How to use the pyvips.Image.identity function in pyvips

def test_hist_ismonotonic(self):
        im = pyvips.Image.identity()
        assert im.hist_ismonotonic()

def test_identity(self):
        im = pyvips.Image.identity()
        assert im.width == 256
        assert im.height == 1
        assert im.bands == 1
        assert im.format == pyvips.BandFormat.UCHAR

        p = im(0, 0)
        assert p[0] == 0.0
        p = im(255, 0)
        assert p[0] == 255.0
        p = im(128, 0)
        assert p[0] == 128.0

        im = pyvips.Image.identity(ushort=True)
        assert im.width == 65536
        assert im.height == 1
        assert im.bands == 1

def test_identity(self):
        im = pyvips.Image.identity()
        assert im.width == 256
        assert im.height == 1
        assert im.bands == 1
        assert im.format == pyvips.BandFormat.UCHAR

        p = im(0, 0)
        assert p[0] == 0.0
        p = im(255, 0)
        assert p[0] == 255.0
        p = im(128, 0)
        assert p[0] == 128.0

        im = pyvips.Image.identity(ushort=True)
        assert im.width == 65536
        assert im.height == 1
        assert im.bands == 1

def test_hist_cum(self):
        im = pyvips.Image.identity()

        sum = im.avg() * 256

        cum = im.hist_cum()

        p = cum(255, 0)
        assert p[0] == sum

def test_hist_plot(self):
        im = pyvips.Image.identity()
        im2 = im.hist_plot()

        assert im2.width == 256
        assert im2.height == 256
        assert im2.format == pyvips.BandFormat.UCHAR
        assert im2.bands == 1

def test_hist_plot(self):
        im = pyvips.Image.identity()
        im2 = im.hist_plot()

        self.assertEqual(im2.width, 256)
        self.assertEqual(im2.height, 256)
        self.assertEqual(im2.format, pyvips.BandFormat.UCHAR)
        self.assertEqual(im2.bands, 1)

def test_identity(self):
        im = pyvips.Image.identity()
        assert im.width == 256
        assert im.height == 1
        assert im.bands == 1
        assert im.format == pyvips.BandFormat.UCHAR

        p = im(0, 0)
        assert p[0] == 0.0
        p = im(255, 0)
        assert p[0] == 255.0
        p = im(128, 0)
        assert p[0] == 128.0

        im = pyvips.Image.identity(ushort=True)
        assert im.width == 65536
        assert im.height == 1
        assert im.bands == 1
        assert im.format == pyvips.BandFormat.USHORT

        p = im(0, 0)
        assert p[0] == 0
        p = im(99, 0)
        assert p[0] == 99
        p = im(65535, 0)
        assert p[0] == 65535

def test_hist_map(self):
        im = pyvips.Image.identity()

        im2 = im.maplut(im)

        self.assertEqual((im - im2).abs().max(), 0.0)

def avgze(image):
    # since we use histograms
    if image.format != "uchar" and image.format != "ushort":
        raise Exception("uchar and ushort images only")

    # take the histogram, and set the count for 0 pixels to 0, removing them
    histze = image.hist_find().insert(pyvips.Image.black(1, 1), 0, 0)

    # number of non-zero pixels in each band
    nnz = [histze[i].avg() * histze.width * histze.height
           for i in range(histze.bands)]

    # multiply by the identity function and we get the sum of non-zero
    # pixels ... for 16-bit images, we need a larger identity
    # function
    totalze = histze * pyvips.Image.identity(ushort=histze.width > 256)

    # find average value in each band
    avgze = [totalze[i].avg() * histze.width * histze.height / nnz[i]
             for i in range(totalze.bands)]

    return avgze