Python environments can be created using the low-level `pkgs.buildEnv` function.
This example shows how to create an environment that has the Pyramid Web Framework.
Saving the following as `default.nix`

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

python3.buildEnv.override {
  extraLibs = [ python3Packages.pyramid ];
  ignoreCollisions = true;
}
```

and running `nix-build` will create

```
/nix/store/cf1xhjwzmdki7fasgr4kz6di72ykicl5-python-2.7.8-env
```

with wrapped binaries in `bin/`.

You can also use the `env` attribute to create local environments with needed
packages installed. This is somewhat comparable to `virtualenv`. For example,
running `nix-shell` with the following `shell.nix`

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

(python3.buildEnv.override {
  extraLibs = with python3Packages; [
    numpy
    requests
  ];
}).env
```

will drop you into a shell where Python will have the
specified packages in its path.

##### `python.buildEnv` arguments {#python.buildenv-arguments}


* `extraLibs`: List of packages installed inside the environment.
* `postBuild`: Shell command executed after the build of environment.
* `ignoreCollisions`: Ignore file collisions inside the environment (default is `false`).
* `permitUserSite`: Skip setting the `PYTHONNOUSERSITE` environment variable in
  wrapped binaries in the environment.

#### `python.withPackages` function {#python.withpackages-function}

The [`python.withPackages`](#python.withpackages-function) function provides a simpler interface to the [`python.buildEnv`](#python.buildenv-function) functionality.
It takes a function as an argument that is passed the set of python packages and returns the list
of the packages to be included in the environment. Using the [`withPackages`](#python.withpackages-function) function, the previous
example for the Pyramid Web Framework environment can be written like this:

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

python.withPackages (ps: [ ps.pyramid ])
```

[`withPackages`](#python.withpackages-function) passes the correct package set for the specific interpreter
version as an argument to the function. In the above example, `ps` equals
`pythonPackages`. But you can also easily switch to using python3:

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

python3.withPackages (ps: [ ps.pyramid ])
```

Now, `ps` is set to `python3Packages`, matching the version of the interpreter.

As [`python.withPackages`](#python.withpackages-function) uses [`python.buildEnv`](#python.buildenv-function) under the hood, it also
supports the `env` attribute. The `shell.nix` file from the previous section can
thus be also written like this:

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

(python3.withPackages (
  ps: with ps; [
    numpy
    requests
  ]
)).env
```

In contrast to [`python.buildEnv`](#python.buildenv-function), [`python.withPackages`](#python.withpackages-function) does not support the
more advanced options such as `ignoreCollisions = true` or `postBuild`. If you
need them, you have to use [`python.buildEnv`](#python.buildenv-function).

Python 2 namespace packages may provide `__init__.py` that collide. In that case
[`python.buildEnv`](#python.buildenv-function) should be used with `ignoreCollisions = true`.

#### Setup hooks {#setup-hooks}

The following are setup hooks specifically for Python packages. Most of these
are used in [`buildPythonPackage`](#buildpythonpackage-function).

- `eggUnpackhook` to move an egg to the correct folder so it can be installed
  with the `eggInstallHook`
- `eggBuildHook` to skip building for eggs.
- `eggInstallHook` to install eggs.
- `pypaBuildHook` to build a wheel using
  [`pypa/build`](https://pypa-build.readthedocs.io/en/latest/index.html) and
  PEP 517/518. Note a build system (e.g. `setuptools` or `flit`) should still
  be added as `build-system`.
- `pypaInstallHook` to install wheels.
- `pytestCheckHook` to run tests with `pytest`. See [example usage](#using-pytestcheckhook).
- `pythonCatchConflictsHook` to fail if the package depends on two different versions of the same dependency.