Documentation: rewrite Python

- Rewrite current documentation
- Add introduction/tutorial
- Convert to markdown

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+# Python
+
+## User Guide
+
+Several versions of Python are available on Nix as well as a high amount of
+packages. The default interpreter is CPython 2.7.
+
+### Using Python
+
+#### Installing Python and packages
+
+It is important to make a distinction between Python packages that are
+used as libraries, and applications that are written in Python.
+
+Applications on Nix are installed typically into your user
+profile imperatively using `nix-env -i`, and on NixOS declaratively by adding the
+package name to `environment.systemPackages` in `/etc/nixos/configuration.nix`.
+Dependencies such as libraries are automatically installed and should not be
+installed explicitly.
+
+The same goes for Python applications and libraries. Python applications can be
+installed in your profile, but Python libraries you would like to use to develop
+cannot. If you do install libraries in your profile, then you will end up with
+import errors.
+
+#### Python environments using `nix-shell`
+
+The recommended method for creating Python environments for development is with
+`nix-shell`. Executing
+
+```sh
+$ nix-shell -p python35Packages.numpy python35Packages.toolz
+```
+
+opens a Nix shell which has available the requested packages and dependencies.
+Now you can launch the Python interpreter (which is itself a dependency)
+
+```sh
+[nix-shell:~] python3
+```
+
+If the packages were not available yet in the Nix store, Nix would download or
+build them automatically. A convenient option with `nix-shell` is the `--run`
+option, with which you can execute a command in the `nix-shell`. Let's say we
+want the above environment and directly run the Python interpreter
+
+```sh
+$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3"
+```
+
+This way you can use the `--run` option also to directly run a script
+
+```sh
+$ nix-shell -p python35Packages.numpy python35Packages.toolz --run "python3 myscript.py"
+```
+
+In fact, for this specific use case there is a more convenient method. You can
+add a [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) to your script
+specifying which dependencies Nix shell needs. With the following shebang, you
+can use `nix-shell myscript.py` and it will make available all dependencies and
+run the script in the `python3` shell.
+
+```py
+#! /usr/bin/env nix-shell
+#! nix-shell -i python3 -p python3Packages.numpy
+
+import numpy
+
+print(numpy.__version__)
+```
+
+Likely you do not want to type your dependencies each and every time. What you
+can do is write a simple Nix expression which sets up an environment for you,
+requiring you only to type `nix-shell`. Say we want to have Python 3.5, `numpy`
+and `toolz`, like before, in an environment. With a `shell.nix` file
+containing
+
+```nix
+with import <nixpkgs> {};
+
+(pkgs.python35.buildEnv.override  {
+  extraLibs = with pkgs.python35Packages; [ numpy toolz ];
+}).env
+```
+executing `nix-shell` gives you again a Nix shell from which you can run Python.
+
+What's happening here?
+
+1. We begin with importing the Nix Packages collections. `import <nixpkgs>` import the `<nixpkgs>` function, `{}` calls it and the `with` statement brings all attributes of `nixpkgs` in the local scope. Therefore we can now use `pkgs`.
+2. Then we create a Python 3.5 environment with `pkgs.buildEnv`. Because we want to use it with a custom set of Python packages, we override it.
+3. The `extraLibs` argument of the original `buildEnv` function can be used to specify which packages should be included. We want `numpy` and `toolz`. Again, we use the `with` statement to bring a set of attributes into the local scope.
+4. And finally, for in interactive use we return the environment.
+
+### Developing with Python
+
+
+Now that you know how to get a working Python environment on Nix, it is time to go forward and start actually developing with Python.
+We will first have a look at how Python packages are packaged on Nix. Then, we will look how you can use development mode with your code.
+
+#### Python packaging on Nix
+
+On Nix all packages are built by functions. The main function in Nix for building Python packages is [`buildPythonPackage`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/python-modules/generic/default.nix).
