[Django] #26963: Improve error message when trying to insert a value that overflows DecimalField

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Jul 27, 2016, 4:02:13 PM7/27/16

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#26963: Improve error message when trying to insert a value that overflows
DecimalField
-------------------------------------+-------------------------------------
Reporter: fdh | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: master
(models, ORM) | Keywords: DecimalField,
Severity: Normal | ValidationError, quantize
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
[https://docs.djangoproject.com/en/1.9/ref/models/fields/#decimalfield)
DecimalField] accepts `max_digit` parameter that specifies the amount of
digit positions used by the `DecimalField`.

Consider the following model that allows for only one digit before the
decimal separator:
{{{#!python
from django.db.models import Model
class MyModel(Model):
my_field = DecimalField(max_digits=2, decimal_places=1)
}}}
A value of `Decimal("10.1")` is not supported, as it requires 3 decimal
places, however its input is accepted until the value is being inserted in
the database.

[https://github.com/django/django/blob/cd2e4293cbf3416af6c478c5aaab711cae88892c/django/db/backends/utils.py#L204
This line] in `django.db.backends.utils.format_number` fails when any
insertion or update is attempted (e.g. using `save()` , `bulk_create`
etc. ), because `quantize` raises an `InvalidOperation` when there are not
enough positions to fit the rounded value (according to `Decimal.quantize`
[https://docs.python.org/3/library/decimal.html#decimal.Decimal.quantize
docs]).

The error message is not very informative (0).

[https://github.com/florisdenhengst/django/commit/0f109a9bca5b20361e11a54c12fa786a4263bc4e
This commit] contains a test that triggers the error. Note that overflow
might also happen because of rounding during the quantization (e.g. values
of 9.99999 could result in 10.0 during quantization, depending on the
[https://docs.python.org/3/library/decimal.html#decimal.Context Context]
used).
For a copy of the commit see (1).

(0) Example error message
{{{
Traceback (most recent call last):
File "/path_to_project/file.py", line 123, in my_failing_function
overflowing_instance.save()
File "/path_to_django/db/models/base.py", line 796, in save
force_update=force_update, update_fields=update_fields)
File "/path_to_django/db/models/base.py", line 824, in save_base
updated = self._save_table(raw, cls, force_insert, force_update,
using, update_fields)
File "/path_to_django/db/models/base.py", line 908, in _save_table
result = self._do_insert(cls._base_manager, using, fields, update_pk,
raw)
File "/path_to_django/db/models/base.py", line 947, in _do_insert
using=using, raw=raw)
File "/path_to_django/db/models/manager.py", line 85, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/path_to_django/db/models/query.py", line 1046, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "/path_to_django/db/models/sql/compiler.py", line 1053, in
execute_sql
for sql, params in self.as_sql():
File "/path_to_django/db/models/sql/compiler.py", line 1006, in as_sql
for obj in self.query.objs
File "/path_to_django/db/models/sql/compiler.py", line 1006, in
<listcomp>
for obj in self.query.objs
File "/path_to_django/db/models/sql/compiler.py", line 1005, in
<listcomp>
[self.prepare_value(field, self.pre_save_val(field, obj)) for field in
fields]
File "/path_to_django/db/models/sql/compiler.py", line 945, in
prepare_value
value = field.get_db_prep_save(value, connection=self.connection)
File "/path_to_django/db/models/fields/__init__.py", line 1587, in
get_db_prep_save
return connection.ops.adapt_decimalfield_value(self.to_python(value),
self.max_digits, self.decimal_places)
File "/path_to_django/db/backends/base/operations.py", line 495, in
adapt_decimalfield_value
return utils.format_number(value, max_digits, decimal_places)
File "/path_to_django/db/backends/utils.py", line 204, in format_number
value = value.quantize(decimal.Decimal(".1") ** decimal_places,
context=context)
decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]
}}}

(1)Test that triggers the error (add to
`django.tests.model_fields.test_decimalfield.DecimalFieldTests`)
{{{#!python
def test_save_with_max_digits_overflow(self):
"""
Ensure overflowing decimals yield a meaningful error.
"""
overflowing_value = Decimal(10 ** 6)
expected_message = "Not enough digit positions in field 'd' to
represent {}".format(overflowing_value) # some meaningful error message
overflowing_instance = Foo(a='a', d=overflowing_value)
with self.assertRaisesMessage(ValidationError, # some meaningful
error
expected_message):
overflowing_instance.save()
}}}

--
Ticket URL: <https://code.djangoproject.com/ticket/26963>
Django <https://code.djangoproject.com/>
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Django

unread,

Jul 27, 2016, 4:28:51 PM7/27/16

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#26963: Improve error message when trying to insert a value that overflows
DecimalField
-------------------------------------+-------------------------------------
Reporter: fdh | Owner: nobody

Type: | Status: closed

Cleanup/optimization |
Component: Database layer | Version: master
(models, ORM) |

Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------

Changes (by timgraham):

* status: new => closed
* needs_better_patch: => 0
* resolution: => invalid
* needs_tests: => 0
* needs_docs: => 0

Comment:

If you want a nice error message, you need to run validation before
calling save: `overflowing_instance.full_clean()`.

Doing so in your test raises `ValidationError: {'d': ['Ensure that there
are no more than 5 digits in total.']}`.