For background, I am somewhat of a self-taught Python developer with only some formal training with a few CS courses in school. 
In my job right now, I am working on a Python program that will automatically parse information from a very large text file (thousands of lines) that's a output result of a simulation software. I would like to be doing test driven development (TDD) but I am having a hard time understanding how to write proper unit tests. 
My trouble is, the output of some of my functions (units) are massive data structures that are parsed versions of the text file. I could go through and create those outputs manually and then test but it would take a lot of time. The whole point of a parser is to save time and create structured outputs. Only testing I've been doing so far is trial and error manually which is also cumbersome. 
So my question is, are there more intuitive ways to create tests for parsers? 
Thank you in advance for any help!
Usually parsers are tested using a regression testing system. You create sample input sets and verify that the output is correct. Then you put the input and output in libraries. Each time you modify the code, you run the regression test system over the library to see if anything changes.
How to write unit tests for text parser?