Comparing Lists with Difflib

Julian, Wed 08 March 2017, Learning

beginners, bestpractices, cleancode, learning, python, pythonic, stdlib, tips

I love finding new things, especially when they end up saving you a boat load of time and effort! I was looking for a way to compare two lists as the code I had seemed quite clunky. Surely there had to be a better way out there! That's when I discovered difflib.

The Julian Way

I'll show you how I was approaching the problem first. Don't judge me!

First, the two blocks of text (made these lists up on the spot):

>>> text1 = """Julian's to-do list:
1. Be awesome.
2. Pybites.
3. Enjoy a beer."""
>>> text2 = """Bob's to-do list:
1. Be awesome!
2. PyBites.
3. Enjoy a beer."""

I then split the these blocks up into strings using splitlines(). This returns a list containing each line:

>>> text1_split = text1.splitlines()
>>> text2_split = text2.splitlines()

This is where I got stuck. I came up with a for loop that checked to see if items from the text1_split list were in the text2_split list. There's an if statement for the checking:

>>> for i in text1_split:
    if i in text2_split:
        print("'%s' is in both lists!" % (i))
        print("'%s' is NOT in both lists!" % (i))

'Julian's to-do list:' is NOT in both lists!
'1. Be awesome.' is NOT in both lists!
'2. Pybites.' is NOT in both lists!
'3. Enjoy a beer.' is in both lists!

The problem is that it doesn't tell me what's in text2_split. All it's confirming is whether the items in text1_split exist in text2_split.

The code was already getting out of hand. From here I'd need to add code to tell me what's in text2_split if there isn't a match and what the differences are. Enter difflib.

Difflib to the Rescue!

Before I show you the command, I'll just say that difflib is actually quite expansive, ie, there's a lot you can do with it. This post is just about the Differ() class.

As before, you have to split the blocks of text into a list of strings/lines:

>>> text1_split = text1.splitlines()
>>> text2_split = text2.splitlines()

I then call Differ().compare() to do the comparison. I store the result of the command in the diff variable. Finally, I print the output, joining the lines with a new line to make it readable.

>>> diff = difflib.Differ().compare(text1_split, text2_split)
>>> print('\n'.join(diff))
- Julian's to-do list:
? ^^^^^^

+ Bob's to-do list:
? ^^^

- 1. Be awesome.
?              ^

+ 1. Be awesome!
?              ^

- 2. Pybites.
?      ^

+ 2. PyBites.
?      ^

  3. Enjoy a beer.

Look familiar? If you're a Linux/Unix fan you'll have likely come across the diff command, in which case you'll have no problem reading this!

What you're seeing here is the differences between each list.

The key to reading the above:

'- ' indicates the difference is in the first list. In this case, the letters "Julian".

'+ ' indicates the difference is in the second list. In this case, the letters "Bob".

'? ' draws your attention to anything that doesn't appear in either list. It appears under every line here (except the last) because the new line we inserted wasn't originally there.

' ' (a blank space) indicates that this line is a perfect match and is in both lists. In this case, line item "3. Enjoy a beer.".

The ^ (caret) symbol appears underneath the differing characters. Note the ^ under the lower and upper case B in the second line item.


This is only the tip of the iceberg as Difflib is pretty big. In fact, we used the difflib.SequenceMatcher class in our Code Challenge 03 - PyBites blog tag analysis to look for similarities between our blog tags.

It's one of those handy stdlib modules you stumble across that can change how you code (another example we wrote about is deque). I think it's brilliant!

Read more on difflib here or use help(difflib) from the Python shell.

Keep Calm and Code in Python!

-- Julian

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