How can I create a ramdisk in Python?

Because file and directory-handling is so low-level and OS dependent, I doubt anything like what you want exists (or is even possible). Your best bet might be to implement a "virtual" file-system-like set of functions, classes, and methods that keep track of the files and directory-hierarchy created and their content.

Callables in such an emulation would need to have the same signature and return the same value(s) as their counterparts in the various Python standard built-ins and modules your application uses.

I suspect this might not be as much work as it sounds -- emulating the standard Python file-system interface -- depending on how much of it you're actually using since you wouldn't necessarily have to imitate all of it. Also, if written in Pure Python™, it would also be portable and easy to maintain and enhance.

One option might be to inject (monkey patch) modified versions of the methods used in the os module as well as the builtins open and file that write to StringIO files instead of to disk. Obviously this substitution should only occur for the module being tested;

How about PyFilesystem?

https://docs.pyfilesystem.org/en/latest/reference/memoryfs.html

https://docs.pyfilesystem.org/en/latest/reference/tempfs.html

The downside is that you have to access the filesystem with PyFilesystem API, but you can also access the real fs with PyFilesystem.