+Let's see how we would build the `toolz` package. According to [`python-packages.nix`](https://raw.githubusercontent.com/NixOS/nixpkgs/master/pkgs/top-level/python-packages.nix) `toolz` is build using
+
+```nix
+toolz = buildPythonPackage rec{
+  name = "toolz-${version}";
+  version = "0.7.4";
+
+  src = pkgs.fetchurl{
+    url = "https://pypi.python.org/packages/source/t/toolz/toolz-${version}.tar.gz";
+    sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
+  };
+
+  meta = {
+    homepage = "http://github.com/pytoolz/toolz/";
+    description = "List processing tools and functional utilities";
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ fridh ];
+  };
+};
+```
+
+What happens here? The function `buildPythonPackage` is called and as argument
+it accepts a set. In this case the set is a recursive set ([`rec`](http://nixos.org/nix/manual/#sec-constructs)).
+One of the arguments is the name of the package, which consists of a basename
+(generally following the name on PyPi) and a version. Another argument, `src`
+specifies the source, which in this case is fetched from an url. `fetchurl` not
+only downloads the target file, but also validates its hash. Furthermore, we
+specify some (optional) [meta information](http://nixos.org/nixpkgs/manual/#chap-meta).
+
+The output of the function is a derivation, which is an attribute with the name
+`toolz` of the set `pythonPackages`. Actually, sets are created for all interpreter versions,
+so `python27Packages`, `python34Packages`, `python35Packages` and `pypyPackages`.
+
+The above example works when you're directly working on
+`pkgs/top-level/python-packages.nix` in the Nixpkgs repository. Often though,
+you will want to test a Nix expression outside of the Nixpkgs tree. If you
+create a `shell.nix` file with the following contents
+
+```nix
+with import <nixpkgs> {};
+
+pkgs.python35Packages.buildPythonPackage rec {
+  name = "toolz-${version}";
+  version = "0.7.4";
+
+  src = pkgs.fetchurl{
+    url = "https://pypi.python.org/packages/source/t/toolz/toolz-${version}.tar.gz";
+    sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
+  };
+
+  meta = {
+    homepage = "http://github.com/pytoolz/toolz/";
+    description = "List processing tools and functional utilities";
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ fridh ];
+  };
+}
+```
+
+and then execute `nix-shell` will result in an environment in which you can use
+Python 3.5 and the `toolz` package. As you can see we had to explicitly mention
+for which Python version we want to build a package.
+
+The above example considered only a single package. Generally you will want to use multiple packages.
+If we create a `shell.nix` file with the following contents
+
+```nix
+with import <nixpkgs> {};
+
+( let
+    toolz = pkgs.python35Packages.buildPythonPackage rec {
+      name = "toolz-${version}";
+      version = "0.7.4";
+
+      src = pkgs.fetchurl{
+        url = "https://pypi.python.org/packages/source/t/toolz/toolz-${version}.tar.gz";
+        sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
+      };
+
+      meta = {
+        homepage = "http://github.com/pytoolz/toolz/";
+        description = "List processing tools and functional utilities";
+        license = licenses.bsd3;
+        maintainers = with maintainers; [ fridh ];
+      };
+    };
+
+  in pkgs.python35.buildEnv.override rec {
+
+    extraLibs = [ pkgs.python35Packages.numpy toolz ];
+}
+).env
+```
+
+and again execute `nix-shell`, then we get a Python 3.5 environment with our
+locally defined package as well as `numpy` which is build according to the
+definition in Nixpkgs. 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`. To introduce our own package in the scope of
+`buildEnv.override` we used a
+[`let`](http://nixos.org/nix/manual/#sec-constructs) expression.
+
+### Handling dependencies
+
+Our example, `toolz`, doesn't have any dependencies on other Python
+packages or system libraries. According to the manual,  `buildPythonPackage`
+uses the arguments `buildInputs` and `propagatedBuildInputs` to specify dependencies. If something is
+exclusively a build-time dependency, then the dependency should be included as a
+`buildInput`, but if it is (also) a runtime dependency, then it should be added
+to `propagatedBuildInputs`. Test dependencies are considered build-time dependencies.
