Unexpected result with += on NumPy arrays

Question

I am creating symmetric matrices/arrays in Python with NumPy, using a standard method:

x = rand(500,500)
x = (x+x.T)
all(x==x.T)
> True

Answer 1

The implementation detail mentioned by others is called buffering. You can read more about it in the docs on array iteration.

If you look at your failing example in a little more detail:

>>> a = np.random.rand(91, 91)
>>> a += a.T
>>> a[:5, -1]
array([ 0.83818399,  1.06489316,  1.23675312,  0.00379798,  1.08967428])
>>> a[-1, :5]
array([ 0.83818399,  1.06489316,  1.75091827,  0.00416305,  1.76315071])

So the first wrong value is 90*91 + 2 = 8192, which, unsurprisingly, is what we get from:

>>> np.getbufsize()
8192

And we can also set it higher, and then:

>>> np.setbufsize(16384)  # Must be a multiple of 16
8192  # returns the previous buffer size
>>> a = np.random.rand(91, 91)
>>> a += a.T
>>> np.all(a == a.T)
True

Although now:

>>> a = np.random.rand(129, 129)
>>> a += a.T
>>> np.all(a == a.T)
False

This is of course a dangerous thing to rely on for correctness, as it is indeed an implementation detail subject to change.

Answer 2

The transpose operation returns a view of the array, which means that no new array is allocated. Which, in turn, means that you are reading and modifying the array at the same time. It's hard to tell why some sizes or some areas of the result work, but most likely it has to do with how numpy deals with array addition (maybe it makes copies of submatrices) and/or array views (maybe for small sizes it does create a new array).

The x = x + x.T operation works because there you are creating a new array and then assigning to x, of course.

Answer 3

The problem is that the addition is not done "at once"; x.T is a view of the x so as you start adding to each element of x, x.T is mutated. This messes up later additions.

The fact it works for sizes below (91, 91) is almost definitely an implementation detail. Using

x = numpy.random.rand(1000, 1000)
x += x.T.copy()
numpy.all(x==x.T)
#>>> True

fixes that, but then you don't really have any space-benefit.