Question

I have a Django model with a decimal field:

class User(models.Model):
    ...
    balance = models.DecimalField(max_digits=10, decimal_places=2, blank=True, null=True)
    ...

I have my app running on my development machine (Python 2.7.1, Mac OSX, Django 1.4, MySQL InnoDB), and on a production server (Python 2.6.5, Ubuntu, Django 1.4, MySQL ISAM)

The problem is on my development machine. This code works without throwing an error:

>>> u = User.objects.get(pk=1)
>>> u.balance
Decimal('9.00')
>>> u.balance=1.44
>>> u.save()
>>> u.balance
1.44

As far as I understand, u.save() should through the following error, which it does on my production instance:

>>> u.save()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/base.py", line 463, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/base.py", line 529, in save_base
    rows = manager.using(using).filter(pk=pk_val)._update(values)
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/query.py", line 557, in _update
    return query.get_compiler(self.db).execute_sql(None)
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/compiler.py", line 986, in execute_sql
    cursor = super(SQLUpdateCompiler, self).execute_sql(result_type)
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/compiler.py", line 808, in execute_sql
    sql, params = self.as_sql()
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/sql/compiler.py", line 951, in as_sql
    val = field.get_db_prep_save(val, connection=self.connection)
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/fields/__init__.py", line 873, in get_db_prep_save
    return connection.ops.value_to_db_decimal(self.to_python(value),
  File "/usr/local/lib/python2.6/dist-packages/django/db/models/fields/__init__.py", line 847, in to_python
    return decimal.Decimal(value)
  File "/usr/lib/python2.6/decimal.py", line 649, in __new__
    "First convert the float to a string")
TypeError: Cannot convert float to Decimal.  First convert the float to a string

So why is my development instance not throwing this error? Problem came up because I checked in some buggy code that passed my unit tests. This error should have been thrown. I put my environment details because it has to be due to one of those differences. I suspect MySQL ISAM vs. InnoDB, but not at all sure.

Thanks!

Answer 1

Your dev environment running Python 2.7 should not throw that error, but your production environment running Python 2.6 should.

From the Python docs on Decimal objects:

Changed in version 2.7: The argument to the constructor is now permitted to be a float instance.