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 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.