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Diffstat (limited to 'doc/src/sgml/plpython.sgml')
| -rw-r--r-- | doc/src/sgml/plpython.sgml | 302 |
1 files changed, 286 insertions, 16 deletions
diff --git a/doc/src/sgml/plpython.sgml b/doc/src/sgml/plpython.sgml index 3e14e94eb7..a4a6119139 100644 --- a/doc/src/sgml/plpython.sgml +++ b/doc/src/sgml/plpython.sgml @@ -46,22 +46,50 @@ <title>PL/Python Functions</title> <para> - Functions in PL/Python are declared via the usual <xref + Functions in PL/Python are declared via the standard <xref linkend="sql-createfunction" endterm="sql-createfunction-title"> - syntax. For example: + 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: + <programlisting> -CREATE FUNCTION myfunc(text) RETURNS text - AS 'return args[0]' - LANGUAGE plpythonu; +CREATE FUNCTION pymax (a integer, b integer) + RETURNS integer +AS $$ + if a > b: + return a + return b +$$ LANGUAGE plpythonu; </programlisting> The Python code that is given as the body of the function definition - gets transformed into a Python function. - For example, the above results in + is transformed into a Python function. For example, the above results in <programlisting> -def __plpython_procedure_myfunc_23456(): - return args[0] +def __plpython_procedure_pymax_23456(): + if a > b: + return a + return b </programlisting> assuming that 23456 is the OID assigned to the function by @@ -69,6 +97,161 @@ def __plpython_procedure_myfunc_23456(): </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. 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 $$ + if e["salary"] > 200000: + return True + if (e["age"] < 30) and (e["salary"] > 100000): + return True + return False +$$ 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> + + <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 @@ -77,13 +260,100 @@ def __plpython_procedure_myfunc_23456(): </para> <para> - The <productname>PostgreSQL</> function parameters are available in - the global <varname>args</varname> list. In the - <function>myfunc</function> example, <varname>args[0]</> contains - whatever was passed in as the text argument. For - <literal>myfunc2(text, integer)</literal>, <varname>args[0]</> - would contain the <type>text</type> argument and - <varname>args[1]</varname> the <type>integer</type> argument. + 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&aid=1483133&group_id=5470&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. </para> <para> |