[zahlman]

The hard part is figuring out what "merge" means for your use case. If there's a definite set of packages that should be in the environment that are all already at definite locations on the local drive, there are many possible approaches (copying, `.pth` files, hard links, and symlinks should also work) to stitching together the venv you want. But you can't just feed the individual package paths to Python, because `sys.path` entries are places that Python will look for the top-level package folders (and top-level module `.py` files - which explains why), not the paths to individual importable things.

More importantly, at runtime you can only have one version of a given package, because the imports are resolved at runtime. Pip won't put multiple versions of the same library into the same environment normally; you can possibly force it to (or more likely, explicitly do it yourself) but then everyone that wants that library will find whichever version gets `import`ed and cached first, which will generally be whichever one is found first on `sys.path` when the first `import` statement is reached at runtime. (Yes, the problem is the same if you only have one venv in the first place, in the sense that your dependency graph could be unsolvable. But naively merging venvs could mean not noticing the problem until, at runtime, something tries to import something else, gets an incompatible version, and fails.)