<!-- $PostgreSQL: pgsql/doc/src/sgml/plpython.sgml,v 1.43 2009/12/19 22:23:21 petere Exp $ -->

<chapter id="plpython">
 <title>PL/Python - Python Procedural Language</title>

 <indexterm zone="plpython"><primary>PL/Python</></>
 <indexterm zone="plpython"><primary>Python</></>

 <para>
  The <application>PL/Python</application> procedural language allows
  <productname>PostgreSQL</productname> functions to be written in the
  <ulink url="http://www.python.org">Python language</ulink>.
 </para>

 <para>
  To install PL/Python in a particular database, use
  <literal>createlang plpythonu <replaceable>dbname</></literal> (but
  see also <xref linkend="plpython-python23">).
 </para>

  <tip>
   <para>
    If a language is installed into <literal>template1</>, all subsequently
    created databases will have the language installed automatically.
   </para>
  </tip>

 <para>
  As of <productname>PostgreSQL</productname> 7.4, PL/Python is only
  available as an <quote>untrusted</> language (meaning it does not
  offer any way of restricting what users can do in it).  It has
  therefore been renamed to <literal>plpythonu</>.  The trusted
  variant <literal>plpython</> might become available again in future,
  if a new secure execution mechanism is developed in Python.
 </para>

 <note>
  <para>
   Users of source packages must specially enable the build of
   PL/Python during the installation process.  (Refer to the
   installation instructions for more information.)  Users of binary
   packages might find PL/Python in a separate subpackage.
  </para>
 </note>

 <sect1 id="plpython-python23">
  <title>Python 2 vs. Python 3</title>

  <para>
   PL/Python supports both the Python 2 and Python 3 language
   variants.  (The PostgreSQL installation instructions might contain
   more precise information about the exact supported minor versions
   of Python.)  Because the Python 2 and Python 3 language variants
   are incompatible in some important aspects, the following naming
   and transitioning scheme is used by PL/Python to avoid mixing them:

   <itemizedlist>
    <listitem>
     <para>
      The PostgreSQL language named <literal>plpython2u</literal>
      implements PL/Python based on the Python 2 language variant.
     </para>
    </listitem>

    <listitem>
     <para>
      The PostgreSQL language named <literal>plpython3u</literal>
      implements PL/Python based on the Python 3 language variant.
     </para>
    </listitem>

    <listitem>
     <para>
      The language named <literal>plpythonu</literal> implements
      PL/Python based on the default Python language variant, which is
      currently Python 2.  (This default is independent of what any
      local Python installations might consider to be
      their <quote>default</quote>, for example,
      what <filename>/usr/bin/python</filename> might be.)  The
      default will probably be changed to Python 3 in a distant future
      release of PostgreSQL, depending on the progress of the
      migration to Python 3 in the Python community.
     </para>
    </listitem>
   </itemizedlist>

   It depends on the build configuration or the installed packages
   whether PL/Python for Python 2 or Python 3 or both are available.
  </para>

  <para>
   This results in the following usage and migration strategy:

   <itemizedlist>
    <listitem>
     <para>
      Existing users and users who are currently not interested in
      Python 3 use the language name <literal>plpythonu</literal> and
      don't have to change anything for the foreseeable future.  It is
      recommended to gradually <quote>future-proof</quote> the code
      via migration to Python 2.6/2.7 to simplify the eventual
      migration to Python 3.
     </para>

     <para>
      In practice, many PL/Python functions will migrate to Python 3
      with few or no changes.
     </para>
    </listitem>

    <listitem>
     <para>
      Users who know that they have heavily Python 2 dependent code
      and don't plan to ever change it can make use of
      the <literal>plpython2u</literal> language name.  This will
      continue to work into the very distant future, until Python 2
      support might be completely dropped by PostgreSQL.
     </para>
    </listitem>

    <listitem>
     <para>
      Users who want to dive into Python 3 can use
      the <literal>plpython3u</literal> language name, which will keep
      working forever by today's standards.  In the distant future,
      when Python 3 might become the default, they might like to
      remove the <quote>3</quote> for aesthetic reasons.
     </para>
    </listitem>

    <listitem>
     <para>
      Daredevils, who want to build a Python-3-only operating system
      environment, can change the build scripts to
      make <literal>plpythonu</literal> be equivalent
      to <literal>plpython3u</literal>, keeping in mind that this
      would make their installation incompatible with most of the rest
      of the world.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   See also the
   document <ulink url="http://docs.python.org/dev/3.0/whatsnew/3.0.html">What's
   New In Python 3.0</ulink> for more information about porting to
   Python 3.
  </para>
 </sect1>

