blit_buffer problem

gistfile1.txt

def change_image(instance, photo, image_slider):
    """open selected image, resize and blit into texture"""
    global currentTexture
    try:
        img = PILImage.open(instance.selection[0])
    except: pass
    img = img.resize((180,240), PILImage.BICUBIC)
    buf = img.tostring()
    with open('buffer.txt', 'w') as f:
        f.write(buf)
    currentTexture = Texture.create(size = (180,240))
    currentTexture.blit_buffer(buf, colorfmt='rgb', bufferfmt= 'ubyte')
    currentTexture.flip_vertical()
    texture1 = currentTexture.get_region(0,0,180,180)
    photo.texture = texture1
    return True