+
+The following example shows which arguments are given to `buildPythonPackage` in
+order to build [`datashape`](https://github.com/blaze/datashape).
+
+```nix
+datashape = buildPythonPackage rec {
+  name = "datashape-${version}";
+  version = "0.4.7";
+
+  src = pkgs.fetchurl {
+    url = "https://pypi.python.org/packages/source/D/DataShape/${name}.tar.gz";
+    sha256 = "14b2ef766d4c9652ab813182e866f493475e65e558bed0822e38bf07bba1a278";
+  };
+
+  buildInputs = with self; [ pytest ];
+  propagatedBuildInputs = with self; [ numpy multipledispatch dateutil ];
+
+  meta = {
+    homepage = https://github.com/ContinuumIO/datashape;
+    description = "A data description language";
+    license = licenses.bsd2;
+    maintainers = with maintainers; [ fridh ];
+  };
+};
+```
+
+We can see several runtime dependencies, `numpy`, `multipledispatch`, and
+`dateutil`. Furthermore, we have one `buildInput`, i.e. `pytest`. `pytest` is a
+test runner and is only used during the `checkPhase` and is therefore not added
+to `propagatedBuildInputs`.
+
+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`.
+
+```nix
+lxml = buildPythonPackage rec {
+  name = "lxml-3.4.4";
+
+  src = pkgs.fetchurl {
+    url = "http://pypi.python.org/packages/source/l/lxml/${name}.tar.gz";
+    sha256 = "16a0fa97hym9ysdk3rmqz32xdjqmy4w34ld3rm3jf5viqjx65lxk";
+  };
+
+  buildInputs = with self; [ pkgs.libxml2 pkgs.libxslt ];
+
+  meta = {
+    description = "Pythonic binding for the libxml2 and libxslt libraries";
+    homepage = http://lxml.de;
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ sjourdois ];
+  };
+};
+```
+
+In this example `lxml` and Nix are able to work out exactly where the relevant
+files of the dependencies are. This is not always the case.
+
+The example below shows bindings to The Fastest Fourier Transform in the West, commonly known as
+FFTW. On Nix we have separate packages of FFTW for the different types of floats
+(`"single"`, `"double"`, `"long-double"`). The bindings need all three types,
+and therefore we add all three as `buildInputs`. The bindings don't expect to
+find each of them in a different folder, and therefore we have to set `LDFLAGS`
+and `CFLAGS`.
+
+```nix
+pyfftw = buildPythonPackage rec {
+  name = "pyfftw-${version}";
+  version = "0.9.2";
+
+  src = pkgs.fetchurl {
+    url = "https://pypi.python.org/packages/source/p/pyFFTW/pyFFTW-${version}.tar.gz";
+    sha256 = "f6bbb6afa93085409ab24885a1a3cdb8909f095a142f4d49e346f2bd1b789074";
+  };
+
+  buildInputs = [ pkgs.fftw pkgs.fftwFloat pkgs.fftwLongDouble];
+
+  propagatedBuildInputs = with self; [ numpy scipy ];
+
+  # Tests cannot import pyfftw. pyfftw works fine though.
+  doCheck = false;
+
+  LDFLAGS="-L${pkgs.fftw}/lib -L${pkgs.fftwFloat}/lib -L${pkgs.fftwLongDouble}/lib"
+  CFLAGS="-I${pkgs.fftw}/include -I${pkgs.fftwFloat}/include -I${pkgs.fftwLongDouble}/include"
+  '';
+
+  meta = {
+    description = "A pythonic wrapper around FFTW, the FFT library, presenting a unified interface for all the supported transforms";
+    homepage = http://hgomersall.github.com/pyFFTW/;
+    license = with licenses; [ bsd2 bsd3 ];
+    maintainer = with maintainers; [ fridh ];
+  };
+};
+```
+Note also the line `doCheck = false;`, we explicitly disabled running the test-suite.