 <sect1 id="plpython-funcs">
  <title>PL/Python Functions</title>

  <para>
   Functions in PL/Python are declared via the standard <xref
   linkend="sql-createfunction" endterm="sql-createfunction-title">
   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, its arguments are passed as elements of the array
   <varname>args[]</varname>; named arguments are also passed 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 result-set statement).
  </para>

  <para>
   For example, a function to return the greater of two integers can be
   defined as:

<programlisting>
CREATE FUNCTION pymax (a integer, b integer)
  RETURNS integer
AS $$
  if a &gt; b:
    return a
  return b
$$ LANGUAGE plpythonu;
</programlisting>

   The Python code that is given as the body of the function definition
   is transformed into a Python function. For example, the above results in:

<programlisting>
def __plpython_procedure_pymax_23456():
  if a &gt; b:
    return a
  return b
</programlisting>

   assuming that 23456 is the OID assigned to the function by
   <productname>PostgreSQL</productname>.
  </para>

  <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.  Use of named parameters is usually more readable.
  </para>

  <para>
   The arguments are set as global variables.  Because of the scoping
   rules of Python, this has the subtle consequence that an argument
   variable cannot be reassigned inside the function to the value of
   an expression that involves the variable name itself, unless the
   variable is redeclared as global in the block.  For example, the
   following won't work:
<programlisting>
CREATE FUNCTION pystrip(x text)
  RETURNS text
AS $$
  x = x.strip()  # error
  return x
$$ LANGUAGE plpythonu;
</programlisting>
   because assigning to <varname>x</varname>
   makes <varname>x</varname> a local variable for the entire block,
   and so the <varname>x</varname> on the right-hand side of the
   assignment refers to a not-yet-assigned local
   variable <varname>x</varname>, not the PL/Python function
   parameter.  Using the <literal>global</literal> statement, this can
   be made to work:
<programlisting>
CREATE FUNCTION pystrip(x text)
  RETURNS text
AS $$
  global x
  x = x.strip()  # ok now
  return x
$$ LANGUAGE plpythonu;
</programlisting>
   But it is advisable not to rely on this implementation detail of
   PL/Python.  It is better to treat the function parameters as
   read-only.
  </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 $$
  if (a is None) or (b is None):
    return None
  if a &gt; b:
    return a
  return b
$$ 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>
   SQL array values are passed into PL/Python as a Python list.  To
   return an SQL array value out of a PL/Python function, return a
   Python sequence, for example a list or tuple:

<programlisting>
CREATE FUNCTION return_arr()
  RETURNS int[]
AS $$
return (1, 2, 3, 4, 5)
$$ LANGUAGE plpythonu;

SELECT return_arr();
 return_arr  
-------------
 {1,2,3,4,5}
(1 row)
</programlisting>

   Note that in Python, strings are sequences, which can have
   undesirable effects that might be familiar to Python programmers:

<programlisting>
CREATE FUNCTION return_str_arr()
  RETURNS varchar[]
AS $$
return "hello"
$$ LANGUAGE plpythonu;

SELECT return_str_arr();
 return_str_arr
----------------
 {h,e,l,l,o}
(1 row)
</programlisting>
  </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
   function. The following examples assume we have:

<programlisting>
CREATE TYPE named_value AS (
  name   text,
  value  integer
);
</programlisting>

   A composite result can be returned as a:

   <variablelist>
    <varlistentry>
     <term>Sequence type (a tuple or list, but not a set because
     it is not indexable)</term>
     <listitem>
      <para>
       Returned sequence objects must have the same number of items as the
       composite result type has fields. The item with index 0 is assigned to
       the first field of the composite type, 1 to the 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 a SQL null for any column, insert <symbol>None</symbol> at
       the corresponding position.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Mapping (dictionary)</term>
     <listitem>
      <para>
       The value for each result type 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>

       Any extra dictionary key/value pairs are ignored. Missing keys are
       treated as errors.
       To return a SQL null value for any column, insert
       <symbol>None</symbol> with the corresponding column name as the key.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>Object (any object providing method <literal>__getattr__</literal>)</term>
     <listitem>
      <para>
       This works the same as a mapping.
       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>

