Hello;
I want to create a custom model field for storing a numpy array. It has
currently been so simple that I fear I am overrlooking something and
getting a very false positive?
<code>
class NumpyArrayField(BinaryField):
dtype = numpy.float32
@classmethod
def load_numpy_array(cls , blob):
return numpy.fromstring(blob , NumpyArrayField.dtype)
def __init__(self , *args , **kwargs):
kwargs['default'] = None
super(NumpyArrayField , self).__init__(*args , **kwargs)
def from_db_value(self, value, expression, connection, context):
if value is None:
return value
return self.load_numpy_array( value )
def to_python(self, value):
if isinstance(value, numpy.ndarray):
return value
if value is None:
return value
return self.load_numpy_array( value )
</code>
As I see it this has (essenially two versions) of code for deserializing
from a database value to a numpy array, but no method for the opposite
operation. However it seemingly works?
I have created a small model with one of these fields:
<code>
class TimeSeries(Model):
start = DateTimeField( )
step = IntegerField( )
data = NumpyArrayField( )
@classmethod
def createArray(cls , size = 0):
return numpy.ndarray( shape = [size] , dtype =
NumpyArrayField.dtype)
def __getitem__(self , index):
if self.data is None:
raise IndexError
else:
return self.data[index]
def __setitem__(self , index , value):
if self.data is None:
raise IndexError
else:
self.data[index] = value
def __len__(self):
if self.data is None:
return 0
else:
shape = self.data.shape
return shape[0]
</code>
And a test:
<code>
class TimeSeriesTest(TestCase):
def test_create(self):
ts = TimeSeries.objects.create( start = timezone.now(),
step = 100 ,
data = TimeSeries.createArray( ))
self.assertEqual( len(ts) , 0 )
with self.assertRaises(IndexError):
ts[0]
ts.addValue( 1 )
self.assertEqual( len(ts) , 1 )
with self.assertRaises(IndexError):
ts[1]
self.assertEqual( ts[0] , 1 )
# How on earth does the NumpyArrayField know how to save itself to
the db?
ts.save()
ts2 = TimeSeries.objects.get( pk = 1 )
self.assertEqual( ts2[0] , 1)
self.assertEqual( len(ts2) , 1 )
</code>
I was expecting the test to fail spectacularly when calling ts.save() -
however it seems to work, and I can even get the instance back from db -
what gives? I am using sqlite and have tried to force the db to create a
file.
I was excpecting to implement the get_prep_db_value() method - but now when
things seemingly work without it I get quite uncertain? Is there some magic
trickery involved which says that this should indeed work - or am I seeing
a completely false positive?
Joakim
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