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><A
NAME="PLPYTHON-FUNCS"
>42.2. PL/Python Functions</A
></H1
><P
> Functions in PL/Python are declared via the
standard <A
HREF="sql-createfunction.html"
>CREATE FUNCTION</A
> syntax:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION <TT
CLASS="REPLACEABLE"
><I
>funcname</I
></TT
> (<TT
CLASS="REPLACEABLE"
><I
>argument-list</I
></TT
>)
RETURNS <TT
CLASS="REPLACEABLE"
><I
>return-type</I
></TT
>
AS $$
# PL/Python function body
$$ LANGUAGE plpythonu;</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
<TT
CLASS="VARNAME"
>args</TT
>; 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 <TT
CLASS="LITERAL"
>return</TT
> or
<TT
CLASS="LITERAL"
>yield</TT
> (in case of a result-set statement). If
you do not provide a return value, Python returns the default
<TT
CLASS="SYMBOL"
>None</TT
>. <SPAN
CLASS="APPLICATION"
>PL/Python</SPAN
> translates
Python's <TT
CLASS="SYMBOL"
>None</TT
> into the SQL null value.
</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 plpythonu;</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 plpythonu;</PRE
><P>
because assigning to <TT
CLASS="VARNAME"
>x</TT
>
makes <TT
CLASS="VARNAME"
>x</TT
> a local variable for the entire block,
and so the <TT
CLASS="VARNAME"
>x</TT
> on the right-hand side of the
assignment refers to a not-yet-assigned local
variable <TT
CLASS="VARNAME"
>x</TT
>, not the PL/Python function
parameter. Using the <TT
CLASS="LITERAL"
>global</TT
> 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 plpythonu;</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.
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