PL/Python: Convert numeric to Decimal

The old implementation converted PostgreSQL numeric to Python float,
which was always considered a shortcoming.  Now numeric is converted to
the Python Decimal object.  Either the external cdecimal module or the
standard library decimal module are supported.

From: Szymon Guz <mabewlun@gmail.com>
From: Ronan Dunklau <rdunklau@gmail.com>
Reviewed-by: Steve Singer <steve@ssinger.info>

doc/src/sgml/plpython.sgml

@@ -310,12 +310,23 @@ $$ LANGUAGE plpythonu;
 
      <listitem>
       <para>
-       PostgreSQL <type>real</type>, <type>double</type>,
-       and <type>numeric</type> are converted to
-       Python <type>float</type>.  Note that for
-       the <type>numeric</type> this loses information and can lead to
-       incorrect results.  This might be fixed in a future
-       release.
+       PostgreSQL <type>real</type> and <type>double</type> are converted to
+       Python <type>float</type>.
+      </para>
+     </listitem>
+
+     <listitem>
+      <para>
+       PostgreSQL <type>numeric</type> is converted to
+       Python <type>Decimal</type>.  This type is imported from
+       the <literal>cdecimal</literal> package if that is available.
+       Otherwise,
+       <literal>decimal.Decimal</literal> from the standard library will be
+       used.  <literal>cdecimal</literal> is significantly faster
+       than <literal>decimal</literal>.  In Python 3.3,
+       however, <literal>cdecimal</literal> has been integrated into the
+       standard library under the name <literal>decimal</literal>, so there is
+       no longer any difference.
       </para>
      </listitem>
 

src/pl/plpython/expected/plpython_types.out

@@ -213,36 +213,69 @@ CONTEXT:  PL/Python function "test_type_conversion_int8"
 (1 row)
 
 CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
-plpy.info(x, type(x))
+# print just the class name, not the type, to avoid differences
+# between decimal and cdecimal
+plpy.info(x, x.__class__.__name__)
 return x
 $$ LANGUAGE plpythonu;
-/* The current implementation converts numeric to float. */
 SELECT * FROM test_type_conversion_numeric(100);
-INFO:  (100.0, <type 'float'>)
+INFO:  (Decimal('100'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
-                        100.0
+                          100
 (1 row)
 
 SELECT * FROM test_type_conversion_numeric(-100);
-INFO:  (-100.0, <type 'float'>)
+INFO:  (Decimal('-100'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
-                       -100.0
+                         -100
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(100.0);
+INFO:  (Decimal('100.0'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+                        100.0
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(100.00);
+INFO:  (Decimal('100.00'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+                       100.00
 (1 row)
 
 SELECT * FROM test_type_conversion_numeric(5000000000.5);
-INFO:  (5000000000.5, <type 'float'>)
+INFO:  (Decimal('5000000000.5'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
                  5000000000.5
 (1 row)
 
+SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
+INFO:  (Decimal('1234567890.0987654321'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+        1234567890.0987654321
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
+INFO:  (Decimal('-1234567890.0987654321'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+       -1234567890.0987654321
+(1 row)
+
 SELECT * FROM test_type_conversion_numeric(null);
-INFO:  (None, <type 'NoneType'>)
+INFO:  (None, 'NoneType')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------

src/pl/plpython/expected/plpython_types_3.out

@@ -213,36 +213,69 @@ CONTEXT:  PL/Python function "test_type_conversion_int8"
 (1 row)
 
 CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
-plpy.info(x, type(x))
+# print just the class name, not the type, to avoid differences
+# between decimal and cdecimal
+plpy.info(x, x.__class__.__name__)
 return x
 $$ LANGUAGE plpython3u;
-/* The current implementation converts numeric to float. */
 SELECT * FROM test_type_conversion_numeric(100);
-INFO:  (100.0, <class 'float'>)
+INFO:  (Decimal('100'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
-                        100.0
+                          100
 (1 row)
 
 SELECT * FROM test_type_conversion_numeric(-100);
-INFO:  (-100.0, <class 'float'>)
+INFO:  (Decimal('-100'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
-                       -100.0
+                         -100
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(100.0);
+INFO:  (Decimal('100.0'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+                        100.0
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(100.00);
+INFO:  (Decimal('100.00'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+                       100.00
 (1 row)
 
 SELECT * FROM test_type_conversion_numeric(5000000000.5);
-INFO:  (5000000000.5, <class 'float'>)
+INFO:  (Decimal('5000000000.5'), 'Decimal')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------
                  5000000000.5
 (1 row)
 
+SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
+INFO:  (Decimal('1234567890.0987654321'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+        1234567890.0987654321
+(1 row)
+
+SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
+INFO:  (Decimal('-1234567890.0987654321'), 'Decimal')
+CONTEXT:  PL/Python function "test_type_conversion_numeric"
+ test_type_conversion_numeric 
+------------------------------
+       -1234567890.0987654321
+(1 row)
+
 SELECT * FROM test_type_conversion_numeric(null);
-INFO:  (None, <class 'NoneType'>)
+INFO:  (None, 'NoneType')
 CONTEXT:  PL/Python function "test_type_conversion_numeric"
  test_type_conversion_numeric 
 ------------------------------

src/pl/plpython/plpy_typeio.c

@@ -16,6 +16,7 @@
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
+#include "utils/numeric.h"
 #include "utils/syscache.h"
 #include "utils/typcache.h"
 
@@ -35,7 +36,7 @@ static void PLy_output_datum_func2(PLyObToDatum *arg, HeapTuple typeTup);
 static PyObject *PLyBool_FromBool(PLyDatumToOb *arg, Datum d);
 static PyObject *PLyFloat_FromFloat4(PLyDatumToOb *arg, Datum d);
 static PyObject *PLyFloat_FromFloat8(PLyDatumToOb *arg, Datum d);
-static PyObject *PLyFloat_FromNumeric(PLyDatumToOb *arg, Datum d);
+static PyObject *PLyDecimal_FromNumeric(PLyDatumToOb *arg, Datum d);
 static PyObject *PLyInt_FromInt16(PLyDatumToOb *arg, Datum d);
 static PyObject *PLyInt_FromInt32(PLyDatumToOb *arg, Datum d);
 static PyObject *PLyLong_FromInt64(PLyDatumToOb *arg, Datum d);
@@ -450,7 +451,7 @@ PLy_input_datum_func2(PLyDatumToOb *arg, Oid typeOid, HeapTuple typeTup)
 			arg->func = PLyFloat_FromFloat8;
 			break;
 		case NUMERICOID:
-			arg->func = PLyFloat_FromNumeric;
+			arg->func = PLyDecimal_FromNumeric;
 			break;
 		case INT2OID:
 			arg->func = PLyInt_FromInt16;
@@ -516,16 +517,37 @@ PLyFloat_FromFloat8(PLyDatumToOb *arg, Datum d)
 }
 
 static PyObject *
-PLyFloat_FromNumeric(PLyDatumToOb *arg, Datum d)
+PLyDecimal_FromNumeric(PLyDatumToOb *arg, Datum d)
 {
-	/*
-	 * Numeric is cast to a PyFloat: This results in a loss of precision Would
-	 * it be better to cast to PyString?
-	 */
-	Datum		f = DirectFunctionCall1(numeric_float8, d);
-	double		x = DatumGetFloat8(f);
+	static PyObject *decimal_constructor;
+	char	   *str;
+	PyObject   *pyvalue;
+
+	/* Try to import cdecimal.  If it doesn't exist, fall back to decimal. */
+	if (!decimal_constructor)
+	{
+		PyObject   *decimal_module;
+
+		decimal_module = PyImport_ImportModule("cdecimal");
+		if (!decimal_module)
+		{
+			PyErr_Clear();
+			decimal_module = PyImport_ImportModule("decimal");
+		}
+		if (!decimal_module)
+			PLy_elog(ERROR, "could not import a module for Decimal constructor");
+
+		decimal_constructor = PyObject_GetAttrString(decimal_module, "Decimal");
+		if (!decimal_constructor)
+			PLy_elog(ERROR, "no Decimal attribute in module");
+	}
+
+	str = DatumGetCString(DirectFunctionCall1(numeric_out, d));
+	pyvalue = PyObject_CallFunction(decimal_constructor, "s", str);
+	if (!pyvalue)
+		PLy_elog(ERROR, "conversion from numeric to Decimal failed");
 
-	return PyFloat_FromDouble(x);
+	return pyvalue;
 }
 
 static PyObject *

src/pl/plpython/sql/plpython_types.sql

@@ -86,14 +86,19 @@ SELECT * FROM test_type_conversion_int8(null);
 
 
 CREATE FUNCTION test_type_conversion_numeric(x numeric) RETURNS numeric AS $$
-plpy.info(x, type(x))
+# print just the class name, not the type, to avoid differences
+# between decimal and cdecimal
+plpy.info(x, x.__class__.__name__)
 return x
 $$ LANGUAGE plpythonu;
 
-/* The current implementation converts numeric to float. */
 SELECT * FROM test_type_conversion_numeric(100);
 SELECT * FROM test_type_conversion_numeric(-100);
+SELECT * FROM test_type_conversion_numeric(100.0);
+SELECT * FROM test_type_conversion_numeric(100.00);
 SELECT * FROM test_type_conversion_numeric(5000000000.5);
+SELECT * FROM test_type_conversion_numeric(1234567890.0987654321);
+SELECT * FROM test_type_conversion_numeric(-1234567890.0987654321);
 SELECT * FROM test_type_conversion_numeric(null);