homepage = "https://github.com/pytoolz/toolz/";
        description = "List processing tools and functional utilities";
        # [...]
      };
    };

  in
  python313.withPackages (
    ps: with ps; [
      numpy
      my_toolz
    ]
  )
).env
```

Executing `nix-shell` will result in an environment in which you can use
Python 3.13 and the `toolz` package. As you can see we had to explicitly mention
for which Python version we want to build a package.

So, what did we do here? Well, we took the Nix expression that we used earlier
to build a Python environment, and said that we wanted to include our own
version of `toolz`, named `my_toolz`. To introduce our own package in the scope
of [`withPackages`](#python.withpackages-function) we used a `let` expression. You can see that we used
`ps.numpy` to select numpy from the nixpkgs package set (`ps`). We did not take
`toolz` from the Nixpkgs package set this time, but instead took our own version
that we introduced with the `let` expression.

#### Handling dependencies {#handling-dependencies}

Our example, `toolz`, does not have any dependencies on other Python packages or system libraries.
[`buildPythonPackage`](#buildpythonpackage-function) uses the the following arguments in the following circumstances:

- `dependencies` - For Python runtime dependencies.
- `build-system` - For Python build-time requirements.
- [`buildInputs`](#var-stdenv-buildInputs) - For non-Python build-time requirements.
- [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs) - For test dependencies

Dependencies can belong to multiple arguments, for example if something is both a build time requirement & a runtime dependency.

The following example shows which arguments are given to [`buildPythonPackage`](#buildpythonpackage-function) in
order to build [`datashape`](https://github.com/blaze/datashape).

```nix
{
  lib,
  buildPythonPackage,
  fetchPypi,

  # build dependencies
  setuptools,

  # dependencies
  numpy,
  multipledispatch,
  python-dateutil,

  # tests
  pytestCheckHook,
}:

buildPythonPackage rec {
  pname = "datashape";
  version = "0.4.7";
  pyproject = true;

  src = fetchPypi {
    inherit pname version;
    hash = "sha256-FLLvdm1MllKrgTGC6Gb0k0deZeVYvtCCLji/B7uhong=";
  };

  build-system = [ setuptools ];

  dependencies = [
    multipledispatch
    numpy
    python-dateutil
  ];

  nativeCheckInputs = [ pytestCheckHook ];

  meta = {
    changelog = "https://github.com/blaze/datashape/releases/tag/${version}";
    homepage = "https://github.com/ContinuumIO/datashape";
    description = "Data description language";
    license = lib.licenses.bsd2;
  };
}
```

We can see several runtime dependencies, `numpy`, `multipledispatch`, and
`python-dateutil`. Furthermore, we have [`nativeCheckInputs`](#var-stdenv-nativeCheckInputs) with `pytestCheckHook`.
`pytestCheckHook` is a test runner hook and is only used during the [`checkPhase`](#ssec-check-phase) and is
therefore not added to `dependencies`.

In the previous case we had only dependencies on other Python packages to consider.
Occasionally you have also system libraries to consider. E.g., `lxml` provides
Python bindings to `libxml2` and `libxslt`. These libraries are only required
when building the bindings and are therefore added as [`buildInputs`](#var-stdenv-buildInputs).

```nix
{
  lib,
  buildPythonPackage,
  fetchPypi,
  setuptools,
  libxml2,
  libxslt,
}:

buildPythonPackage rec {
  pname = "lxml";
  version = "3.4.4";
  pyproject = true;

  src = fetchPypi {
    inherit pname version;
    hash = "sha256-s9NiusRxFydHzaNRMjjxFcvWxfi45jGb9ql6eJJyQJk=";
  };

  build-system = [ setuptools ];

  buildInputs = [
    libxml2
    libxslt
  ];

  # tests are meant to be ran "in-place" in the same directory as src
  doCheck = false;

  pythonImportsCheck = [
    "lxml"
    "lxml.etree"
  ];

  meta = {
    changelog = "https://github.com/lxml/lxml/releases/tag/lxml-${version}";
    description = "Pythonic binding for the libxml2 and libxslt libraries";