import numpy as np
import matplotlib.pyplot as plt
from scipy import misc
import cv2

carsample=cv2.imread('P1010001.jpg')
#cv2.imshow('original image',carsample)
gray_carsample=cv2.cvtColor(carsample, cv2.COLOR_BGR2GRAY)

#blur the image
blur=cv2.GaussianBlur(gray_carsample,(5,5),0)

#find the sobel gradient
sobelx=cv2.Sobel(blur, cv2.CV_8U, 1, 0, ksize=3)

#Otsu thresholding
ret2,th2=cv2.threshold(sobelx, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)
#cv2.imshow('otsu image', th2)

se=cv2.getStructuringElement(cv2.MORPH_RECT,(23,2))
closing=cv2.morphologyEx(th2, cv2.MORPH_CLOSE, se)
cv2.imshow("closed images",closing)


contours, hierarchy=cv2.findContours(closing, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
print len(contours)
'''
cv2.drawContours(carsample, contours, -1, (0,255,0), 3)
cv2.imshow("contoured image", carsample)
'''


'''
for i in contours:
    rect=cv2.minAreaRect(i)
    box=cv2.boxPoints(rect)
    box=np.int0(box)
    cv2.drawContours(carsample, [box], 0, (0,0,255),2)
'''
cv2.waitKey(0)
cv2.destroyAllWindows()
plt.show()



###############################
the output:
##############################
OpenCV Error: Assertion failed (_contours.kind() == _InputArray::STD_VECTOR_VECTOR && _contours.channels() == 2 && _contours.depth() == CV_32S) in findContours, file /home/kislay/Documents/Opop/opencv/modules/imgproc/src/contours.cpp, line 1708
Traceback (most recent call last):
  File "try.py", line 25, in <module>
    contours, hierarchy=cv2.findContours(closing, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
cv2.error: /home/kislay/Documents/Opop/opencv/modules/imgproc/src/contours.cpp:1708: error: (-215) _contours.kind() == _InputArray::STD_VECTOR_VECTOR && _contours.channels() == 2 && _contours.depth() == CV_32S in function findContours