may be nice to encode those dependencies in source to make the script re-usable
without that bit of knowledge. That can be done by using `nix-shell` as a
[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)), like so:

```python
#!/usr/bin/env nix-shell
#!nix-shell -i python3 -p "python3.withPackages(ps: [ ps.numpy ])"
import numpy as np
a = np.array([1,2])
b = np.array([3,4])
print(f"The dot product of {a} and {b} is: {np.dot(a, b)}")
```

Then we execute it, without requiring any environment setup at all!

```sh
$ ./foo.py
The dot product of [1 2] and [3 4] is: 11
```

If the dependencies are not available on the host where `foo.py` is executed, it
will build or download them from a Nix binary cache prior to starting up, prior
that it is executed on a machine with a multi-user nix installation.

This provides a way to ship a self bootstrapping Python script, akin to a
statically linked binary, where it can be run on any machine (provided nix is
installed) without having to assume that `numpy` is installed globally on the
system.

By default it is pulling the import checkout of Nixpkgs itself from our nix
channel, which is nice as it cache aligns with our other package builds, but we
can make it fully reproducible by pinning the `nixpkgs` import:

```python
#!/usr/bin/env nix-shell
#!nix-shell -i python3 -p "python3.withPackages (ps: [ ps.numpy ])"
#!nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/e51209796c4262bfb8908e3d6d72302fe4e96f5f.tar.gz
import numpy as np
a = np.array([1,2])
b = np.array([3,4])
print(f"The dot product of {a} and {b} is: {np.dot(a, b)}")
```

This will execute with the exact same versions of Python 3.10, numpy, and system
dependencies a year from now as it does today, because it will always use
exactly git commit `e51209796c4262bfb8908e3d6d72302fe4e96f5f` of Nixpkgs for all
of the package versions.

This is also a great way to ensure the script executes identically on different
servers.

##### Load environment from `.nix` expression {#load-environment-from-.nix-expression}

We've now seen how to create an ad-hoc temporary shell session, and how to
create a single script with Python dependencies, but in the course of normal
development we're usually working in an entire package repository.

As explained [in the `nix-shell` section](https://nixos.org/manual/nix/stable/command-ref/nix-shell) of the Nix manual, `nix-shell` can also load an expression from a `.nix` file.
Say we want to have Python 3.12, `numpy` and `toolz`, like before,
in an environment. We can add a `shell.nix` file describing our dependencies:

```nix
with import <nixpkgs> { };
(python312.withPackages (
  ps: with ps; [
    numpy
    toolz
  ]
)).env
```

And then at the command line, just typing `nix-shell` produces the same
environment as before. In a normal project, we'll likely have many more
dependencies; this can provide a way for developers to share the environments
with each other and with CI builders.

What's happening here?

1. We begin with importing the Nix Packages collections. `import <nixpkgs>`
   imports the `<nixpkgs>` function, `{}` calls it and the `with` statement
   brings all attributes of `nixpkgs` in the local scope. These attributes form
   the main package set.
2. Then we create a Python 3.12 environment with the [`withPackages`](#python.withpackages-function) function, as before.
3. The [`withPackages`](#python.withpackages-function) function expects us to provide a function as an argument
   that takes the set of all Python packages and returns a list of packages to
   include in the environment. Here, we select the packages `numpy` and `toolz`
   from the package set.

To combine this with `mkShell` you can:

```nix
with import <nixpkgs> { };
let
  pythonEnv = python312.withPackages (ps: [
    ps.numpy
    ps.toolz
  ]);
in
mkShell {
  packages = [
    pythonEnv

    black
    mypy

    libffi
    openssl
  ];
}
```

This will create a unified environment that has not just our Python interpreter