I want to know why you are giving this error in the variable?

import numpy as np      # conda install numpy
import cv2 as cv        # conda install opencv
import matplotlib.pyplot as plt
#from PIL import Image  # para rodar no console
def showfig(image, ucmap):
    imgplot=plt.imshow(image, ucmap)

    img = cv.imread("imgs/pfig01.jpg",0)
hf = cv.calcHist([img],[0],None,[256],[0,256])

plt.figure(figsize=(13,5)) 
plt.subplot(121),plt.imshow(img,"gray"),plt.title('Input')
plt.subplot(122),plt.plot(hf),plt.title('Hist')
plt.show()

You're giving this error

NameError                                 Traceback (most recent call last)
<ipython-input-15-03db2af2cf65> in <module>()
      7 
      8     img = cv.imread("imgs/pfig01.jpg",0)
----> 9 hf = cv.calcHist([img],[0],None,[256],[0,256])
     10 
     11 plt.figure(figsize=(13,5))

NameError: name 'img' is not defined