<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _subtractSky(image,skyValue,memmap=False):
 """ subtract the given sky value from each the data array that has been passed. image is a fits object that contains the data and header for one image extension """

    try:
        np.subtract(image.data,skyValue,image.data)

    except IOError:
        print("Unable to perform sky subtraction on data array")
        raise IOError