  <para>
   If you do not provide a return value, Python returns the default
   <symbol>None</symbol>. <application>PL/Python</application> translates
   Python's <symbol>None</symbol> into the SQL null value.
  </para>

  <para>
   A <application>PL/Python</application> function can also return sets of
   scalar or composite types. There are several ways to achieve this because
   the returned object is internally turned into an iterator. The following
   examples assume we have composite type:

<programlisting>
CREATE TYPE greeting AS (
  how text,
  who text
);
</programlisting>
   
   A set result can be returned from a:

   <variablelist>
    <varlistentry>
     <term>Sequence type (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://bugs.python.org/issue1483133">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
         when using an iterator to return a set result.
         Unpatched versions of Fedora 4 contain this bug.
         It does not happen in production versions of Python or on patched
         versions of Fedora 4.
        </para>
       </warning>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   The global dictionary <varname>SD</varname> is available to store
   data between function calls.  This variable is private static data.
   The global dictionary <varname>GD</varname> is public data,
   available to all Python functions within a session.  Use with
   care.<indexterm><primary>global data</><secondary>in
   PL/Python</></indexterm>
  </para>

  <para>
   Each function gets its own execution environment in the
   Python interpreter, so that global data and function arguments from
   <function>myfunc</function> are not available to
   <function>myfunc2</function>.  The exception is the data in the
   <varname>GD</varname> dictionary, as mentioned above.
  </para>
 </sect1>

 <sect1 id="plpython-trigger">
  <title>Trigger Functions</title>

  <indexterm zone="plpython-trigger">
   <primary>trigger</primary>
   <secondary>in PL/Python</secondary>
  </indexterm>

  <para>
   When a function is used as a trigger, the dictionary
   <literal>TD</literal> contains trigger-related values.
   <literal>TD["event"]</> contains
   the event as a string (<literal>INSERT</>, <literal>UPDATE</>,
   <literal>DELETE</>, <literal>TRUNCATE</>, or <literal>UNKNOWN</>).
   <literal>TD["when"]</> contains one of <literal>BEFORE</>,
   <literal>AFTER</>, or <literal>UNKNOWN</>.
   <literal>TD["level"]</> contains one of <literal>ROW</>,
   <literal>STATEMENT</>, or <literal>UNKNOWN</>.
   For a row-level trigger, the trigger
   rows are in <literal>TD["new"]</> and/or <literal>TD["old"]</>
   depending on the trigger event.
   <literal>TD["name"]</> contains the trigger name,
   <literal>TD["table_name"]</> contains the name of the table on which the trigger occurred,
   <literal>TD["table_schema"]</> contains the schema of the table on which the trigger occurred,
   and <literal>TD["relid"]</> contains the OID of the table on
   which the trigger occurred.  If the <command>CREATE TRIGGER</> command
   included arguments, they are available in <literal>TD["args"][0]</> to
   <literal>TD["args"][<replaceable>n</>-1]</>.
  </para>

  <para>
   If <literal>TD["when"]</literal> is <literal>BEFORE</> and
   <literal>TD["level"]</literal> is <literal>ROW</>, you can
   return <literal>None</literal> or <literal>"OK"</literal> from the
   Python function to indicate the row is unmodified,
   <literal>"SKIP"</> to abort the event, or <literal>"MODIFY"</> to
   indicate you've modified the row.
   Otherwise the return value is ignored.
  </para>
 </sect1>

 <sect1 id="plpython-database">
  <title>Database Access</title>

  <para>
   The PL/Python language module automatically imports a Python module
   called <literal>plpy</literal>.  The functions and constants in
   this module are available to you in the Python code as
   <literal>plpy.<replaceable>foo</replaceable></literal>.  At present
   <literal>plpy</literal> implements the functions
   <literal>plpy.debug(<replaceable>msg</>)</literal>,
   <literal>plpy.log(<replaceable>msg</>)</literal>,
   <literal>plpy.info(<replaceable>msg</>)</literal>,
   <literal>plpy.notice(<replaceable>msg</>)</literal>,
   <literal>plpy.warning(<replaceable>msg</>)</literal>,
   <literal>plpy.error(<replaceable>msg</>)</literal>, and
   <literal>plpy.fatal(<replaceable>msg</>)</literal>.<indexterm><primary>elog</><secondary>in PL/Python</></indexterm>
   <function>plpy.error</function> and 
   <function>plpy.fatal</function> actually raise a Python exception
   which, if uncaught, propagates out to the calling query, causing
   the current transaction or subtransaction to be aborted. 
   <literal>raise plpy.ERROR(<replaceable>msg</>)</literal> and
   <literal>raise plpy.FATAL(<replaceable>msg</>)</literal> are
   equivalent to calling
   <function>plpy.error</function> and
   <function>plpy.fatal</function>, respectively.
   The other functions only generate messages of different
   priority levels.
   Whether messages of a particular priority are reported to the client,
   written to the server log, or both is controlled by the
   <xref linkend="guc-log-min-messages"> and
   <xref linkend="guc-client-min-messages"> configuration
   variables. See <xref linkend="runtime-config"> for more information.
  </para>

