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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>45.3. Data Values</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="plpython-funcs.html" title="45.2. PL/Python Functions" /><link rel="next" href="plpython-sharing.html" title="45.4. Sharing Data" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">45.3. Data Values</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpython-funcs.html" title="45.2. PL/Python Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpython.html" title="Chapter 45. PL/Python - Python Procedural Language">Up</a></td><th width="60%" align="center">Chapter 45. PL/Python - Python Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpython-sharing.html" title="45.4. Sharing Data">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLPYTHON-DATA"><div class="titlepage"><div><div><h2 class="title" style="clear: both">45.3. Data Values</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="plpython-data.html#id-1.8.11.11.3">45.3.1. Data Type Mapping</a></span></dt><dt><span class="sect2"><a href="plpython-data.html#id-1.8.11.11.4">45.3.2. Null, None</a></span></dt><dt><span class="sect2"><a href="plpython-data.html#PLPYTHON-ARRAYS">45.3.3. Arrays, Lists</a></span></dt><dt><span class="sect2"><a href="plpython-data.html#id-1.8.11.11.6">45.3.4. Composite Types</a></span></dt><dt><span class="sect2"><a href="plpython-data.html#id-1.8.11.11.7">45.3.5. Set-Returning Functions</a></span></dt></dl></div><p>
   Generally speaking, the aim of PL/Python is to provide
   a <span class="quote">“<span class="quote">natural</span>”</span> mapping between the PostgreSQL and the
   Python worlds.  This informs the data mapping rules described
   below.
  </p><div class="sect2" id="id-1.8.11.11.3"><div class="titlepage"><div><div><h3 class="title">45.3.1. Data Type Mapping</h3></div></div></div><p>
    When a PL/Python function is called, its arguments are converted from
    their PostgreSQL data type to a corresponding Python type:

    </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
       PostgreSQL <code class="type">boolean</code> is converted to Python <code class="type">bool</code>.
      </p></li><li class="listitem"><p>
       PostgreSQL <code class="type">smallint</code> and <code class="type">int</code> are
       converted to Python <code class="type">int</code>.
       PostgreSQL <code class="type">bigint</code> and <code class="type">oid</code> are converted
       to <code class="type">long</code> in Python 2 and to <code class="type">int</code> in
       Python 3.
      </p></li><li class="listitem"><p>
       PostgreSQL <code class="type">real</code> and <code class="type">double</code> are converted to
       Python <code class="type">float</code>.
      </p></li><li class="listitem"><p>
       PostgreSQL <code class="type">numeric</code> is converted to
       Python <code class="type">Decimal</code>.  This type is imported from
       the <code class="literal">cdecimal</code> package if that is available.
       Otherwise,
       <code class="literal">decimal.Decimal</code> from the standard library will be
       used.  <code class="literal">cdecimal</code> is significantly faster
       than <code class="literal">decimal</code>.  In Python 3.3 and up,
       however, <code class="literal">cdecimal</code> has been integrated into the
       standard library under the name <code class="literal">decimal</code>, so there is
       no longer any difference.
      </p></li><li class="listitem"><p>
       PostgreSQL <code class="type">bytea</code> is converted to
       Python <code class="type">str</code> in Python 2 and to <code class="type">bytes</code>
       in Python 3.  In Python 2, the string should be treated as a
       byte sequence without any character encoding.
      </p></li><li class="listitem"><p>
       All other data types, including the PostgreSQL character string
       types, are converted to a Python <code class="type">str</code>.  In Python
       2, this string will be in the PostgreSQL server encoding; in
       Python 3, it will be a Unicode string like all strings.
      </p></li><li class="listitem"><p>
       For nonscalar data types, see below.
      </p></li></ul></div><p>
   </p><p>
    When a PL/Python function returns, its return value is converted to the
    function's declared PostgreSQL return data type as follows:

    </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
       When the PostgreSQL return type is <code class="type">boolean</code>, the
       return value will be evaluated for truth according to the
       <span class="emphasis"><em>Python</em></span> rules.  That is, 0 and empty string
       are false, but notably <code class="literal">'f'</code> is true.
      </p></li><li class="listitem"><p>
       When the PostgreSQL return type is <code class="type">bytea</code>, the
       return value will be converted to a string (Python 2) or bytes
       (Python 3) using the respective Python built-ins, with the
       result being converted to <code class="type">bytea</code>.
      </p></li><li class="listitem"><p>
       For all other PostgreSQL return types, the return value is converted
       to a string using the Python built-in <code class="literal">str</code>, and the
       result is passed to the input function of the PostgreSQL data type.
       (If the Python value is a <code class="type">float</code>, it is converted using
       the <code class="literal">repr</code> built-in instead of <code class="literal">str</code>, to
       avoid loss of precision.)
      </p><p>
       Strings in Python 2 are required to be in the PostgreSQL server
       encoding when they are passed to PostgreSQL.  Strings that are
       not valid in the current server encoding will raise an error,
       but not all encoding mismatches can be detected, so garbage
       data can still result when this is not done correctly.  Unicode
       strings are converted to the correct encoding automatically, so
       it can be safer and more convenient to use those.  In Python 3,
       all strings are Unicode strings.
      </p></li><li class="listitem"><p>
       For nonscalar data types, see below.
      </p></li></ul></div><p>

    Note that logical mismatches between the declared PostgreSQL
    return type and the Python data type of the actual return object
    are not flagged; the value will be converted in any case.
   </p></div><div class="sect2" id="id-1.8.11.11.4"><div class="titlepage"><div><div><h3 class="title">45.3.2. Null, None</h3></div></div></div><p>
   If an SQL null value<a id="id-1.8.11.11.4.2.1" class="indexterm"></a> is passed to a
   function, the argument value will appear as <code class="symbol">None</code> in
   Python. For example, the function definition of <code class="function">pymax</code>
   shown in <a class="xref" href="plpython-funcs.html" title="45.2. PL/Python Functions">Section 45.2</a> will return the wrong answer for null
   inputs. We could add <code class="literal">STRICT</code> to the function definition
   to make <span class="productname">PostgreSQL</span> 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:

</p><pre class="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;
</pre><p>

   As shown above, to return an SQL null value from a PL/Python
   function, return the value <code class="symbol">None</code>. This can be done whether the
   function is strict or not.
  </p></div><div class="sect2" id="PLPYTHON-ARRAYS"><div class="titlepage"><div><div><h3 class="title">45.3.3. Arrays, Lists</h3></div></div></div><p>
   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 list:

</p><pre class="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)
</pre><p>

   Multidimensional arrays are passed into PL/Python as nested Python lists.
   A 2-dimensional array is a list of lists, for example. When returning
   a multi-dimensional SQL array out of a PL/Python function, the inner
   lists at each level must all be of the same size. For example:

</p><pre class="programlisting">
CREATE FUNCTION test_type_conversion_array_int4(x int4[]) RETURNS int4[] AS $$
plpy.info(x, type(x))
return x
$$ LANGUAGE plpythonu;

SELECT * FROM test_type_conversion_array_int4(ARRAY[[1,2,3],[4,5,6]]);
INFO:  ([[1, 2, 3], [4, 5, 6]], &lt;type 'list'&gt;)
 test_type_conversion_array_int4 
---------------------------------
 {{1,2,3},{4,5,6}}
(1 row)
</pre><p>

   Other Python sequences, like tuples, are also accepted for
   backwards-compatibility with PostgreSQL versions 9.6 and below, when
   multi-dimensional arrays were not supported. However, they are always
   treated as one-dimensional arrays, because they are ambiguous with
   composite types. For the same reason, when a composite type is used in a
   multi-dimensional array, it must be represented by a tuple, rather than a
   list.
  </p><p>
   Note that in Python, strings are sequences, which can have
   undesirable effects that might be familiar to Python programmers:

</p><pre class="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)
</pre><p>
  </p></div><div class="sect2" id="id-1.8.11.11.6"><div class="titlepage"><div><div><h3 class="title">45.3.4. Composite Types</h3></div></div></div><p>
   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
   <code class="symbol">None</code> in the mapping. Here is an example:

