Arguments to cv2::imshow

The function has the following docstring: imshow(winname, mat) -> None. You can see the doc string by typing cv2.imshow.__doc__ in the interpreter.

Try cv2.imshow('Image', cvimage).

tl;dr : In original question, first argument of "window name" was missing. "imshow" takes two parameters and only one was supplied.

The question technically asks how to convert a NumPy Array (analogous to CV2 array) into a Mat object (CV). For anyone who is interested, this can be done by:

mat_array = cv.fromarray(numpy_array)

where mat_array is a Mat object, and numpy_array is a NumPy array or image. I would suggest staying away from older CV structures where possible. Numpy arrays offer much better performance than implemenations in native Python

Mat object was needed because C/C++ lacked a standard/native implementation of matrices.

However, numpy's array is a perfect replacement for that functionality. Hence, the cv2 module accepts numpy.arrays wherever a matrix is indicated in the docs.