Data Values

PostgreSQL 9.6.5 Documentation
Prev | Up | Chapter 44. PL/Python - Python Procedural Language | Next

# 44.3. Data Values

Generally speaking, the aim of PL/Python is to provide a "natural" mapping between the PostgreSQL and the Python worlds. This informs the data mapping rules described below.

## 44.3.1. Data Type Mapping

When a PL/Python function is called, its arguments are converted from their PostgreSQL data type to a corresponding Python type:

- PostgreSQL `boolean` is converted to Python `bool`.

- PostgreSQL `smallint` and `int` are converted to Python `int`. PostgreSQL `bigint` and `oid` are converted to `long` in Python 2 and to `int` in Python 3.

- PostgreSQL `real` and `double` are converted to Python `float`.

- PostgreSQL `numeric` is converted to Python `Decimal`. This type is imported from the `cdecimal` package if that is available. Otherwise, `decimal.Decimal` from the standard library will be used. `cdecimal` is significantly faster than `decimal`. In Python 3.3 and up, however, `cdecimal` has been integrated into the standard library under the name `decimal`, so there is no longer any difference.

- PostgreSQL `bytea` is converted to Python `str` in Python 2 and to `bytes` in Python 3. In Python 2, the string should be treated as a byte sequence without any character encoding.

- All other data types, including the PostgreSQL character string types, are converted to a Python `str`. In Python 2, this string will be in the PostgreSQL server encoding; in Python 3, it will be a Unicode string like all strings.

- For nonscalar data types, see below.

When a PL/Python function returns, its return value is converted to the function's declared PostgreSQL return data type as follows:

- When the PostgreSQL return type is `boolean`, the return value will be evaluated for truth according to the Python rules. That is, 0 and empty string are false, but notably `'f'` is true.

- When the PostgreSQL return type is `bytea`, 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 `bytea`.

- For all other PostgreSQL return types, the return value is converted to a string using the Python built-in `str`, and the result is passed to the input function of the PostgreSQL data type. (If the Python value is a `float`, it is converted using the `repr` built-in instead of `str`, to avoid loss of precision.)

  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.

- For nonscalar data types, see below.

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.

## 44.3.2. Null, None

If an SQL null value is passed to a function, the argument value will appear as `None` in Python. For example, the function definition of `pymax` shown in Section 44.2 will return the wrong answer for null inputs. We could add `STRICT` to the function definition to make PostgreSQL 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:

    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;

As shown above, to return an SQL null value from a PL/Python function, return the value `None`. This can be done whether the function is strict or not.

## 44.3.3. Arrays, Lists

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:

    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)

Note that in Python, strings are sequences, which can have undesirable effects that might be familiar to Python programmers:

    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)

## 44.3.4. Composite Types

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 `None` in the mapping. Here is an example:

    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;

There are multiple ways to return row or composite types from a Python function. The following examples assume we have:

    CREATE TYPE named_value AS (
      name   text,
      value  integer
    );

A composite result can be returned as a:

Sequence type (a tuple or list, but not a set because it is not indexable)

  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:

      CREATE FUNCTION make_pair (name text, value integer)
        RETURNS named_value
      AS $$
        return [ name, value ]
        # or alternatively, as tuple: return ( name, value )
      $$ LANGUAGE plpythonu;

  To return a SQL null for any column, insert `None` at the corresponding position.

Mapping (dictionary)

  The value for each result type column is retrieved from the mapping with the column name as key. Example:

      CREATE FUNCTION make_pair (name text, value integer)
        RETURNS named_value
      AS $$
        return { "name": name, "value": value }
      $$ LANGUAGE plpythonu;

  Any extra dictionary key/value pairs are ignored. Missing keys are treated as errors. To return a SQL null value for any column, insert `None` with the corresponding column name as the key.

Object (any object providing method `__getattr__`)

  This works the same as a mapping. Example:

      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;

Functions with `OUT` parameters are also supported. For example:

    CREATE FUNCTION multiout_simple(OUT i integer, OUT j integer) AS $$
    return (1, 2)
    $$ LANGUAGE plpythonu;

    SELECT * FROM multiout_simple();

## 44.3.5. Set-returning Functions

A PL/Python 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:

    CREATE TYPE greeting AS (
      how text,
      who text
    );

A set result can be returned from a:

Sequence type (tuple, list, set)

      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;

Iterator (any object providing `__iter__` and `next` methods)

      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;

Generator (`yield`)

      CREATE FUNCTION greet (how text)
        RETURNS SETOF greeting
      AS $$
        for who in [ "World", "PostgreSQL", "PL/Python" ]:
          yield ( how, who )
      $$ LANGUAGE plpythonu;

  Warning
  Due to Python bug #1483133, some debug versions of Python 2.4 (configured and compiled with option `--with-pydebug`) are known to crash the PostgreSQL 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.

Set-returning functions with `OUT` parameters (using `RETURNS SETOF record`) are also supported. For example:

    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);

Prev | Home | Next
PL/Python Functions | Up | Sharing Data