Edited Question:
I guess I worded my previous question improperly, I actually want to get away from "unit tests" and create automated, modular system tests that build off of each other to test the application as whole.  Many parts are dependent upon the previous pages and subsequent pages cannot be reached without first performing the necessary steps on the previous pages.  
For example (and I am sorry I cannot give the actual code), I want to sign into an app, then insert some data, then show that the data was sent successfully.  It is more involved than that, however, I would like to make the web driver portion, 'Module 1.x'. Then the sign in portion, 'Module 2.x'.  The data portion, 'Module 3.x'.  Finally, success portion, 'Module 4.x'.  I was hoping to achieve this so that I could eventually say, "ok, for this test, I need it to be a bit more complicated so let's do, IE (ie. Module 1.4), sign in (ie. Module 2.1), add a name (ie Module 3.1), add an address (ie. Module 3.2), add a phone number (ie Module 3.3), then check for success (ie Module 4.1).  So, I need all of these strung together.  (This is extremely simplified and just an example of what I need to occur.  Even in the case of the unit tests, I am unable to simply skip to a page to check that the elements are present without completing the required prerequisite information.) The issue that I am running into with the lengthy tests that I have created is that each one requires multiple edits when something is changed and then multiplied by the number of drivers, in this case Chrome, IE, Edge and Firefox (a factor of 4).  Maybe my approach is totally wrong but this is new ground for me, so any advice is much appreciated. Thank you again for your help!
Previous Question:
I have found many answers for creating unit tests, however, I am unable to find any advice on how to make said tests sequential.  
I really want to make modular tests that can be reused when the same action is being performed repeatedly.  I have tried various ways to achieve this but I have been unsuccessful.  Currently I have several lengthy tests that reuse much of the same code in each test, but I have to adjust each one individually with any new changes.  
So, I really would like to have .py files that only contain a few lines of code for the specific task that I am trying to complete, while re-using the same browser instance that is already open and on the page where the previous portion of the test left off.  Hoping to achieve this by 'calling' the smaller/modular test files.  
Any help and/or examples are greatly appreciated.  Thank you for your time and assistance with this issue.
Respectfully, 
Billiamaire
You don't really want your tests to be sequential. That breaks one of the core rules of unit tests where they should be able to be run in any order.
You haven't posted any code so it's hard to know what to suggest but if you aren't using the page object model, I would suggest that you start. There are a lot of resources on the web for this but the basics are that you create a single class per page or widget. That class would hold all the code and locators that pertains to that page. This will help with the modular aspect of what you are seeking because in your script you just instantiate the page object and then consume the API. The details of interacting with the page, the logic, etc. all lives in the page object is exposed via the API it provides.
Changes/updates are easy. If the login page changes, you edit the page object for the login page and you're done. If the page objects are properly implemented and the changes to the page aren't severe, many times you won't need to change the scripts at all.
A simple example would be the login page. In the login class for that page, you would have a login() method that takes username and password. The login() method would handle entering the username and password into the appropriate fields and clicking the sign in button, etc.
Is it possible to make sequential tests in Python-Selenium tests?