doc/python: don't recommend all-packages.nix for Python applications

}

```

This is then added to `all-packages.nix` just as any other application would be.

```nix

{

  luigi = callPackage ../applications/networking/cluster/luigi { };

}

```

This is then added to `pkgs/by-name` just as any other application would be.

Since the package is an application, a consumer doesn't need to care about

Python versions or modules, which is why they don't go in `python3Packages`.

A distinction is made between applications and libraries, however, sometimes a

package is used as both. In this case the package is added as a library to

`python-packages.nix` and as an application to `all-packages.nix`. To reduce

`python-packages.nix` and as an application to `pkgs/by-name`. To reduce

duplication the `toPythonApplication` can be used to convert a library to an

application.

The Nix expression shall use [`buildPythonPackage`](#buildpythonpackage-function) and be called from

`python-packages.nix`. A reference shall be created from `all-packages.nix` to

`python-packages.nix`. A reference shall be created from `pkgs/by-name` to

the attribute in `python-packages.nix`, and the `toPythonApplication` shall be

applied to the reference:

```nix

{

  youtube-dl = with python3Packages; toPythonApplication youtube-dl;

}

  python3Packages,

}:

python3Packages.toPythonApplication python3Packages.youtube-dl

```

#### `toPythonModule` function {#topythonmodule-function}

In some cases, such as bindings, a package is created using

[`stdenv.mkDerivation`](#sec-using-stdenv) and added as attribute in `all-packages.nix`. The Python

[`stdenv.mkDerivation`](#sec-using-stdenv) and added as attribute in `pkgs/by-name` or in `all-packages.nix`. The Python

bindings should be made available from `python-packages.nix`. The

`toPythonModule` function takes a derivation and makes certain Python-specific

modifications.