> pip doesn't resolve dependencies of dependencies.

This is simply incorrect. In fact the reason it gets stuck on resolution sometimes is exactly because it resolved transitive dependencies and found that they were mutually incompatible.

Here's an example which will also help illustrate the rest of my reply. I make a venv for Python 3.8, and set up a new project with a deliberately poorly-thought-out pyproject.toml:

[project]
name="example"
version="0.1.0"
dependencies=["pandas==2.0.3", "numpy==1.17.3"]

I've specified the oldest version of Numpy that has a manylinux wheel for Python 3.8 and the newest version of Pandas similarly. These are both acceptable for the venv separately, but mutually incompatible on purpose.

When I try to `pip install -e .` in the venv, Pip happily explains (granted the first line is a bit strange):

ERROR: Cannot install example and example==0.1.0 because these package versions have conflicting dependencies.

The conflict is caused by:
example 0.1.0 depends on numpy==1.17.3
pandas 2.0.3 depends on numpy>=1.20.3; python_version < "3.10"

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip to attempt to solve the dependency conflict

If I change the Numpy pin to 1.20.3, that's the version that gets installed. (`python-dateutil`, `pytz`, `six` and `tzdata` are also installed.) If I remove the Numpy requirement completely and start over, Numpy 1.24.4 is installed instead - the latest version compatible with Pandas' transitive specification of the dependency. Similarly if I unpin Pandas and ask for any version - Pip will try to install the latest version it can, and it turns out that the latest Pandas version that declares compatibility with 3.8, indeed allows for fetching 3.8-compatible dependencies. (Good job not breaking it, Pandas maintainers! Although usually this is trivial, because your dependencies are also actively maintained.)

> pip will only respect version pinning for dependencies you explicitly specify. So for example, say I am using pandas and I pin it to version X. If a dependency of pandas (say, numpy) isn't pinned as well, the underlying version of numpy can still change when I reinstall dependencies.

Well, sure; Pip can't respect a version pin that doesn't exist anywhere in your project. If the specific version of Pandas you want says that it's okay with a range of Numpy versions, then of course Pip has freedom to choose one of those versions. If that matters, you explicitly specify it. Other programs like uv can't fix this. They can only choose different resolution strategies, such as "don't update the transitive dependency if the environment already contains a compatible version", versus "try to use the most recent versions of everything that meet the specified compatibility requirements".

> To get around this with pip you would need an additional tool like pip-tools, which allows you to pin all dependencies, explicit and nested, to a lock file for true reproducibility.

No, you just use Pip's options to determine what's already in the environment (`pip list`, `pip freeze` etc.) and pin everything that needs pinning (whether with a Pip requirements file or with `pyproject.toml`). Nothing prevents you from listing your transitive dependencies in e.g. the [project.dependencies] of your pyproject.toml, and if you pin them, Pip will take that constraint into consideration. Lock files are for when you need to care about alternate package sources, checking hashes etc.; or for when you want an explicit representation of your dependency graph in metadata for the sake of other tooling.

> This assumes the Python version you need is available from your package manager's repo. This won't work if you want a Python version either newer or older than what is available.

I have built versions 3.5 through 3.13 inclusive from source and have them installed in /opt and the binaries symlinked in /usr/local/bin. It's not difficult at all.

> True, but it's not best practice to do that because while the tool gets installed globally, it is not necessarily linked to a specific python version, and so it's extremely brittle.

What brittleness are you talking about? There's no reason why the tool needs to run in the same environment as the code it's operating on. You can install it in its own virtual environment, too. Since tools generally are applications, I use Pipx for this (which really just wraps a bit of environment management around Pip). It works great; for example I always have the standard build-frontend `build` (as `pyproject-build`) and the uploader `twine` available. They run from a guaranteed-compatible Python.

And they would if they were installed for the system Python, too. (I just, you know, don't want to do that because the system Python is the system package manager's responsibility.) The separate environment don't matter because the tool's code and the operated-on project's code don't even need to run at the same time, let alone in the same process. In fact, it would make no sense to be running the code while actively trying to build or upload it.

> And it gets even more complex if you need different tools that have different Python version requirements.

No, you just let each tool have the virtual environment it requires. And you can update them in-place in those environments, too.