  <para>
   Additionally, the <literal>plpy</literal> module provides two
   functions called <function>execute</function> and
   <function>prepare</function>.  Calling
   <function>plpy.execute</function> with a query string and an
   optional limit argument causes that query to be run and the result
   to be returned in a result object.  The result object emulates a
   list or dictionary object.  The result object can be accessed by
   row number and column name.  It has these additional methods:
   <function>nrows</function> which returns the number of rows
   returned by the query, and <function>status</function> which is the
   <function>SPI_execute()</function> return value.  The result object
   can be modified.
  </para>

  <para>
   For example:
<programlisting>
rv = plpy.execute("SELECT * FROM my_table", 5)
</programlisting>
   returns up to 5 rows from <literal>my_table</literal>.  If
   <literal>my_table</literal> has a column
   <literal>my_column</literal>, it would be accessed as:
<programlisting>
foo = rv[i]["my_column"]
</programlisting>
  </para>

  <para>
   <indexterm><primary>preparing a query</><secondary>in PL/Python</></indexterm>
   The second function, <function>plpy.prepare</function>, prepares
   the execution plan for a query.  It is called with a query string
   and a list of parameter types, if you have parameter references in
   the query.  For example:
<programlisting>
plan = plpy.prepare("SELECT last_name FROM my_users WHERE first_name = $1", [ "text" ])
</programlisting>
   <literal>text</literal> is the type of the variable you will be
   passing for <literal>$1</literal>.  After preparing a statement, you
   use the function <function>plpy.execute</function> to run it:
<programlisting>
rv = plpy.execute(plan, [ "name" ], 5)
</programlisting>
   The third argument is the limit and is optional.
  </para>

  <para>
   When you prepare a plan using the PL/Python module it is
   automatically saved.  Read the SPI documentation (<xref
   linkend="spi">) for a description of what this means.
   In order to make effective use of this across function calls
   one needs to use one of the persistent storage dictionaries
   <literal>SD</literal> or <literal>GD</literal> (see
   <xref linkend="plpython-funcs">). For example:
<programlisting>
CREATE FUNCTION usesavedplan() RETURNS trigger AS $$
    if SD.has_key("plan"):
        plan = SD["plan"]
    else:
        plan = plpy.prepare("SELECT 1")
        SD["plan"] = plan
    # rest of function
$$ LANGUAGE plpythonu;
</programlisting>
  </para>
 </sect1>

<![IGNORE[
 <!-- NOT CURRENTLY SUPPORTED -->

 <sect1 id="plpython-trusted">
  <title>Restricted Environment</title>

  <para>
   The current version of <application>PL/Python</application>
   functions as a trusted language only; access to the file system and
   other local resources is disabled.  Specifically,
   <application>PL/Python</application> uses the Python restricted
   execution environment, further restricts it to prevent the use of
   the file <function>open</> call, and allows only modules from a
   specific list to be imported.  Presently, that list includes:
   <literal>array</>, <literal>bisect</>, <literal>binascii</>,
   <literal>calendar</>, <literal>cmath</>, <literal>codecs</>,
   <literal>errno</>, <literal>marshal</>, <literal>math</>, <literal>md5</>,
   <literal>mpz</>, <literal>operator</>, <literal>pcre</>,
   <literal>pickle</>, <literal>random</>, <literal>re</>, <literal>regex</>,
   <literal>sre</>, <literal>sha</>, <literal>string</>, <literal>StringIO</>,
   <literal>struct</>, <literal>time</>, <literal>whrandom</>, and
   <literal>zlib</>.
  </para>
 </sect1>

]]>

</chapter>
