I think that will end up installing the subdependency version of whatever is last in the requirements.txt. You need a dependency resolver to deal with problems with conflicting versions.
pip handles the simple cases: if you install a new pkgA that depends on 'pkgB<3', it installs the latest appropriate version of that, e.g. 'pkgB==2.5.6'. This works even if you already installed 'pkgB==3.0.2', it will uninstall that first. The problem is if some 'pkgC' depends on 'pkgB>=3'. You probably want for pip (or similar) to figure out that an older version of 'pkgC' is compatible with 'pkgB>2'.
But I actually don't want it to be too smart. Better to keep your dependencies minimal and explicit, and manually specify older 'pkgC' if you need to. I have a few non-trivial services in production, the most complex one with 16 total dependencies + sub-dependencies. That is quite manageable.
So, I strongly recommend manually curating the most appropriate versions of the few tastefully chosen dependencies you really need. Then, pip+venv can easily reproduce that exact set of dependencies anytime. I also do something very similar to this with C applications, and Go. Sub-dependencies should be a big factor in how you choose your direct dependencies.
The problem with doing things this way is that you’re not going to know there’s a problem until there’s an issue in your tests (hopefully) or production. You’ll eventually install something new, it’ll update some subdependencies to a version that another library doesn’t support, and then things get broken. Pip-tools is easy to use and it tells you there’s a problem before it’s too late.
But I actually don't want it to be too smart. Better to keep your dependencies minimal and explicit, and manually specify older 'pkgC' if you need to. I have a few non-trivial services in production, the most complex one with 16 total dependencies + sub-dependencies. That is quite manageable.
So, I strongly recommend manually curating the most appropriate versions of the few tastefully chosen dependencies you really need. Then, pip+venv can easily reproduce that exact set of dependencies anytime. I also do something very similar to this with C applications, and Go. Sub-dependencies should be a big factor in how you choose your direct dependencies.