function. For example, the above results in

def __plpython_procedure_pymax_23456():

if a &gt; b:

return a

return b

<para>

The <productname>PostgreSQL</> function parameters are available in

the global <varname>args</varname> list. In the

<function>pymax</function> example, <varname>args[0]</varname> contains

whatever was passed in as the first argument and

<varname>args[1]</varname> contains the second argument's value. Alternatively,

one can use named parameters as shown in the example above. This greatly simplifies

the reading and writing of <application>PL/Python</application> code.

</para>

<para>

If an SQL null value<indexterm><primary>null value</primary><secondary

sortas="PL/Python">PL/Python</secondary></indexterm> is passed to a

function, the argument value will appear as <symbol>None</symbol> in

Python. The above function definition will return the wrong answer for null

inputs. We could add <literal>STRICT</literal> to the function definition

to make <productname>PostgreSQL</productname> do something more reasonable:

if a null value is passed, the function will not be called at all,

but will just return a null result automatically. Alternatively,

we could check for null inputs in the function body:

<programlisting>

CREATE FUNCTION pymax (a integer, b integer)

RETURNS integer

AS $$

if (a is None) or (b is None):

return None

if a > b:

return a

return b

$$ LANGUAGE plpythonu;

</programlisting>

As shown above, to return an SQL null value from a PL/Python

function, return the value <symbol>None</symbol>. This can be done whether the

function is strict or not.

</para>

<para>

Composite-type arguments are passed to the function as Python mappings. The

element names of the mapping are the attribute names of the composite type.

If an attribute in the passed row has the null value, it has the value

<symbol>None</symbol> in the mapping. Here is an example:

<programlisting>

CREATE TABLE employee (

name text,

salary integer,

age integer

);

CREATE FUNCTION overpaid (e employee)

RETURNS boolean

AS $$

if e["salary"] &gt; 200000:

return True

if (e["age"] &lt; 30) and (e["salary"] &gt; 100000):

return True

return False

$$ LANGUAGE plpythonu;

</programlisting>

</para>

<para>

There are multiple ways to return row or composite types from a Python

scripts. In following examples we assume to have:

<programlisting>

CREATE TABLE named_value (

name text,

value integer

);

</programlisting>

or

<programlisting>

CREATE TYPE named_value AS (

name text,

value integer

);

</programlisting>

<variablelist>

<varlistentry>

<term>Sequence types (tuple or list), but not <literal>set</literal> (because

it is not indexable)</term>

<listitem>

<para>

Returned sequence objects must have the same number of items as

composite types have fields. Item with index 0 is assigned to the first field

of the composite type, 1 to second and so on. For example:

<programlisting>

CREATE FUNCTION make_pair (name text, value integer)

RETURNS named_value

AS $$

return [ name, value ]

# or alternatively, as tuple: return ( name, value )

$$ LANGUAGE plpythonu;

</programlisting>

To return SQL null in any column, insert <symbol>None</symbol> at

the corresponding position.

</para>

</listitem>

<varlistentry>

<term>Mapping (dictionary)</term>

<listitem>

<para>

Value for a composite type's column is retrieved from the mapping with

the column name as key. Example:

<programlisting>

CREATE FUNCTION make_pair (name text, value integer)

RETURNS named_value

AS $$

return { "name": name, "value": value }

$$ LANGUAGE plpythonu;

</programlisting>

Additional dictionary key/value pairs are ignored. Missing keys are

treated as errors, i.e. to return an SQL null value for any column, insert

<symbol>None</symbol> with the corresponding column name as the key.

</para>

</listitem>

<varlistentry>

<term>Object (any object providing method <literal>__getattr__</literal>)</term>

<listitem>

<para>

Example:

<programlisting>

CREATE FUNCTION make_pair (name text, value integer)

RETURNS named_value

AS $$

class named_value:

def __init__ (self, n, v):

self.name = n

self.value = v

return named_value(name, value)

# or simply

class nv: pass

nv.name = name

nv.value = value

return nv

$$ LANGUAGE plpythonu;

</programlisting>

</para>

</listitem>

</varlistentry>

</variablelist>

</para>

A <application>PL/Python</application> function can also return sets of

scalar or composite types. There are serveral ways to achieve this because

the returned object is internally turned into an iterator. For following

examples, let's assume to have composite type:

<programlisting>

CREATE TYPE greeting AS (

how text,

who text

);

</programlisting>

Currently known iterable types are:

<variablelist>

<varlistentry>

<term>Sequence types (tuple, list, set)</term>

<listitem>

<para>

<programlisting>

CREATE FUNCTION greet (how text)

RETURNS SETOF greeting

AS $$

# return tuple containing lists as composite types

# all other combinations work also

return ( [ how, "World" ], [ how, "PostgreSQL" ], [ how, "PL/Python" ] )

$$ LANGUAGE plpythonu;

</programlisting>

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Iterator (any object providing <symbol>__iter__</symbol> and

<symbol>next</symbol> methods)</term>

<listitem>

<para>

<programlisting>

CREATE FUNCTION greet (how text)

RETURNS SETOF greeting

AS $$

class producer:

def __init__ (self, how, who):

self.how = how

self.who = who

self.ndx = -1

def __iter__ (self):

return self

def next (self):

self.ndx += 1

if self.ndx == len(self.who):

raise StopIteration

return ( self.how, self.who[self.ndx] )

return producer(how, [ "World", "PostgreSQL", "PL/Python" ])

$$ LANGUAGE plpythonu;

</programlisting>

</para>

</listitem>

</varlistentry>

<varlistentry>

<term>Generator (<literal>yield</literal>)</term>

<listitem>

<para>

<programlisting>

CREATE FUNCTION greet (how text)

RETURNS SETOF greeting

AS $$

for who in [ "World", "PostgreSQL", "PL/Python" ]:

yield ( how, who )

$$ LANGUAGE plpythonu;

</programlisting>

<warning>

<para>

Currently, due to Python

<ulink url="http://sourceforge.net/tracker/index.php?func=detail&amp;aid=1483133&amp;group_id=5470&amp;atid=105470">bug #1483133</ulink>,

some debug versions of Python 2.4

(configured and compiled with option <literal>--with-pydebug</literal>)

are known to crash the <productname>PostgreSQL</productname> server.

Unpatched versions of Fedora 4 contain this bug.

It does not happen in production version of Python or on patched

versions of Fedora 4.

</para>

</warning>

</para>

</listitem>

</varlistentry>

</variablelist>

Whenever new iterable types are added to Python language,

<application>PL/Python</application> is ready to use it.