+
+
+#### Develop local package
+
+As a Python developer you're likely aware of [development mode](http://pythonhosted.org/setuptools/setuptools.html#development-mode) (`python setup.py develop`);
+instead of installing the package this command creates a special link to the project code.
+That way, you can run updated code without having to reinstall after each and every change you make.
+Development mode is also available on Nix as [explained](http://nixos.org/nixpkgs/manual/#ssec-python-development) in the Nixpkgs manual.
+Let's see how you can use it.
+
+In the previous Nix expression the source was fetched from an url. We can also refer to a local source instead using
+
+```nix
+src = ./path/to/source/tree;
+```
+
+If we create a `shell.nix` file which calls `buildPythonPackage`, and if `src`
+is a local source, and if the local source has a `setup.py`, then development
+mode is activated.
+
+In the following example we create a simple environment that
+has a Python 3.5 version of our package in it, as well as its dependencies and
+other packages we like to have in the environment, all specified with `propagatedBuildInputs`.
+Indeed, we can just add any package we like to have in our environment to `propagatedBuildInputs`.
+
+```nix
+with import <nixpkgs>;
+with pkgs.python35Packages;
+
+buildPythonPackage rec {
+  name = "mypackage";
+  src = ./path/to/package/source;
+  propagatedBuildInputs = [ pytest numpy pkgs.libsndfile ];
+};
+```
+
+It is important to note that due to how development mode is implemented on Nix it is not possible to have multiple packages simultaneously in development mode.
+
+
+### Organising your packages
+
+So far we discussed how you can use Python on Nix, and how you can develop with
+it. We've looked at how you write expressions to package Python packages, and we
+looked at how you can create environments in which specified packages are
+available.
+
+At some point you'll likely have multiple packages which you would
+like to be able to use in different projects. In order to minimise unnecessary
+duplication we now look at how you can maintain yourself a repository with your
+own packages. The important functions here are `import` and `callPackage`.
+
+### Including a derivation using `callPackage`
+
+Earlier we created a Python environment using `buildEnv`, and included the
+`toolz` package via a `let` expression.
+Let's split the package definition from the environment definition.
+
+We first create a function that builds `toolz` in `~/path/to/toolz/release.nix`
+
+```nix
+{ pkgs, buildPythonPackage }:
+
+buildPythonPackage rec {
+  name = "toolz-${version}";
+  version = "0.7.4";
+
+  src = pkgs.fetchurl{
+    url = "https://pypi.python.org/packages/source/t/toolz/toolz-${version}.tar.gz";
+    sha256 = "43c2c9e5e7a16b6c88ba3088a9bfc82f7db8e13378be7c78d6c14a5f8ed05afd";
+  };
+
+  meta = {
+    homepage = "http://github.com/pytoolz/toolz/";
+    description = "List processing tools and functional utilities";
+    license = licenses.bsd3;
+    maintainers = with maintainers; [ fridh ];
+  };
+};
+```
+
+It takes two arguments, `pkgs` and `buildPythonPackage`.
+We now call this function using `callPackage` in the definition of our environment
+
+```nix
+with import <nixpkgs> {};
+
+( let
+    toolz = pkgs.callPackage ~/path/to/toolz/release.nix { pkgs=pkgs; buildPythonPackage=pkgs.python35Packages.buildPythonPackage; };
+  in pkgs.python35.buildEnv.override rec {
+    extraLibs = [ pkgs.python35Packages.numpy  toolz ];
+}
+).env
+```
+
+Important to remember is that the Python version for which the package is made
+depends on the `python` derivation that is passed to `buildPythonPackage`. Nix
+tries to automatically pass arguments when possible, which is why generally you
+don't explicitly define which `python` derivation should be used. In the above
+example we use `buildPythonPackage` that is part of the set `python35Packages`,
+and in this case the `python35` interpreter is automatically used.
