Python 3 SEP

import numpy as np
import matplotlib.pyplot as plt
from astropy.io import fits
fits_image_filename = fits.util.get_testdata_filepath('test0.fits')

fits_image_filename

data2 = fits.open(fits_image_filename)

data =data2[-3].data

m, s = np.mean(data), np.std(data)
plt.imshow(data, interpolation='nearest', cmap='gray', vmin=m-s, vmax=m+s, origin='lower')
plt.colorbar();

import sep

data_sep = data.byteswap().newbyteorder().astype('float')
bkg = sep.Background(data_sep)

m, s = np.mean(bkg), np.std(bkg)
plt.imshow(bkg, interpolation='nearest', cmap='gray', vmin=m-s, vmax=m+s, origin='lower')
plt.colorbar();