How to compare PDFs using Python

Problem:How do you compare two PDF files programmatically using Python?

Adobe makes it easy to compare the changes in two PDF files. However as testers, we sometimes need to compare a lot of PDF files (especially reports!) against some preset baselines. In these cases, it helps to have a script that can compare PDF files and tell you if they differ in any way.

There are several options. We like DiffPDF, pdf2text, the pdf-diff python module. Each option comes with its own set of pros and cons. Most of these solutions do a good job of comparing the text in the PDF files. However we noticed that they are somewhat lacking when it comes to comparing graphs and charts. In this post, we show you one more approach which is useful if you have a lot of graphs and charts in your PDF files.

WARNING: You should be using this kind of automated check as a last resort. Ask your developers for other ways to check the data/content of the PDF files before using this approach.


Steps involved

We will be using image comparison to verify if the two PDF files are identical or not. To do so, we need to:
1. Get setup with ImageMagick and Ghostscript
2. Convert each page of the PDF file into one image
3. Compare corresponding images and save the resulting difference image for every page
4. Stitch all the resulting difference images into a single PDF file
5. Use the utility to compare two PDF files

I have created a class PDF_Image_Compare which can be used to compare two PDFs. The class will help you compare two PDF files, list out which pages differ and give you a overlaid images of the two PDF files. Below few steps will explain the different methods and modules which are required to compare two PDF files.


Step 1. Get setup with ImageMagick and Ghostscript
The first step is to convert the PDF file to a different format like jpg. We will use ImageMagick, which in turn uses Ghostscript. To do this you need to:
a. Download and install ImageMagick which is a software suite to create, edit, compose, or convert bitmap images
b. ImageMagick needs Ghostscript which is an interpreter for the PostScript language and for PDF.
c. Add both ImageMagick and GhostScript to your path environment variable.
d. Verify you are setup correctly by using the “convert” utility. Open a command prompt and run the command ‘convert file.pdf file.jpg’ to convert file.pdf into a file.jpg.


Step 2. Convert each page of the PDF file into one image
We plan to use the difference method in Imagechops module which returns the absolute value of the difference between the two images. However we can’t use it directly on a PDF file. So we first have to convert the PDF into a list of images. To do so, we will call convert from Python using the subprocess module.

    def get_image_list_from_pdf(self,pdf_file):
        "Return a list of images that resulted from running convert on a given pdf"
        pdf_name = pdf_file.split(os.sep)[-1].split('.pdf')[0]
        pdf_dir = pdf_file.split(pdf_name)[0]
        jpg = pdf_file.split('.pdf')[0]+'.jpg'
        # Convert the pdf file to jpg file
        self.call_convert(pdf_file,jpg)
        #Get all the jpg files after calling convert and store it in a list
        image_list = []        
        file_list = os.listdir(pdf_dir)
        for f in file_list:
            if f[-4:]=='.jpg' and pdf_name in f:
                #Make sure the file names of both pdf are not similar
                image_list.append(f)
 
        print('Total of %d jpgs produced after converting the pdf file: %s'%(len(image_list),pdf_file))
        return image_list
 
 
    def call_convert(self,src,dest):
        "Call convert to convert pdf to jpg"
        print('About to call convert on %s'%src)
        try:
            subprocess.check_call(["convert",src,dest], shell=True)
        except Exception,e:
            print('Convert exception ... could be an ImageMagick bug')
            print(e)
        print('Finished calling convert on %s'%src)

Step 3. Compare corresponding images and save the resulting difference image for every page
Now we can use the ImageChops.difference() method to compare the images from the list of images created.
The below method would help you achieve this. It will return result_flag as True if the images match and False if they do not. The method will also print out the image pairs that differ.

def create_diff_image(self,pdf1_list,pdf2_list,diff_image_dir):
        "Creates the diffed images in diff image directory and generates a pdf by calling call convert"
        result_flag = True
        for pdf1_img,pdf2_img in zip(pdf1_list,pdf2_list):
            diff_filename = diff_image_dir + os.sep+'diff_' + pdf2_img
            try:
                pdf2_image = Image.open(self.download_dir +os.sep+ pdf2_img)
                pdf1_image = Image.open(self.download_dir+os.sep + pdf1_img)
                diff = ImageChops.difference(pdf2_image,pdf1_image)
                diff.save(diff_filename)
 
                if (ImageChops.difference(pdf2_image,pdf1_image).getbbox() is None):
                    result_flag = result_flag & True
                else:
                    result_flag = result_flag & False
                    print ('The file didnt match for: \n>>%s\nand\n>>%s'%(self.download_dir +os.sep+ pdf2_img,self.download_dir +os.sep+ pdf1_img))
            except Exception,e:
                print('Error when trying to open image')
                result_flag = result_flag & False
 
        return result_flag

Step 4. Stitch all the resulting difference images into a single PDF file
We found it useful to present the final difference as one PDF file rather than as a series of images. This makes it easier for the human interpreting the results to quickly identify and summarize the differences. To do so, add the below code to the create_diff_image method above.

