# 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: {
        twisted = python-super.twisted.overridePythonAttrs (oldAttrs: {
          src = super.fetchPypi {
            pname = "Twisted";
            version = "19.10.0";
            hash = "sha256-c5S6fycq5yKnTz2Wnc9Zm8TvCTvDkgOHSKSQ8XJKUV0=";
            extension = "tar.bz2";
          };
        });
      };
    };
  };
}
```

`python3Packages.twisted` is now globally overridden.
All packages and also all NixOS services that reference `twisted`
(such as `services.buildbot-worker`) now use the new definition.
Note that `python-super` refers to the old package set and `python-self`
to the new, overridden version.

To modify only a Python package set instead of a whole Python derivation, use
this snippet:

```nix
{
  myPythonPackages = python3Packages.override {
    overrides = self: super: {
      twisted = <...>;
    };
  };
}
```

### How to override a Python package using overlays? {#how-to-override-a-python-package-using-overlays}

Use the following overlay template:

```nix
self: super: {
  python = super.python.override {
    packageOverrides = python-self: python-super: {
      twisted = python-super.twisted.overrideAttrs (oldAttrs: {
        src = super.fetchPypi {
          pname = "Twisted";
          version = "19.10.0";
          hash = "sha256-c5S6fycq5yKnTz2Wnc9Zm8TvCTvDkgOHSKSQ8XJKUV0=";
          extension = "tar.bz2";
        };
      });
    };
  };
}
```

### How to override a Python package for all Python versions using extensions? {#how-to-override-a-python-package-for-all-python-versions-using-extensions}

The following overlay overrides the call to [`buildPythonPackage`](#buildpythonpackage-function) for the
`foo` package for all interpreters by appending a Python extension to the
`pythonPackagesExtensions` list of extensions.

```nix
final: prev: {
  pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
    (python-final: python-prev: {
      foo = python-prev.foo.overridePythonAttrs (oldAttrs: {
        # ...
      });
    })
  ];
}
```

### How to use Intel’s MKL with numpy and scipy? {#how-to-use-intels-mkl-with-numpy-and-scipy}

MKL can be configured using an overlay. See the section "[Using overlays to
configure alternatives](#sec-overlays-alternatives-blas-lapack)".

### What inputs do `setup_requires`, `install_requires` and `tests_require` map to? {#what-inputs-do-setup_requires-install_requires-and-tests_require-map-to}

In a `setup.py` or `setup.cfg` it is common to declare dependencies:

* `setup_requires` corresponds to `build-system`