Dependencies not installed - and requirements.txt

I’m trying to discover how dependencies are resolved when you have both a file and a requirements.txt file. I am using Pyramid and the application is built as a package but the requirements are not included through the file. Will CircleCI install the requirements.txt AND run or does it choose based on the things it sees? Also, if both are run, is there a way to ensure that the dependencies are installed before another step in the pipeline? For instance, ensuring that the dependencies are resolved before the database step?