DecimalField.quantize() fails when using cdecimal if rounding parameter is not specified

[kbussell]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Coming up with a test case to run as part of the test suite is tough because the decimal module needs to be monkey-patched with cdecimal early in the flow. I realize this isn't necessarily a common case, but the fix (coming soon in a pr) is straightforward.

Steps to reproduce

In [1]: import sys

In [2]: import cdecimal

In [3]: sys.modules['decimal'] = cdecimal

In [4]: from django.conf import settings

In [5]: settings.configure(
   ...:         DEBUG_PROPAGATE_EXCEPTIONS=True,
   ...:         DATABASES={
   ...:             'default': {
   ...:                 'ENGINE': 'django.db.backends.sqlite3',
   ...:                 'NAME': ':memory:'
   ...:             }
   ...:         },
   ...:         SITE_ID=1,
   ...:         SECRET_KEY='not very secret in tests',
   ...:         USE_I18N=True,
   ...:         USE_L10N=True,
   ...:         STATIC_URL='/static/',
   ...:         ROOT_URLCONF='tests.urls',
   ...:         TEMPLATES=[
   ...:             {
   ...:                 'BACKEND': 'django.template.backends.django.DjangoTemplates',
   ...:                 'APP_DIRS': True,
   ...:                 'OPTIONS': {
   ...:                     "debug": True,  # We want template errors to raise
   ...:                 }
   ...:             },
   ...:         ],
   ...:         MIDDLEWARE=(
   ...:             'django.middleware.common.CommonMiddleware',
   ...:             'django.contrib.sessions.middleware.SessionMiddleware',
   ...:             'django.contrib.auth.middleware.AuthenticationMiddleware',
   ...:             'django.contrib.messages.middleware.MessageMiddleware',
   ...:         ),
   ...:         INSTALLED_APPS=(
   ...:             'django.contrib.admin',
   ...:             'django.contrib.auth',
   ...:             'django.contrib.contenttypes',
   ...:             'django.contrib.sessions',
   ...:             'django.contrib.sites',
   ...:             'django.contrib.staticfiles',
   ...:             'rest_framework',
   ...:             'rest_framework.authtoken',
   ...:             'tests.importable',
   ...:             'tests',
   ...:         ),
   ...:         PASSWORD_HASHERS=(
   ...:             'django.contrib.auth.hashers.MD5PasswordHasher',
   ...:         ),
   ...:     )

In [6]: from rest_framework import serializers

In [7]: f = serializers.DecimalField(max_digits=4, decimal_places=2)

In [8]: f.quantize(cdecimal.Decimal('1.234'))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-728ed1ea55db> in <module>()
----> 1 f.quantize(cdecimal.Decimal('1.234'))

/Users/keith/src/open/django-rest-framework/rest_framework/fields.py in quantize(self, value)
   1141             decimal.Decimal('.1') ** self.decimal_places,
   1142             rounding=self.rounding,
-> 1143             context=context
   1144         )
   1145

TypeError: an integer is required

In [9]:

Expected behavior

I would expect that one does not need to specify the rounding parameter in the field declaration for it to work.

Actual behavior

Without specifying the rounding parameter explicitly, calls to DecimalField.quantize() will fail with a TypeError because the rounding parameter is expected to be an integer (and defaults to None in the code).

[rpkilby]

I'm closing this for now per #5741 (comment), as we generally don't consider this an issue. However, I am leaving the PR open for consideration.