Larger floats incorrectly pass DecimalField validation

[ryankask]

Larger floats are not failing DecimalField validation.

For example:

>>> serializers.DecimalField(max_digits=3, decimal_places=1).run_validation(200000000000.0)
>>> Decimal('2E+11')

I would expect a ValidationError to be raised since that number has more than 3 digits. Django's form validation throws the expected error:

>>> forms.DecimalField(max_digits=3, decimal_places=1).clean(200000000000.0)
ValidationError: [u'Ensure that there are no more than 3 digits in total.']

ValidationError is raised if you drop a zero from the above sample input:

>>> serializers.DecimalField(max_digits=3, decimal_places=1).run_validation(20000000000.0)
ValidationError: [u'Ensure that there are no more than 3 digits in total.']

Similarly, the exception is also raised using the original number but as a Decimal and int:

>>> serializers.DecimalField(max_digits=3, decimal_places=1).run_validation(200000000000)
ValidationError: [u'Ensure that there are no more than 3 digits in total.']
>>> serializers.DecimalField(max_digits=3, decimal_places=1).run_validation(Decimal('200000000000.0'))
ValidationError: [u'Ensure that there are no more than 3 digits in total.']

A key line recently changed in #2948 but it doesn't seem correct and deviates from Django's forms.DecimalField validation which otherwise appears to have been copied verbatim:

decimals = exponent * decimal.Decimal(-1) if exponent < 0 else 0

I don't really understand the original issue addressed in #2948 so I don't know why the line changed. I'm more than happy to work on the issue if I understand the original issue.

Below is a patch with a failing test:

diff --git a/tests/test_fields.py b/tests/test_fields.py
index 0427873..cf41a5b 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -773,6 +773,7 @@ class TestDecimalField(FieldValues):
         (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."])
     )

[tomchristie]

Thanks Ryan.

[tomchristie]

Now resolved, with some much more transparent & obvious logic.

[Audace]

@tomchristie @ryankask - I see this was added to a release milestone back in 2015. I'm still running into this issue on version 3.11.0. Did the fix end up getting released? Code below:

class RecommendationSerializer(serializers.Serializer):
    total_owed = serializers.DecimalField(decimal_places=2, max_digits=8, min_value='1.00',
                                          rounding=ROUND_DOWN)
    term_length = serializers.IntegerField(min_value=1)

    class Meta:
        fields = ['total_owed', 'term_length']


serializer = RecommendationSerializer(data={'total_owed': '12.333333333333333', 'term_length': 6})
serializer.is_valid(raise_exception=True)

>>> {"total_owed": ["Ensure that there are no more than 8 digits in total."]}

[xordoquy]

@Audace you get the expected validation error. The discussion group is the best place to take this discussion and other usage questions. Thanks!