### How to consume Python modules using pip in a virtual environment like I am used to on other Operating Systems? {#how-to-consume-python-modules-using-pip-in-a-virtual-environment-like-i-am-used-to-on-other-operating-systems}

While this approach is not very idiomatic from Nix perspective, it can still be
useful when dealing with pre-existing projects or in situations where it's not
feasible or desired to write derivations for all required dependencies.

This is an example of a `default.nix` for a `nix-shell`, which allows to consume
a virtual environment created by `venv`, and install Python modules through
`pip` the traditional way.

Create this `default.nix` file, together with a `requirements.txt` and
execute `nix-shell`.

```nix
with import <nixpkgs> { };

let
  pythonPackages = python3Packages;
in
pkgs.mkShell rec {
  name = "impurePythonEnv";
  venvDir = "./.venv";
  buildInputs = [
    # A Python interpreter including the 'venv' module is required to bootstrap
    # the environment.
    pythonPackages.python

    # This executes some shell code to initialize a venv in $venvDir before
    # dropping into the shell
    pythonPackages.venvShellHook

    # Those are dependencies that we would like to use from nixpkgs, which will
    # add them to PYTHONPATH and thus make them accessible from within the venv.
    pythonPackages.numpy
    pythonPackages.requests

    # In this particular example, in order to compile any binary extensions they may
    # require, the Python modules listed in the hypothetical requirements.txt need
    # the following packages to be installed locally:
    taglib
    openssl
    git
    libxml2
    libxslt
    libzip
    zlib
  ];

  # Run this command, only after creating the virtual environment
  postVenvCreation = ''
    unset SOURCE_DATE_EPOCH
    pip install -r requirements.txt
  '';

  # Now we can execute any commands within the virtual environment.
  # This is optional and can be left out to run pip manually.
  postShellHook = ''
    # allow pip to install wheels
    unset SOURCE_DATE_EPOCH
  '';

}
```

In case the supplied venvShellHook is insufficient, or when Python 2 support is
needed, you can define your own shell hook and adapt to your needs like in the
following example:

```nix
with import <nixpkgs> { };

let
  venvDir = "./.venv";
  pythonPackages = python3Packages;
in
pkgs.mkShell rec {
  name = "impurePythonEnv";
  buildInputs = [
    pythonPackages.python
    # Needed when using python 2.7
    # pythonPackages.virtualenv
    # ...
  ];

  # This is very close to how venvShellHook is implemented, but
  # adapted to use 'virtualenv'
  shellHook = ''
    SOURCE_DATE_EPOCH=$(date +%s)

    if [ -d "${venvDir}" ]; then
      echo "Skipping venv creation, '${venvDir}' already exists"
    else
      echo "Creating new venv environment in path: '${venvDir}'"
      # Note that the module venv was only introduced in python 3, so for 2.7
      # this needs to be replaced with a call to virtualenv
      ${pythonPackages.python.interpreter} -m venv "${venvDir}"
    fi

    # Under some circumstances it might be necessary to add your virtual
    # environment to PYTHONPATH, which you can do here too;
    # PYTHONPATH=$PWD/${venvDir}/${pythonPackages.python.sitePackages}/:$PYTHONPATH

    source "${venvDir}/bin/activate"

    # As in the previous example, this is optional.
    pip install -r requirements.txt
  '';
}
```

Note that the `pip install` is an imperative action. So every time `nix-shell`
is executed it will attempt to download the Python modules listed in
requirements.txt. However these will be cached locally within the `virtualenv`
folder and not downloaded again.

### How to override a Python package from `configuration.nix`? {#how-to-override-a-python-package-from-configuration.nix}

If you need to change a package's attribute(s) from `configuration.nix` you could do:

```nix
{
  nixpkgs.config.packageOverrides = super: {
    python3 = super.python3.override {
      packageOverrides = python-self: python-super: {