+
+
+
+## Reference
+
+### Interpreters
+
+Versions 2.6, 2.7, 3.3, 3.4 and 3.5 of the CPython interpreter are available on
+Nix and are available as `python26`, `python27`, `python33`, `python34` and
+`python35`. The PyPy interpreter is also available as `pypy`. Currently, the
+aliases `python` and `python3` correspond to respectively `python27` and
+`python35`. The Nix expressions for the interpreters can be found in
+`pkgs/development/interpreters/python`.
+
+
+#### Missing modules standard library
+
+The interpreters `python26` and `python27` do not include modules that
+require external dependencies. This is done in order to reduce the closure size.
+The following modules need to be added as `buildInput` explicitly:
+
+* `python.modules.bsddb`
+* `python.modules.curses`
+* `python.modules.curses_panel`
+* `python.modules.crypt`
+* `python.modules.gdbm`
+* `python.modules.sqlite3`
+* `python.modules.tkinter`
+* `python.modules.readline`
+
+For convenience `python27Full` and `python26Full` are provided with all
+modules included.
+
+All packages depending on any Python interpreter get appended
+`out/{python.sitePackages}` to `$PYTHONPATH` if such directory
+exists.
+
+#### Attributes on interpreters packages
+
+Each interpreter has the following attributes:
+
+- `libPrefix`. Name of the folder in `${python}/lib/` for corresponding interpreter.
+- `interpreter`. Alias for `${python}/bin/${executable}`.
+- `buildEnv`. Function to build python interpreter environments with extra packages bundled together. See section *python.buildEnv function* for usage and documentation.
+- `sitePackages`. Alias for `lib/${libPrefix}/site-packages`.
+- `executable`. Name of the interpreter executable, ie `python3.4`.
+
+### Building packages and applications
+
+Python packages (libraries) and applications that use `setuptools` or
+`distutils` are typically built with respectively the `buildPythonPackage` and
+`buildPythonApplication` functions.
+
+All Python packages reside in `pkgs/top-level/python-packages.nix` and all
+applications elsewhere. Some packages are also defined in
+`pkgs/development/python-modules`. It is important that these packages are
+called in `pkgs/top-level/python-packages.nix` and not elsewhere, to guarantee
+the right version of the package is built.
+
+Based on the packages defined in `pkgs/top-level/python-packages.nix` an
+attribute set is created for each available Python interpreter. The available
+sets are
+
+* `pkgs.python26Packages`
+* `pkgs.python27Packages`
+* `pkgs.python33Packages`
+* `pkgs.python34Packages`
+* `pkgs.python35Packages`
+* `pkgs.pypyPackages`
+
+and the aliases
+
+* `pkgs.pythonPackages` pointing to `pkgs.python27Packages`
+* `pkgs.python3Packages` pointing to `pkgs.python35Packages`
+
+#### `buildPythonPackage` function
+
+The `buildPythonPackage` function is implemented in
+`pkgs/development/python-modules/generic/default.nix`
+
+and can be used as:
+
+    twisted = buildPythonPackage {
+      name = "twisted-8.1.0";
+
+      src = pkgs.fetchurl {
+        url = http://tmrc.mit.edu/mirror/twisted/Twisted/8.1/Twisted-8.1.0.tar.bz2;
+        sha256 = "0q25zbr4xzknaghha72mq57kh53qw1bf8csgp63pm9sfi72qhirl";
+      };
+
+      propagatedBuildInputs = [ self.ZopeInterface ];
+
+      meta = {
+        homepage = http://twistedmatrix.com/;
+        description = "Twisted, an event-driven networking engine written in Python";
+        license = stdenv.lib.licenses.mit; };
+      };
+
+The `buildPythonPackage` mainly does four things:
+
+* In the `buildPhase`, it calls `${python.interpreter} setup.py bdist_wheel` to build a wheel binary zipfile.
+* In the `installPhase`, it installs the wheel file using `pip install *.whl`.