        #Create a pdf out of all the jpgs created
        diff_pdf_name = 'diff_'+pdf2_img.split('.jpg')[0]+'.pdf'
        self.call_convert(diff_image_dir+os.sep+'*.jpg', self.download_dir+os.sep+diff_pdf_name)
 
        if os.path.exists(diff_pdf_name):
            print('Successfully created the difference pdf: %s'%(diff_pdf_name))

Step 5. Use the utility to compare two PDF files

 
if __name__== '__main__':
    #Lets accept command line options for the location of two PDF files from the user 
    #We have chosen to use the Python module optparse 
    usage = "usage: %prog --f1 <pdf1> --f2 <pdf2>\nE.g.: %prog --f1 'D:\Image Compare\Sample.pdf' --f2 'D:\Image Compare\Test.pdf'\n---"
    parser = OptionParser(usage=usage)
    parser.add_option("--f1","--pdf1",dest="pdf1",help="The location of pdf file1",default=None)
    parser.add_option("--f2","--pdf2",dest="pdf2",help="The location of pdf file2",default=None)
    (options,args) = parser.parse_args()
 
    test_obj = PDF_Image_Compare(pdf1=options.pdf1,pdf2=options.pdf2)
    result_flag = test_obj.get_pdf_diff()
    if result_flag == True:
        print ('The two PDF matched properly')
    else:
        print ('The PDFs didnt match properly, check the diff file generated')

Putting it all together

Here is how our utility to compare two PDFs look.

from PIL import Image, ImageChops
import os,time,PythonMagick,subprocess,shutil
from optparse import OptionParser
 
class PDF_Image_Compare:    
    "Compare's two pdf files"
    def __init__(self,pdf1,pdf2):
        "Constructor: Initialises file1 and file 2"
        self.download_dir = os.getcwd()
        self.pdf1 = pdf1
        self.pdf2 = pdf2
 
 
    def get_image_list_from_pdf(self,pdf_file):
        "Return a list of images that resulted from running convert on a given pdf"
        pdf_name = pdf_file.split(os.sep)[-1].split('.pdf')[0]
        pdf_dir = pdf_file.split(pdf_name)[0]
        jpg = pdf_file.split('.pdf')[0]+'.jpg'
        # Convert the pdf file to jpg file
        self.call_convert(pdf_file,jpg)
        #Get all the jpg files after calling convert and store it in a list
        image_list = []        
        file_list = os.listdir(pdf_dir)
        for f in file_list:
            if f[-4:]=='.jpg' and pdf_name in f:
                #Make sure the file names of both pdf are not similar
                image_list.append(f)
 
        print('Total of %d jpgs produced after converting the pdf file: %s'%(len(image_list),pdf_file))
        return image_list
 
 
    def call_convert(self,src,dest):
        "Call convert to convert pdf to jpg"
        print('About to call convert on %s'%src)
        try:
            subprocess.check_call(["convert",src,dest], shell=True)
        except Exception,e:
            print('Convert exception ... could be an ImageMagick bug')
            print(e)
        print('Finished calling convert on %s'%src)
 
 
    def create_diff_image(self,pdf1_list,pdf2_list,diff_image_dir):
        "Creates the diffed images in diff image directory and generates a pdf by calling call convert"
        result_flag = True
        for pdf1_img,pdf2_img in zip(pdf1_list,pdf2_list):
            diff_filename = diff_image_dir + os.sep+'diff_' + pdf2_img
            try:
                pdf2_image = Image.open(self.download_dir +os.sep+ pdf2_img)
                pdf1_image = Image.open(self.download_dir+os.sep + pdf1_img)
                diff = ImageChops.difference(pdf2_image,pdf1_image)
                diff.save(diff_filename)
 
                if (ImageChops.difference(pdf2_image,pdf1_image).getbbox() is None):
                    result_flag = result_flag & True
                else:
                    result_flag = result_flag & False
                    print ('The file didnt match for: \n>>%s\nand\n>>%s'%(self.download_dir +os.sep+ pdf2_img,self.download_dir +os.sep+ pdf1_img))
            except Exception,e:
                print('Error when trying to open image')
                result_flag = result_flag & False
        #Create a pdf out of all the jpgs created
        diff_pdf_name = 'diff_'+pdf2_img.split('.jpg')[0]+'.pdf'
        self.call_convert(diff_image_dir+os.sep+'*.jpg', self.download_dir+os.sep+diff_pdf_name)
 
        if os.path.exists(diff_pdf_name):
            print('Successfully created the difference pdf: %s'%(diff_pdf_name))
 
