Add python tutorial (flatpak#102)

Author: cpba

docs/python.rst

@@ -1,2 +1,80 @@
-Python
-======
+Building python modules
+-----------------------
+
+Python applications that use supported buildsystems such as meson,
+cmake, or autotools can be built in the same way as regular c
+applications. However, many python apps and dependencies use custom
+install scripts or are expected to be installed through setuptools and
+pip.
+
+For these cases, ``flatpak-builder`` provides the ``simple``
+buildsystem. Rather than trying to automate the process like the other
+buildsystems, ``simple`` takes a ``build-commands`` array of strings and
+only executes the commands given there.
+
+For example, this makes building the popular requests module rather
+straightforward:
+
+::
+
+    {
+      "name": "requests",
+      "buildsystem": "simple",
+      "build-commands": [
+        "pip3 install --prefix=/app --no-deps ."
+      ],
+      "sources": [
+        {
+          "type": "archive",
+          "url": "https://files.pythonhosted.org/packages/source/r/requests/requests-2.18.4.tar.gz",
+          "sha256": "9c443e7324ba5b85070c4a818ade28bfabedf16ea10206da1132edaa6dda237e"
+        }
+      ]
+    }
+
+``name`` is the name of the module, in practice this also means the name
+folder in which your module will be built.
+
+``build-commands`` is an array with the commands you need to build and
+install your module. In this case we are running pip to do it, but the
+parameters are important. ``--prefix=/app`` is necessary, because
+otherwise pip would try to install it under ``/usr/`` and (because
+``/usr/`` is mounted read-only inside the sandbox) it would fail.
+
+``--no-deps`` is also relevant, flatpak-builder downloads all
+``sources`` before it starts building, and doesn't allow network access
+during once a build starts. This means that any attempts to download any
+further dependencies past that point would fail. It is used here for
+illustrative purposes, but it can be detrimental because it will make
+pip quiet about real errors. If you must install multiple dependencies,
+it is better to do it in one go using the method in the next section.
+
+Building multiple python dependencies
+-------------------------------------
+
+You'll notice that, even though it installs fine, it doesn't actually
+work. This is because requests has a number of dependencies that we've
+neglected to install:
+
+-  certifi
+-  chardet
+-  idna
+-  urllib3
+
+Four dependencies aren't too much work, we could install all these using
+the same method and it would work just fine. However, anything more
+complex than this would quickly become tedious.
+
+For these cases, we can use
+`flatpak-pip-generator <https://github.com/flatpak/flatpak-builder-tools/tree/master/pip>`_,
+this is a python script that takes a package name and uses pip to track
+its dependencies, tarball urls and hashes.
+
+Using it is as simple as:
+
+::
+
+    $ python3 flatpak-pip-generator requests
+
+This will output a file called ``python3-requests.json`` containing json
+data that can be directly included among your manifest's modules.