Python detect paragraphs with opencv

detect_para.py

import cv2
import numpy as np

def para_detect(file_name):
    img = cv2.imread(file_name)

    img_final = cv2.imread(file_name)
    img2gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    ret, mask = cv2.threshold(img2gray, 180, 255, cv2.THRESH_BINARY)
    image_final = cv2.bitwise_and(img2gray, img2gray, mask=mask)
    ret, new_img = cv2.threshold(image_final, 180, 255, cv2.THRESH_BINARY_INV)  # for black text , cv.THRESH_BINARY_INV
    '''
            line  8 to 12  : Remove noisy portion 
    '''
    kernel = cv2.getStructuringElement(cv2.MORPH_CROSS, (10,
                                                         3))  # to manipulate the orientation of dilution , large x means horizonatally dilating  more, large y means vertically dilating more
    dilated = cv2.dilate(new_img, kernel, iterations=5)  # dilate , more the iteration more the dilation
    output = cv2.bitwise_not(dilated, dilated)

    ais = []
    for contour in contours:
        # get rectangle bounding contour
        [x, y, w, h] = cv2.boundingRect(contour)
        a = w * h
        ais.append(a)
 
    ais.sort(reverse=True)
    for contour in contours:
        [x, y, w, h] = cv2.boundingRect(contour)
        # Don't plot small false positives that aren't text
        #if w < 2050 and h < 450:
        #    continue
        if w*h not in ais[:2]:
            continue
#    cv2.imshow('w', img)
#    cv2.waitKey()
    # write original image with added contours to disk
    cv2.imwrite('output4.png', img)

file_name = 'n.png'
para_detect(file_name)

contours is not defined.