python - numpy array with dtype Decimal?

30
votes

Are Decimal dtypes available in numpy?

>>> import decimal, numpy
>>> d = decimal.Decimal('1.1') 
>>> s = [['123.123','23'],['2323.212','123123.21312']]
>>> ss = numpy.array(s, dtype=numpy.dtype(decimal.Decimal))
>>> a = numpy.array(s, dtype=float)
>>> type(d)
<class 'decimal.Decimal'>
>>> type(ss[1,1])
<class 'str'>
>>> type(a[1,1])
<class 'numpy.float64'>

I suppose numpy.array doesn't support every dtype, but I sort of thought that it would at least let a dtype propagate as far as it could as long as the right operations were defined. Am I missing something? Is there some way for this to work?

3
pythonnumpydecimal
There are three answer below at the moment I am writing this and all are useful. I've marked one as correct. Depending on what you want, it seems that you can first 'cast' the array as dtype=float and then dtype=Decimal and it will function properly. If you need to go from string to Decimal, then I think it's an element wise conversion that is needed. - mathtick
Before reading all that, ask to yourself "Do I really need more than, let's say, 10 digits precision ?". If not, float64 is good and you can avoid most of workaround and, as underlined here, you'll take full advantage of numpy. - comte

3 Answers

4
votes

IMPORTANT CAVEAT: THIS IS A BAD ANSWER

So I answered this question before I really understood the point of it. The answer was accepted, and has some upvotes, but you would probably do best to skip to the next one.

Original answer:

It seems that Decimal is available:

>>> import decimal, numpy
>>> d = decimal.Decimal('1.1')
>>> a = numpy.array([d,d,d],dtype=numpy.dtype(decimal.Decimal))
>>> type(a[1])
<class 'decimal.Decimal'>

I'm not sure exactly what you are trying to accomplish, your example is more complicated than is necessary for simply creating a decimal numpy array.

28
votes

Numpy doesn't recognize decimal.Decimal as a specific type. The closest it can get is the most general dtype, object. So when converting the elements to the desired dtype, the conversion is a no-op.

>>> ss.dtype
dtype('object')

Keep in mind that because the elements of the array are Python objects, you won't get much of a speed up using them. For example, if you try to add this to any other array, the others elements will have to be boxed back into python objects and added via the normal Python addition code. You might gain some speed in that the iteration will be in C, but not that much.

11
votes

Unfortunately, you have to cast each of your items to Decimal when you create the numpy.array. Something like

s = [['123.123','23'],['2323.212','123123.21312']]
decimal_s = [[decimal.Decimal(x) for x in y] for y in s]
ss = numpy.array(decimal_s)