Add rounding parameter to DecimalField (#5562)

* Adding rounding parameter to DecimalField.

* Using standard `assert` instead of `self.fail()`.

* add testcase and PEP8 multilines fix

* flake8 fixes

* Use decimal module constants in tests.

* Add docs note for `rounding` parameter.

docs/api-guide/fields.md

@@ -269,6 +269,7 @@ Corresponds to `django.db.models.fields.DecimalField`.
 - `max_value` Validate that the number provided is no greater than this value.
 - `min_value` Validate that the number provided is no less than this value.
 - `localize` Set to `True` to enable localization of input and output based on the current locale. This will also force `coerce_to_string` to `True`. Defaults to `False`. Note that data formatting is enabled if you have set `USE_L10N=True` in your settings file.
+- `rounding` Sets the rounding mode used when quantising to the configured precision. Valid values are [`decimal` module rounding modes][python-decimal-rounding-modes]. Defaults to `None`.
 
 #### Example usage
 
@@ -680,3 +681,4 @@ The [django-rest-framework-hstore][django-rest-framework-hstore] package provide
 [django-rest-framework-gis]: https://github.com/djangonauts/django-rest-framework-gis
 [django-rest-framework-hstore]: https://github.com/djangonauts/django-rest-framework-hstore
 [django-hstore]: https://github.com/djangonauts/django-hstore
+[python-decimal-rounding-modes]: https://docs.python.org/3/library/decimal.html#rounding-modes

rest_framework/fields.py

@@ -997,7 +997,7 @@ class DecimalField(Field):
     MAX_STRING_LENGTH = 1000  # Guard against malicious string inputs.
 
     def __init__(self, max_digits, decimal_places, coerce_to_string=None, max_value=None, min_value=None,
-                 localize=False, **kwargs):
+                 localize=False, rounding=None, **kwargs):
         self.max_digits = max_digits
         self.decimal_places = decimal_places
         self.localize = localize
@@ -1029,6 +1029,12 @@ class DecimalField(Field):
             self.validators.append(
                 MinValueValidator(self.min_value, message=message))
 
+        if rounding is not None:
+            valid_roundings = [v for k, v in vars(decimal).items() if k.startswith('ROUND_')]
+            assert rounding in valid_roundings, (
+                'Invalid rounding option %s. Valid values for rounding are: %s' % (rounding, valid_roundings))
+        self.rounding = rounding
+
     def to_internal_value(self, data):
         """
         Validate that the input is a decimal number and return a Decimal
@@ -1121,6 +1127,7 @@ class DecimalField(Field):
             context.prec = self.max_digits
         return value.quantize(
             decimal.Decimal('.1') ** self.decimal_places,
+            rounding=self.rounding,
             context=context
         )
 

tests/test_fields.py

@@ -3,7 +3,7 @@ import os
 import re
 import unittest
 import uuid
-from decimal import Decimal
+from decimal import ROUND_DOWN, ROUND_UP, Decimal
 
 import django
 import pytest
@@ -1092,8 +1092,21 @@ class TestNoDecimalPlaces(FieldValues):
     field = serializers.DecimalField(max_digits=6, decimal_places=None)
 
 
-# Date & time serializers...
+class TestRoundingDecimalField(TestCase):
+    def test_valid_rounding(self):
+        field = serializers.DecimalField(max_digits=4, decimal_places=2, rounding=ROUND_UP)
+        assert field.to_representation(Decimal('1.234')) == '1.24'
 
+        field = serializers.DecimalField(max_digits=4, decimal_places=2, rounding=ROUND_DOWN)
+        assert field.to_representation(Decimal('1.234')) == '1.23'
+
+    def test_invalid_rounding(self):
+        with pytest.raises(AssertionError) as excinfo:
+            serializers.DecimalField(max_digits=1, decimal_places=1, rounding='ROUND_UNKNOWN')
+        assert 'Invalid rounding option' in str(excinfo.value)
+
+
+# Date & time serializers...
 class TestDateField(FieldValues):
     """
     Valid and invalid values for `DateField`.