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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>46.1. PL/Python Functions</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 Vsnapshot" /><link rel="prev" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language" /><link rel="next" href="plpython-data.html" title="46.2. Data Values" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">46.1. PL/Python Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Up</a></td><th width="60%" align="center">Chapter 46. PL/Python — Python Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="plpython-data.html" title="46.2. Data Values">Next</a></td></tr></table><hr /></div><div class="sect1" id="PLPYTHON-FUNCS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">46.1. PL/Python Functions</h2></div></div></div><p>
Functions in PL/Python are declared via the
standard <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a> syntax:
</p><pre class="programlisting">
CREATE FUNCTION <em class="replaceable"><code>funcname</code></em> (<em class="replaceable"><code>argument-list</code></em>)
RETURNS <em class="replaceable"><code>return-type</code></em>
AS $$
# PL/Python function body
$$ LANGUAGE plpython3u;
</pre><p>
</p><p>
The body of a function is simply a Python script. When the function
is called, its arguments are passed as elements of the list
<code class="varname">args</code>; named arguments are also passed as
ordinary variables to the Python script. Use of named arguments is
usually more readable. The result is returned from the Python code
in the usual way, with <code class="literal">return</code> or
<code class="literal">yield</code> (in case of a result-set statement). If
you do not provide a return value, Python returns the default
<code class="symbol">None</code>. <span class="application">PL/Python</span> translates
Python's <code class="symbol">None</code> into the SQL null value. In a procedure,
the result from the Python code must be <code class="symbol">None</code> (typically
achieved by ending the procedure without a <code class="literal">return</code>
statement or by using a <code class="literal">return</code> statement without
argument); otherwise, an error will be raised.
</p><p>
For example, a function to return the greater of two integers can be
defined as:
</p><pre class="programlisting">
CREATE FUNCTION pymax (a integer, b integer)
RETURNS integer
AS $$
if a > b:
return a
return b
$$ LANGUAGE plpython3u;
</pre><p>
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:
</p><pre class="programlisting">
def __plpython_procedure_pymax_23456():
if a > b:
return a
return b
</pre><p>
assuming that 23456 is the OID assigned to the function by
<span class="productname">PostgreSQL</span>.
</p><p>
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:
</p><pre class="programlisting">
CREATE FUNCTION pystrip(x text)
RETURNS text
AS $$
x = x.strip() # error
return x
$$ LANGUAGE plpython3u;
</pre><p>
because assigning to <code class="varname">x</code>
makes <code class="varname">x</code> a local variable for the entire block,
and so the <code class="varname">x</code> on the right-hand side of the
assignment refers to a not-yet-assigned local
variable <code class="varname">x</code>, not the PL/Python function
parameter. Using the <code class="literal">global</code> statement, this can
be made to work:
</p><pre class="programlisting">
CREATE FUNCTION pystrip(x text)
RETURNS text
AS $$
global x
x = x.strip() # ok now
return x
$$ LANGUAGE plpython3u;
</pre><p>
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.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="plpython.html" title="Chapter 46. PL/Python — Python Procedural Language">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="plpython-data.html" title="46.2. Data Values">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 46. PL/Python — Python Procedural Language </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 46.2. Data Values</td></tr></table></div></body></html>
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