+* In the `postFixup` phase, the `wrapPythonPrograms` bash function is called to wrap all programs in the `$out/bin/*` directory to include `$PYTHONPATH` and `$PATH` environment variables.
+* In the `installCheck` phase, `${python.interpreter} setup.py test` is ran.
+
+As in Perl, dependencies on other Python packages can be specified in the
+`buildInputs` and `propagatedBuildInputs` attributes.  If something is
+exclusively a build-time dependency, use `buildInputs`; if it’s (also) a runtime
+dependency, use `propagatedBuildInputs`.
+
+By default tests are run because `doCheck = true`. Test dependencies, like
+e.g. the test runner, should be added to `buildInputs`.
+
+By default `meta.platforms` is set to the same value
+as the interpreter unless overriden otherwise.
+
+##### `buildPythonPackage` parameters
+
+All parameters from `mkDerivation` function are still supported.
+
+* `namePrefix`: Prepended text to `${name}` parameter. Defaults to `"python3.3-"` for Python 3.3, etc. Set it to `""` if you're packaging an application or a command line tool.
+* `disabled`: If `true`, package is not build for particular python interpreter version. Grep around `pkgs/top-level/python-packages.nix` for examples.
+* `setupPyBuildFlags`: List of flags passed to `setup.py build_ext` command.
+* `pythonPath`: List of packages to be added into `$PYTHONPATH`. Packages in `pythonPath` are not propagated (contrary to `propagatedBuildInputs`).
+* `preShellHook`: Hook to execute commands before `shellHook`.
+* `postShellHook`: Hook to execute commands after `shellHook`.
+* `makeWrapperArgs`: A list of strings. Arguments to be passed to `makeWrapper`, which wraps generated binaries. By default, the arguments to `makeWrapper` set `PATH` and `PYTHONPATH` environment variables before calling the binary. Additional arguments here can allow a developer to set environment variables which will be available when the binary is run. For example, `makeWrapperArgs = ["--set FOO BAR" "--set BAZ QUX"]`.
+* `installFlags`: A list of strings. Arguments to be passed to `pip install`. To pass options to `python setup.py install`, use `--install-option`. E.g., `installFlags=["--install-option='--cpp_implementation'"].
+
+#### `buildPythonApplication` function
+
+The `buildPythonApplication` function is practically the same as `buildPythonPackage`.
+The difference is that `buildPythonPackage` by default prefixes the names of the packages with the version of the interpreter.
+Because with an application we're not interested in multiple version the prefix is dropped.
+
+#### python.buildEnv function
+
+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`
+
+    with import {};
+
+    python.buildEnv.override {
+      extraLibs = [ pkgs.pythonPackages.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`
+
+    with import {};
+
+    (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
+
+* `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`).
+
+### Development mode
+
+Development or editable mode is supported. To develop Python packages
+`buildPythonPackage` has additional logic inside `shellPhase` to run `pip
+install -e . --prefix $TMPDIR/`for the package.
+
+Warning: `shellPhase` is executed only if `setup.py` exists.
+
+Given a `default.nix`:
+
+    with import {};
+
+    buildPythonPackage { name = "myproject";
+
+    buildInputs = with pkgs.pythonPackages; [ pyramid ];
+
+    src = ./.; }
+
+Running `nix-shell` with no arguments should give you
+the environment in which the package would be build with
+`nix-build`.
+
+Shortcut to setup environments with C headers/libraries and python packages:
+
+    $ nix-shell -p pythonPackages.pyramid zlib libjpeg git
+
+Note: There is a boolean value `lib.inNixShell` set to `true` if nix-shell is invoked.
+
+### Tools
+
+Packages inside nixpkgs are written by hand. However many tools exist in
+community to help save time. No tool is preferred at the moment.
+
+- [python2nix](https://github.com/proger/python2nix) by Vladimir Kirillov
+- [pypi2nix](https://github.com/garbas/pypi2nix) by Rok Garbas
+- [pypi2nix](https://github.com/offlinehacker/pypi2nix) by Jaka Hudoklin
+
+## FAQ
+
+### How to solve circular dependencies?
