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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="PLPYTHON-DATA"
>42.3. Data Values</A
></H1
><P
> Generally speaking, the aim of PL/Python is to provide
a <SPAN
CLASS="QUOTE"
>"natural"</SPAN
> mapping between the PostgreSQL and the
Python worlds. This informs the data mapping rules described
below.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN59476"
>42.3.1. Data Type Mapping</A
></H2
><P
> Function arguments are converted from their PostgreSQL type to a
corresponding Python type:
<P
></P
></P><UL
><LI
><P
> PostgreSQL <TT
CLASS="TYPE"
>boolean</TT
> is converted to Python <TT
CLASS="TYPE"
>bool</TT
>.
</P
></LI
><LI
><P
> PostgreSQL <TT
CLASS="TYPE"
>smallint</TT
> and <TT
CLASS="TYPE"
>int</TT
> are
converted to Python <TT
CLASS="TYPE"
>int</TT
>.
PostgreSQL <TT
CLASS="TYPE"
>bigint</TT
> is converted
to <TT
CLASS="TYPE"
>long</TT
> in Python 2 and to <TT
CLASS="TYPE"
>int</TT
> in
Python 3.
</P
></LI
><LI
><P
> PostgreSQL <TT
CLASS="TYPE"
>real</TT
>, <TT
CLASS="TYPE"
>double</TT
>,
and <TT
CLASS="TYPE"
>numeric</TT
> are converted to
Python <TT
CLASS="TYPE"
>float</TT
>. Note that for
the <TT
CLASS="TYPE"
>numeric</TT
> this loses information and can lead to
incorrect results. This might be fixed in a future
release.
</P
></LI
><LI
><P
> PostgreSQL <TT
CLASS="TYPE"
>bytea</TT
> is converted to
Python <TT
CLASS="TYPE"
>str</TT
> in Python 2 and to <TT
CLASS="TYPE"
>bytes</TT
>
in Python 3. In Python 2, the string should be treated as a
byte sequence without any character encoding.
</P
></LI
><LI
><P
> All other data types, including the PostgreSQL character string
types, are converted to a Python <TT
CLASS="TYPE"
>str</TT
>. 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
><P
> For nonscalar data types, see below.
</P
></LI
></UL
><P>
</P
><P
> Function return values are converted to the declared PostgreSQL
return data type as follows:
<P
></P
></P><UL
><LI
><P
> When the PostgreSQL return type is <TT
CLASS="TYPE"
>boolean</TT
>, the
return value will be evaluated for truth according to the
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Python</I
></SPAN
> rules. That is, 0 and empty string
are false, but notably <TT
CLASS="LITERAL"
>'f'</TT
> is true.
</P
></LI
><LI
><P
> When the PostgreSQL return type is <TT
CLASS="TYPE"
>bytea</TT
>, 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 <TT
CLASS="TYPE"
>bytea</TT
>.
</P
></LI
><LI
><P
> For all other PostgreSQL return types, the returned Python
value is converted to a string using the Python
built-in <TT
CLASS="LITERAL"
>str</TT
>, and the result is passed to the
input function of the PostgreSQL data type.
</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
><P
> For nonscalar data types, see below.
</P
></LI
></UL
><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"
><H2
CLASS="SECT2"
><A
NAME="AEN59526"
>42.3.2. Null, None</A
></H2
><P
> If an SQL null value is passed to a
function, the argument value will appear as <TT
CLASS="SYMBOL"
>None</TT
> in
Python. For example, the function definition of <CODE
CLASS="FUNCTION"
>pymax</CODE
>
shown in <A
HREF="plpython-funcs.html"
>Section 42.2</A
> will return the wrong answer for null
inputs. We could add <TT
CLASS="LITERAL"
>STRICT</TT
> 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 $$
if (a is None) or (b is None):
return None
if a > b:
return a
return b
$$ LANGUAGE plpythonu;</PRE
><P>
As shown above, to return an SQL null value from a PL/Python
function, return the value <TT
CLASS="SYMBOL"
>None</TT
>. This can be done whether the
function is strict or not.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="PLPYTHON-ARRAYS"
>42.3.3. Arrays, Lists</A
></H2
><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 sequence, for example a list or tuple:
</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>
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"
><H2
CLASS="SECT2"
><A
NAME="AEN59544"
>42.3.4. Composite Types</A
></H2
><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
<TT
CLASS="SYMBOL"
>None</TT
> 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 $$
if e["salary"] > 200000:
return True
if (e["age"] < 30) and (e["salary"] > 100000):
return True
return False
$$ 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
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Sequence type (a tuple or list, but not a set because
it is not indexable)</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 <TT
CLASS="SYMBOL"
>None</TT
> at
the corresponding position.
</P
></DD
><DT
>Mapping (dictionary)</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
<TT
CLASS="SYMBOL"
>None</TT
> with the corresponding column name as the key.
</P
></DD
><DT
>Object (any object providing method <TT
CLASS="LITERAL"
>__getattr__</TT
>)</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 <TT
CLASS="LITERAL"
>OUT</TT
> 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
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN59573"
>42.3.5. Set-returning Functions</A
></H2
><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
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Sequence type (tuple, list, set)</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
>Iterator (any object providing <TT
CLASS="SYMBOL"
>__iter__</TT
> and
<TT
CLASS="SYMBOL"
>next</TT
> methods)</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
>Generator (<TT
CLASS="LITERAL"
>yield</TT
>)</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>
<DIV
CLASS="WARNING"
><P
></P
><TABLE
CLASS="WARNING"
BORDER="1"
WIDTH="90%"
><TR
><TD
ALIGN="CENTER"
><B
>Warning</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> Due to Python
<A
HREF="http://bugs.python.org/issue1483133"
TARGET="_top"
>bug #1483133</A
>,
some debug versions of Python 2.4
(configured and compiled with option <TT
CLASS="LITERAL"
>--with-pydebug</TT
>)
are known to crash the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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.
</P
></TD
></TR
></TABLE
></DIV
>
</P
></DD
></DL
></DIV
><P>
</P
><P
> Set-returning functions with <TT
CLASS="LITERAL"
>OUT</TT
> parameters
(using <TT
CLASS="LITERAL"
>RETURNS SETOF record</TT
>) 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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