file_list = glob.glob(source_dir + '/*.TXT')
for num, file_path in enumerate(file_list):
    data = np.genfromtxt(file_path, delimiter=',', skip_header=3,
        skip_footer=18)
data = np.vstack((data.T, np.ones(data.shape[0]) * num)).T
if num == 0:
    Output = data
else:
    Output = np.vstack((Output, data))
Pythonic way to import data from multiple files into an array