Question: As I run my code I get these warnings, always in groups of four, sporadically. I have tried to locate the source by placing debug messages before and after certain statements to pin-point its origin.

```
Warning: invalid value encountered in double_scalars
Warning: invalid value encountered in double_scalars
Warning: invalid value encountered in double_scalars
Warning: invalid value encountered in double_scalars

```

Is this is a Numpy warning, and what is a double scalar?

From Numpy I use

```
min(), argmin(), mean() and random.randn()

```

I also use Matplotlib

Answer: 
It looks like a floating-point calculation error. Check the [numpy.seterr](http://docs.scipy.org/doc/numpy/reference/generated/numpy.seterr.html) function to get more information about where it happens.
I encount this while I was calculating `np.var(np.array([]))`. `np.var` will divide size of the array which is zero in this case.