        return result_flag
 
 
    def cleanup(self,diff_image_dir,pdf1_list,pdf2_list):
        "Clean up all the image files created"
        print('Cleaning up all the intermediate jpg files created when comparing the pdf')
        for pdf1_img,pdf2_img in zip(pdf1_list,pdf2_list):
            try:
                os.remove(self.download_dir +os.sep+ pdf1_img)
                os.remove(self.download_dir +os.sep+ pdf2_img)
            except Exception,e:
                print('Unable to delete jpg file')
                print(e)
        print('Nuking the temporary image_diff directory')
        try:
            time.sleep(5)
            shutil.rmtree(diff_image_dir)
        except Exception,e:
            print('Could not delete the image_diff directory')
            print(e)
 
 
    def get_pdf_diff(self,cleanup=True):
        "Create a difference pdf by overlaying the two pdfs and generating an image difference.Returns True if the file matches else returns false"
 
        #Get the list of images using get_image_list_from_pdf which inturn calls convert on a given pdf  
        pdf1_list = self.get_image_list_from_pdf(self.pdf1)
        pdf2_list = self.get_image_list_from_pdf(self.pdf2)
 
        #If diff directory already does exist - delete it 
        #Easier to simply nuke the folder and create it again than to check if its empty
        diff_image_dir = self.download_dir + os.sep+'diff_images'
        if os.path.exists(diff_image_dir):
            print('diff_images directory exists ... about to nuke it')
            shutil.rmtree(diff_image_dir)
 
        #Create a new and empty diff directory
        os.mkdir(diff_image_dir)
        print('diff_images directory created')
        print('Total pages in pdf2: %d'%len(pdf2_list))
        print('Total pages in pdf1 : %d'%len(pdf1_list))
 
        #Verify that there are equal number pages in pdf1 and pdf2
        if len(pdf2_list)==len(pdf1_list) and len(pdf2_list) !=0:
            print('Check SUCCEEDED: There are an equal number of jpgs created from the pdf generated from pdf2 and pdf1')
            print('Total pages in images: %d'%len(pdf2_list))
            pdf1_list.sort()
            pdf2_list.sort()
 
            #Create the diffed images
            result_flag = self.create_diff_image(pdf1_list,pdf2_list,diff_image_dir)
        else:
            print('Check FAILED: There are an unequal number of jpgs created from the pdf generated from pdf2 and pdf1')
            print('Total pages in image2 : %d'%len(pdf2_list))
            print('Total pages in image1: %d'%len(pdf1_list))
            print('ERROR: Skipping image comparison between %s and %s'%(self.pdf1,self.pdf2))
 
        if cleanup:
            #Delete all the image files created
            self.cleanup(diff_image_dir,pdf1_list,pdf2_list)            
 
        return result_flag
 
if __name__== '__main__':
    #Lets accept command line options for the location of two PDF files from the user 
    #We have chosen to use the Python module optparse 
    usage = "usage: %prog --f1 <pdf1> --f2 <pdf2>\nE.g.: %prog --f1 'D:\Image Compare\Sample.pdf' --f2 'D:\Image Compare\Test.pdf'\n---"
    parser = OptionParser(usage=usage)
    parser.add_option("--f1","--pdf1",dest="pdf1",help="The location of pdf file1",default=None)
    parser.add_option("--f2","--pdf2",dest="pdf2",help="The location of pdf file2",default=None)
    (options,args) = parser.parse_args()
 
    test_obj = PDF_Image_Compare(pdf1=options.pdf1,pdf2=options.pdf2)
    result_flag = test_obj.get_pdf_diff()
    if result_flag == True:
        print ('The two PDF matched properly')
    else:
        print ('The PDFs didnt match properly, check the diff file generated')

Run the utility file using command prompt

You can use the utility file any way you want. Below screenshot shows you how to compare two PDF files using command prompt by passing the location of the PDF files.

Running the Test


We have used this utility at a couple of our clients. We feel it fills a specific need (comparing image heavy PDF files) pretty well. Hope this post helps you do the same.

52 thoughts on “How to compare PDFs using Python

  1. C:\Users\sneha\subpython>python demo1.py
    Traceback (most recent call last):
    File “C:\Users\sneha\subpython\demo1.py”, line 121, in
    result_flag = test_obj.get_pdf_diff()
    File “C:\Users\sneha\subpython\demo1.py”, line 79, in get_pdf_diff
    pdf1_list = self.get_image_list_from_pdf(self.pdf1)
    File “C:\Users\sneha\subpython\demo1.py”, line 12, in get_image_list_from_pdf
    pdf_name = pdf_file.split(os.sep)[-1].split(‘.pdf’)[0]
    AttributeError: ‘NoneType’ object has no attribute ‘split’
    …….Any solution for these errors

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