[Python] How to obtain a jpeg resolution in python 3, without any alien libraries?

I'm a rookie in python programming. I can't find an answer, how to obtain jpeg resolution in python version 3. without using any external libraries. I've tried this code, but result is unpredictable. And I don't understand why..

def jpeg_res(filename):

# open image for reading in binary mode
with open(filename,'rb') as img_file:

# height of image (in 2 bytes) is at 164th position
img_file.seek(163)

# read the 2 bytes
a = img_file.read(2)

# calculate height
height = (a[0] << 8) + a[1]

# next 2 bytes is width
a = img_file.read(2)

# calculate width
width = (a[0] << 8) + a[1]

print("The resolution of the image is",width,"x",height)

jpeg_res("img1.jpg")


I'm using python version 3.5 working on MS Windows 7 (64 bit)

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