How can I install some nix packages?

Question

I know how to install some nix packages with nix-env but there are some of them I have no clue how to install,

for example:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/python-modules/pygit2/default.nix

How to install that package?

How can I know the full name of the package just looking at the .nix file?**

Answer 1

How to install that package?

nix-env -iA nixos.python3Packages.pygit2. You can replace python3Packages with your python version of choice, e.g. python36Packages instead.

How can I know the full name of the package just looking at the .nix file?

You just cannot.
The reason is that the attribute path of the .nix file is determined by how it is referenced from <nixpkgs>/pkgs/all-packages/top-level.nix. In this case, the top-level.nix file makes all python packages accessible through the pythonPackages or python3packages attribute. The packages themselves are listed in <nixpkgs>/pkgs/all-packages/python-packages.nix where each entry now finally calls the nix expression file of the package, like the one you linked to above.

This makes the full attribute path of the package pkgs.python3Packages.pygit2. A great tool to browse around the nixkpgs attribute set is nix repl "<nixpkgs/nixos>", which drops you into a nix interpreter with tab-completion. When using nix-env for imperative software installation, you still have to substite pkgs by nixos in the package's attribute path. I recommend you looking into declarative software management instead anyways, as there you can use the normal attribute path of the package.