I am creating a python system that needs to handle many files. Each of the file has more than 10 thousand lines of text data. 
Because DB (like mysql) can not be used in that environment, when file is uploaded by a user, I think I will save all the data of the uploaded file in in-memory-SQLite so that I can use SQL to fetch specific data from there. 
Then, when all operations by program are finished, save the processed data in a file. This is the file users will receive from the system.
But some websites say SQLite shouldn't be used in production. But in my case, I just save them temporarily in memory to use SQL for the data. Is there any problem for using SQLite in production even in this scenario?
Edit:
The data in in-memory-DB doesn't need to be shared between processes. It just creates tables, process data, then discard all data and tables after saving the processed data in file. I just think saving everything in list makes search difficult and slow. So using SQLite is still a problem?
I'm not familiar with the specific context of your system, but if what you're looking for is a SQL database that is 

light
Access is from a single process and a single thread. 
If the system crashes in the middle, you have a good way to recover from it (either backing up the last stable version of the database or just create it from scratch). 

If you meet all these criteria, using SQLite is production is fine. OSX, for example, uses sqlite for a few purposes (e.g. ./var/db/auth.db).
in-memory sqlite in production with python