Edge case DecimalField serialization

[szymonhab]

Hey, i've noticed a strange behavior in Decimal serialization

In theory i shouldn't expect any error here. Especially that this code fired after running for few months on production - only when serialized value is greater or equal to 1.0

Same bug occurs for serializers.DecimalField(decimal_places=3, max_digits=4) configuration - when value is greater or equal to 10

In theory this isn't a problem when someone serializes verified data from psql, this time I serialize raw python objects to json payload and decimals should be rounded up somehow?

Possible solution could be to just cut decimal part that can't fit into the configured places

In [51]: class TestDecimal(serializers.Serializer):
    ...:     something = serializers.DecimalField(decimal_places=4, max_digits=4)
    ...: 

In [52]: TestDecimal({"something": "0.100123"}).data  # i know that there should be an object - added dict only to shorten the code
Out[52]: {'something': '0.1001'}

In [53]: TestDecimal({"something": "1.000"}).data
---------------------------------------------------------------------------
InvalidOperation                          Traceback (most recent call last)
Cell In[53], line 1
----> 1 TestDecimal({"something": "1.000"}).data

File /usr/local/lib/python3.10/site-packages/rest_framework/serializers.py:555, in Serializer.data(self)
    553 @property
    554 def data(self):
--> 555     ret = super().data
    556     return ReturnDict(ret, serializer=self)

File /usr/local/lib/python3.10/site-packages/rest_framework/serializers.py:253, in BaseSerializer.data(self)
    251 if not hasattr(self, '_data'):
    252     if self.instance is not None and not getattr(self, '_errors', None):
--> 253         self._data = self.to_representation(self.instance)
    254     elif hasattr(self, '_validated_data') and not getattr(self, '_errors', None):
    255         self._data = self.to_representation(self.validated_data)

File /usr/local/lib/python3.10/site-packages/rest_framework/serializers.py:522, in Serializer.to_representation(self, instance)
    520         ret[field.field_name] = None
    521     else:
--> 522         ret[field.field_name] = field.to_representation(attribute)
    524 return ret

File /usr/local/lib/python3.10/site-packages/rest_framework/fields.py:1081, in DecimalField.to_representation(self, value)
   1078 if not isinstance(value, decimal.Decimal):
   1079     value = decimal.Decimal(str(value).strip())
-> 1081 quantized = self.quantize(value)
   1083 if not coerce_to_string:
   1084     return quantized

File /usr/local/lib/python3.10/site-packages/rest_framework/fields.py:1100, in DecimalField.quantize(self, value)
   1098 if self.max_digits is not None:
   1099     context.prec = self.max_digits
-> 1100 return value.quantize(
   1101     decimal.Decimal('.1') ** self.decimal_places,
   1102     rounding=self.rounding,
   1103     context=context
   1104 )

InvalidOperation: [<class 'decimal.InvalidOperation'>]

[alexintersog]

Facing same issue

[alexintersog]

Problem was size of number with decimals trimmed was > max_digits in serializer. Resolved by increasing max_digits