</p><pre class="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;
</pre><p>
  </p><p>
   There are multiple ways to return row or composite types from a Python
   function. The following examples assume we have:

</p><pre class="programlisting">
CREATE TYPE named_value AS (
  name   text,
  value  integer
);
</pre><p>

   A composite result can be returned as a:

   </p><div class="variablelist"><dl class="variablelist"><dt><span class="term">Sequence type (a tuple or list, but not a set because
     it is not indexable)</span></dt><dd><p>
       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:

</p><pre class="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;
</pre><p>

       To return a SQL null for any column, insert <code class="symbol">None</code> at
       the corresponding position.
      </p><p>
       When an array of composite types is returned, it cannot be returned as a list,
       because it is ambiguous whether the Python list represents a composite type,
       or another array dimension.
      </p></dd><dt><span class="term">Mapping (dictionary)</span></dt><dd><p>
       The value for each result type column is retrieved from the mapping
       with the column name as key. Example:

</p><pre class="programlisting">
CREATE FUNCTION make_pair (name text, value integer)
  RETURNS named_value
AS $$
  return { "name": name, "value": value }
$$ LANGUAGE plpythonu;
</pre><p>

       Any extra dictionary key/value pairs are ignored. Missing keys are
       treated as errors.
       To return a SQL null value for any column, insert
       <code class="symbol">None</code> with the corresponding column name as the key.
      </p></dd><dt><span class="term">Object (any object providing method <code class="literal">__getattr__</code>)</span></dt><dd><p>
       This works the same as a mapping.
       Example:

</p><pre class="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
$$ LANGUAGE plpythonu;
</pre><p>
      </p></dd></dl></div><p>
  </p><p>
    Functions with <code class="literal">OUT</code> parameters are also supported.  For example:
</p><pre class="programlisting">
CREATE FUNCTION multiout_simple(OUT i integer, OUT j integer) AS $$
return (1, 2)
$$ LANGUAGE plpythonu;

SELECT * FROM multiout_simple();
</pre><p>
   </p><p>
    Output parameters of procedures are passed back the same way.  For example:
</p><pre class="programlisting">
CREATE PROCEDURE python_triple(INOUT a integer, INOUT b integer) AS $$
return (a * 3, b * 3)
$$ LANGUAGE plpythonu;

CALL python_triple(5, 10);
</pre><p>
   </p></div><div class="sect2" id="id-1.8.11.11.7"><div class="titlepage"><div><div><h3 class="title">45.3.5. Set-Returning Functions</h3></div></div></div><p>
   A <span class="application">PL/Python</span> 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:

</p><pre class="programlisting">
CREATE TYPE greeting AS (
  how text,
  who text
);
</pre><p>

   A set result can be returned from a:

   </p><div class="variablelist"><dl class="variablelist"><dt><span class="term">Sequence type (tuple, list, set)</span></dt><dd><p>
</p><pre class="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;
</pre><p>
      </p></dd><dt><span class="term">Iterator (any object providing <code class="symbol">__iter__</code> and
      <code class="symbol">next</code> methods)</span></dt><dd><p>
</p><pre class="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" ])
$$ LANGUAGE plpythonu;
</pre><p>
      </p></dd><dt><span class="term">Generator (<code class="literal">yield</code>)</span></dt><dd><p>
</p><pre class="programlisting">
CREATE FUNCTION greet (how text)
  RETURNS SETOF greeting
AS $$
  for who in [ "World", "PostgreSQL", "PL/Python" ]:
    yield ( how, who )
$$ LANGUAGE plpythonu;
</pre><p>

      </p></dd></dl></div><p>
  </p><p>
    Set-returning functions with <code class="literal">OUT</code> parameters
    (using <code class="literal">RETURNS SETOF record</code>) are also
    supported.  For example:
</p><pre class="programlisting">
CREATE FUNCTION multiout_simple_setof(n integer, OUT integer, OUT integer) RETURNS SETOF record AS $$
return [(1, 2)] * n
$$ LANGUAGE plpythonu;

SELECT * FROM multiout_simple_setof(3);
</pre><p>
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