Recently as part of my professional service activities, I was tasked to ask a substantial number of people to register in an online database with relevant information. Getting people to do such things can tack some coaxing to reach completion. That's well known. However, the time consuming part of the task is matching the lists to see who hasn't completed registering. This can be a substantial task as I would have to manually search for each of the 70 members in what is a rather long list (in the thousands). Each search isn't that long, but nobody in my team wanted to do all of the searches. It seemed rather barbaric that it couldn't be automated. So, after a bit of work, I found that it took precisely 4 lines of code in Python.

import pandas
list1 = pandas.read_csv('list1.csv') # my member database. I care
             # that they have registered on list2
list2 = pandas.read_csv('list2.csv') # list of everyone who has
             # registered. This includes people not in my member
             # database. I don't need those people in my output
delinquent = list1[~list1['Email'].isin(list2['Email'])] # all data
             # of people in my member list who have not signed up
delinquent(np.array(['Phone', 'Last Name', 'First Name'])) # print
             # the useful information

As you can anticipate, a lot is happening in a very small space. However, it's pretty easy if you parse is step by step. Admittedly, the last line would be simpler if I did it in multiple steps. On the other hand, at least I didn't execute all of this in one line of code.

Step 0

I simply put the name of my file inside the file so have it. Take my block and make a text file with the name You can use another name, but you need to adapt my later instructions.

Step 1

We start with the line:

import pandas

This imports pandas, the Python Data Analysis Library. This enables us to import datafiles in comma separated value format very easily, as illustrated in the next two lines.

Step 2

list1 = pandas.read_csv('list1.csv')

imports the list of members of my group. I want to know who in this list is not in the second list. Further, I'd like to get all of the information about them. I could pull only partial information. I'll explain that in Step 5.

Note that I either have to execute in the same directory (folder) as the files list1.csv and list2.csv, or I need to insert paths to these files. On my computer, the complete path (including the filename) is /Users/jslater/Downloads/list1.csv. You either need to figure this out, or just run the script in the same folder.

Don't forget, you either have to name your file list1.csv or modify the code with your filename.

Step 3

I need to get the database system everyone is registering on to export all registrants to a CSV file. This is on their end to put their data in a file for you. Hopefully they have a method for you to do it on your own. If I can filter the before the export, without losing any of my people, that's fine. However, it's not necessary.

Once I do that, I can import the registrants database with

list2 = pandas.read_csv('list2.csv')

Note that we have to assure filename and path are correct/accommodated.

Step 4

We have one line left, and that's where all the work is done. Let's do this in steps, peeling from the inside out.


One of the columns is named Email. You have to adjust for what you think is an appropriate identifier, whether it's a username, social security number, phone number, etc. It needs to satisfy 3 requirements: be reliably correct, be unique, and never be absent.

The result of this snippet is to create a list of all email addresses (my chosen identifier) from my registrants list. This lists a bunch of people who I don't need to know about.


This is an operator that operates on the stuff passed to it (the stuff before the period). It's receiving a list and answering, for each member in the list, whether or not that member is in list1['email']. So,


will give me True for each member of list1 who is in list 2, and False otherwise. The result is another list, corresponding one-to-one with the members of list1. Now recall that we wanted to find out who is not registered. This is where the ~ comes in to play by inverting the results of isin.


will give me a list that has False for each member of list1 who is in list 2, and True otherwise. So, we now have a list that states when it is true that a member hasn't signed up.

Step 5

Now all I need to do is pull out of list1 the information for those people who are not registered (the result of the prior step).


For the sake of convenience, if I want to do anything more with this, I should probably assign it to a variable, such as

delinquent = list1[~list1['Email'].isin(list2['Email'])]
delinquent(np.array(['Phone', 'Last Name', 'First Name']))

which will list phone numbers for those who haven't registered. Phone needs to be the first line header for the phone numbers, along with Last Name and First Name.

If this helps you, or if it can be improved, please send me a note.


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