I am currently interning at a place where they've asked me to make a standalone python program to do something (say X).
Now, that program is to be run by some commands sent by their proprietary software which is written in their proprietary language. Now the reason I'm saying proprietary so many times is because they aren't ready to take me anywhere near their code. I am just supposed to make a Python code that does X based on the input given by their software.
So is there a way I can make an API and wrap it around my code so as to let the software control it? Also I need to make the whole thing standalone (maybe an installer of some kind) so that they don't have to install Python and the accompanying modules (like opencv) just to run my script?
All I could get out of them was "there are dll files that will be calling your app and we want an executable"
Any programm can execute any other program (if it has the appropriate rights) so there is no real distinction between "python file" and "python executable" (that is because python is an interpreted language. The python source files and the "final python program" are "identical" (asuming cpython), in contrast to e.g. a C program where the source files and the executable are vastly different).
If you are on windows there is the additional problem that the user must have installed python to execute .py files. There are some ways to mitigate that problem - there are python libraries that "freeze" the python interpreter and your code into a single .exe file (from the comment by Bakuriu see e.g. freeze) . You could bundle the python interpreter with your code. You can just say your users to install python (if the amount of users is low that might be the good way).
"API" is just a fancy way of saying "this is how you communicate with my programm". This might be how you call a library (e.g. what functions a python module exports) or this might be an HTTP API or which command line arguments are passed or which protocoll over an TCP socket is spoken. Without knowing which API you are supposed to implement you cannot fulfill your job.
Without knowing further specifications (what inputdoes the other program give to yours, how does it call your programm?) it's very hard to say anything more helpful.