From 38a1b3ec6b62ea0e5bfcb0de2043067d8e333c95 Mon Sep 17 00:00:00 2001
From: Tom Christie <tom@tomchristie.com>
Date: Thu, 6 Aug 2015 09:51:00 +0100
Subject: [PATCH] Rationalize decimal logic. Closes #3222.

---
 rest_framework/fields.py | 38 ++++++++++++++++++++++++--------------
 tests/test_fields.py     |  6 ++++--
 2 files changed, 28 insertions(+), 16 deletions(-)

diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index aa264b2aa..37c902954 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -797,6 +797,11 @@ class DecimalField(Field):
         self.max_value = max_value
         self.min_value = min_value
 
+        if self.max_digits is not None and self.decimal_places is not None:
+            self.max_whole_digits = self.max_digits - self.decimal_places
+        else:
+            self.max_whole_digits = None
+
         super(DecimalField, self).__init__(**kwargs)
 
         if self.max_value is not None:
@@ -840,24 +845,29 @@ class DecimalField(Field):
         values or to enhance it in any way you need to.
         """
         sign, digittuple, exponent = value.as_tuple()
-        decimals = exponent * decimal.Decimal(-1) if exponent < 0 else 0
 
-        # digittuple doesn't include any leading zeros.
-        digits = len(digittuple)
-        if decimals > digits:
-            # We have leading zeros up to or past the decimal point.  Count
-            # everything past the decimal point as a digit.  We do not count
-            # 0 before the decimal point as a digit since that would mean
-            # we would not allow max_digits = decimal_places.
-            digits = decimals
-        whole_digits = digits - decimals
+        if exponent >= 0:
+            # 1234500.0
+            total_digits = len(digittuple) + exponent
+            whole_digits = total_digits
+            decimal_places = 0
+        elif len(digittuple) > abs(exponent):
+            # 123.45
+            total_digits = len(digittuple)
+            whole_digits = total_digits - abs(exponent)
+            decimal_places = abs(exponent)
+        else:
+            # 0.001234
+            total_digits = abs(exponent)
+            whole_digits = 0
+            decimal_places = total_digits
 
-        if self.max_digits is not None and digits > self.max_digits:
+        if self.max_digits is not None and total_digits > self.max_digits:
             self.fail('max_digits', max_digits=self.max_digits)
-        if self.decimal_places is not None and decimals > self.decimal_places:
+        if self.decimal_places is not None and decimal_places > self.decimal_places:
             self.fail('max_decimal_places', max_decimal_places=self.decimal_places)
-        if self.max_digits is not None and self.decimal_places is not None and whole_digits > (self.max_digits - self.decimal_places):
-            self.fail('max_whole_digits', max_whole_digits=self.max_digits - self.decimal_places)
+        if self.max_whole_digits is not None and whole_digits > self.max_whole_digits:
+            self.fail('max_whole_digits', max_whole_digits=self.max_whole_digits)
 
         return value
 
diff --git a/tests/test_fields.py b/tests/test_fields.py
index 042787357..6adba66a7 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -766,15 +766,17 @@ class TestDecimalField(FieldValues):
         0: Decimal('0'),
         12.3: Decimal('12.3'),
         0.1: Decimal('0.1'),
-        '2E+2': Decimal('200'),
+        '2E+1': Decimal('20'),
     }
     invalid_inputs = (
         ('abc', ["A valid number is required."]),
         (Decimal('Nan'), ["A valid number is required."]),
         (Decimal('Inf'), ["A valid number is required."]),
         ('12.345', ["Ensure that there are no more than 3 digits in total."]),
+        (200000000000.0, ["Ensure that there are no more than 3 digits in total."]),
         ('0.01', ["Ensure that there are no more than 1 decimal places."]),
-        (123, ["Ensure that there are no more than 2 digits before the decimal point."])
+        (123, ["Ensure that there are no more than 2 digits before the decimal point."]),
+        ('2E+2', ["Ensure that there are no more than 2 digits before the decimal point."])
     )
     outputs = {
         '1': '1.0',