+
+Consider the packages `A` and `B` that depend on each other. When packaging `B`,
+a solution is to override package `A` not to depend on `B` as an input. The same
+should also be done when packaging `A`.
+
+### How to override a Python package?
+
+Recursively updating a package can be done with `pkgs.overridePackages` as explained in the Nixpkgs manual.
+Python attribute sets are created for each interpreter version. We will therefore override the attribute set for the interpreter version we're interested.
+In the following example we change the name of the package `pandas` to `foo`.
+```
+newpkgs = pkgs.overridePackages(self: super: rec {
+  python35Packages = super.python35Packages.override {
+    self = python35Packages // { pandas = python35Packages.pandas.override{name="foo";};};
+  };
+});
+```
+This can be tested with
+```
+with import <nixpkgs> {};
+
+(let
+
+newpkgs = pkgs.overridePackages(self: super: rec {
+  python35Packages = super.python35Packages.override {
+    self = python35Packages // { pandas = python35Packages.pandas.override{name="foo";};};
+  };
+});
+in newpkgs.python35.buildEnv.override{
+  extraLibs = [newpkgs.python35Packages.blaze ];
+}).env
+```
+A typical use case is to switch to another version of a certain package. For example, in the Nixpkgs repository we have multiple versions of `django` and `scipy`.
+In the following example we use a different version of `scipy`. All packages in `newpkgs` will now use the updated `scipy` version.
+```
+with import <nixpkgs> {};
+
+(let
+
+newpkgs = pkgs.overridePackages(self: super: rec {
+  python35Packages = super.python35Packages.override {
+    self = python35Packages // { scipy = python35Packages.scipy_0_16;};
+  };
+});
+in pkgs.python35.buildEnv.override{
+  extraLibs = [newpkgs.python35Packages.blaze ];
+}).env
+```
+The requested package `blaze` depends upon `pandas` which itself depends on `scipy`.
+
+
+### `install_data` / `data_files` problems
+
+If you get the following error:
+
+    could not create '/nix/store/6l1bvljpy8gazlsw2aw9skwwp4pmvyxw-python-2.7.8/etc':
+    Permission denied
+
+This is a [known bug](https://bitbucket.org/pypa/setuptools/issue/130/install_data-doesnt-respect-prefix) in setuptools.
+Setuptools `install_data` does not respect `--prefix`. An example of such package using the feature is `pkgs/tools/X11/xpra/default.nix`.
+As workaround install it as an extra `preInstall` step:
+
+    ${python.interpreter} setup.py install_data --install-dir=$out --root=$out
+    sed -i '/ = data\_files/d' setup.py
+
+
+###  Rationale of non-existent global site-packages
+
+On most operating systems a global `site-packages` is maintained. This however
+becomes problematic if you want to run multiple Python versions or have multiple
+versions of certain libraries for your projects. Generally, you would solve such
+issues by creating virtual environments using `virtualenv`.
+
+On Nix each package has an isolated dependency tree which, in the case of
+Python, guarantees the right versions of the interpreter and libraries or
+packages are available. There is therefore no need to maintain a global `site-packages`.
+
+If you want to create a Python environment for development, then the recommended
+method is to use `nix-shell`, either with or without the `python.buildEnv`
+function.
+
+
+## Contributing
+
+### Contributing guidelines
+
+Following rules are desired to be respected:
+
+* Make sure package builds for all python interpreters. Use `disabled` argument to `buildPythonPackage` to set unsupported interpreters.
+* If tests need to be disabled for a package, make sure you leave a comment about reasoning.
+* Packages in `pkgs/top-level/python-packages.nix` are sorted quasi-alphabetically to avoid merge conflicts.
+* Python libraries are supposed to be in `python-packages.nix` and packaged with `buildPythonPackage`. Python applications live outside of `python-packages.nix` and are packaged with `buildPythonApplication`.