Demystifying Python

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This is a guest post from Franco A. Alvarado is a freelance ebook developer and a project manager at a Boston-based academic publisher. 


Maybe you’ve seen this script before:

import re 
i = 0 
def replace(m): 
    global i
    i += 1
    return str(i) 
file = open('toc.ncx', 'rb').read() 
file = re.sub(r'(?<=<navPoint id="navpoint)(\d+)', replace, file)
i = 0
file = re.sub(r'(?<= playOrder=")(\d+)', replace, file)
open('toc.ncx', 'wb').write(file)

It might also be the case that you’ve never actually opened up the file (called, on my computer at least, renumber.py), but you’ve used it. If you haven’t used it, this renumbers your NCX. Ancient, I know, but perhaps you’ve wondered what else Python can do.

Python is a fairly straightforward programming language (which incidentally helped me wrap my mind around Javascript). Basically, you execute a code and it does something. I use Terminal on my Mac. You just enter in python script.py and you’ve done something. Hopefully something you meant to do.

You just enter in python script.py and you’ve done something. Hopefully something you meant to do.

Specifically, I’ve used it to clean up EPUBs generated from a CMS that – long story short – did not have all the metadata that it needed to have. Rather than manually enter in all the metadata (including the ISBN and author name), drop in the cover, and update the NCX and NAV, I saved myself about an hour per title by writing this script, which takes 1 second to run. And sure that sounds like a very specific problem, but I bet you can think of things you do repetitively and, more importantly, things that you might mess up somehow. I’ve used it to automate sending emails to authors. Time-saving is really secondary to the amount of fine-grain control you get from running a script.

Time-saving is really secondary to the amount of fine-grain control you get from running a script.

Let’s walk through how to run renumber.py because a lot of people know it, it’s very short, and the other one is about 120 lines long. (But it is available on Github).

So, first make a folder. (Well, really first, install Python). Put your offending misnumbered NCX into this folder. Open your favorite text editor (I recommend SublimeText 3) and make a file called renumber.py.

I’ve rewritten the script again below, with some comments.

import re 
    # Import the regular expressions module. This lets you use the familiar regex syntax
i = 0 
    # Set up a variable called i and assign it a value of 0. This is a number.
    # Below we will define a function
def replace(m): 
    global i
    # Call that i variable we defined above
    i += 1
    # Add one to it
    return str(i) 
    # And return that new number as a string

The difference between a number 1 and a string called “1” is that one is a number and can add and subtract and multiply and all that whereas “1” acts as text rather than a functional number.

file = open('toc.ncx', 'rb').read() 
    # this creates another variable called file and assigns it the value of that ncx file in the folder.

file = re.sub(r'(?<=<navPoint id="navpoint)(\d+)', replace, file)
    # this redefines that variable as a regular expression search and replace.

So this part above is really key. The regex here is searching for a navpoint with one or more digits: (?<=<navPoint id="navpoint)(\d+). The replacement text, however, is (and this is key) the function that we defined earlier. This is not like normal regex where the replacing text is just another string of letters and \1, \2, \3.

With our i starting at 0, i will now equal i + 1 = i, which is 0 + 1 = i, which is i = 1. So, that is our first replacement. Then, with 1 as our new number, when the regex continues to the next digit (\d+), it will replace that with the function again, which is i + 1 = i, or 1 + 1 = 2. And on and on.

For the rest of it, we just do the same for the playOrder numbers, overwrite the file, and save it.

i = 0
file = re.sub(r'(?<= playOrder=")(\d+)', replace, file)
open('toc.ncx', 'wb').write(file)

And that is it! This might be a bit broad and I don’t know this script as well since I did not write it, but I hope that demystifies Python a bit and encourages some of you to explore how it works.


Header image: https://www.flickr.com/photos/ejorpin/6763569379/in/photostream/

4 Responses to “Demystifying Python”

  1. Štěpán Böhm says:

    Thanks for that post. I must try it because some years ago I try to write something similar – I unzip epub, do some regex changes, but I was unable to zip epub back again.

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