`import numpy` adds the numpy namespace for runtime stuff and `cimport numpy` add the numpy c-api stuff. Even if Cython itself can figure out which one every `np.` refers to, I prefer to `cimport numpy as cnp`. This makes it clear what's going on, and is commonly found in [style guides](http://scikit-image.org/docs/dev/contribute.html#stylistic-guidelines) for projects that write a lot of Cython.
> Explicit is better than implicit.