update docs for pinning Python

- clarify that runtime.txt is only for env files that don't support runtime specification
- specify default version of Python and when it changed
- more links to pinning docs

docs/source/config_files.rst

@@ -23,15 +23,14 @@ Below is a list of supported configuration files (roughly in the order of build
    :local:
    :depth: 1
 
-.. _environment-yml:
+.. _environment.yml:
 
 ``environment.yml`` - Install a Python environment
 ==================================================
 
-``environment.yml`` is the standard configuration file used by Anaconda, conda,
-and miniconda that lets you install packages in the data analytics stack (it
-primarily installs Python packages, though can be used to install a range of
-non-Python packages as well).
+``environment.yml`` is the standard configuration file used by `conda <https://conda.io>`_
+that lets you install any kind of package,
+including Python, R, and C/C++ packages.
 
 .. note::
 
@@ -41,16 +40,18 @@ non-Python packages as well).
    file.
 
 You can also specify which Python version to install in your built environment
-with ``environment.yml``. By default, ``repo2docker`` **installs
-Python 3.6** with your ``environment.yml`` unless you include the version of
-Python in the file.  ``conda`` supports Python versions 3.6, 3.5, 3.4, and 2.7.
-``repo2docker`` support is best with Python 3.6, 3.5, and 2.7.
+with ``environment.yml``. By default, ``repo2docker`` installs
+|default_python| with your ``environment.yml`` unless you include the version of
+Python in this file.  ``conda`` supports all versions of Python,
+though ``repo2docker`` support is best with Python 3.7, 3.6, 3.5 and 2.7.
 
 .. warning::
    If you include a Python version in a ``runtime.txt`` file in addition to your
    ``environment.yml``, your ``runtime.txt`` will be ignored.
 
 
+.. _requirements.txt:
+
 ``requirements.txt`` - Install a Python environment
 ===================================================
 
@@ -60,6 +61,8 @@ environment. Our
 on GitHub shows a typical requirements file.
 
 
+.. _setup.py:
+
 ``setup.py`` - Install Python packages
 ======================================
 
@@ -68,6 +71,8 @@ To install your repository like a Python package, you may include a
 ``pip install -e .``.
 
 
+.. _REQUIRE:
+
 ``REQUIRE`` - Install a Julia environment
 =========================================
 
@@ -76,6 +81,8 @@ Julia repository with ``REQUIRE`` and ``environment.yml``,
 visit `binder-examples/julia-python <https://github.com/binder-examples/julia-python>`_.
 
 
+.. _install.R:
+
 ``install.R`` - Install an R/RStudio environment
 ================================================
 
@@ -85,6 +92,8 @@ To set the date of the snapshot add a runtime.txt_.
 For an example ``install.R`` file, visit our `example install.R file <https://github.com/binder-examples/r/blob/master/install.R>`_.
 
 
+.. _apt.txt:
+
 ``apt.txt`` - Install packages with apt-get
 ===========================================
 
@@ -95,6 +104,8 @@ We use ``apt.txt``, for example, to install LaTeX in our
 `example apt.txt for LaTeX <https://github.com/binder-examples/latex/blob/master/apt.txt>`_.
 
 
+.. _DESCRIPTION:
+
 ``DESCRIPTION`` - Install an R package
 ======================================
 
@@ -106,6 +117,9 @@ You also need to have a ``runtime.txt`` file that is formatted as
 ``r-<YYYY>-<MM>-<DD>``, where YYYY-MM-DD is a snapshot of MRAN that will be
 used for your R installation.
 
+
+.. _manifest.xml:
+
 ``manifest.xml`` - Install Stencila
 ===================================
 
@@ -154,21 +168,29 @@ If you only need to run things once during the build phase use :ref:`postBuild`.
 
 
 .. TODO: Discuss runtime limits, best practices, etc.
-   Also, point to an example.
 
 .. _runtime.txt:
 
 ``runtime.txt`` - Specifying runtimes
 =====================================
 
-This allows you to control the runtime of Python or R.
+Sometimes you want to specify the version of the runtime
+(e.g. the version of Python or R),
+but the environment specification format don't let you specify this information
+(e.g. requirements.txt or install.R).
+For these cases, we have a special file, ``runtime.txt``.
+
+.. note::
+
+   ``runtime.txt`` is only supported when used with environment specifications
+   that do not already support specifying the runtime
+   (e.g. when using ``environment.yml`` for conda or ``REQUIRE`` for Julia,
+   ``runtime.txt`` will be ignored).
 
 To use python-2.7: add ``python-2.7`` in runtime.txt file.
-The repository will run in a virtualenv with
+The repository will run in an env with
 Python 2 installed. To see a full example repository, visit our
 `Python2 example <https://github.com/binder-examples/python2_runtime/blob/master/runtime.txt>`_.
-**Python versions in** ``runtime.txt`` **are ignored when** ``environment.yml`` **is
-present in the same folder**.
 
 repo2docker uses R libraries pinned to a specific snapshot on
 `MRAN <https://mran.microsoft.com/documents/rro/reproducibility>`_.

docs/source/faq.rst

@@ -9,7 +9,8 @@ and have found an answer, send a PR to add it here!
 How should I specify another version of Python?
 -----------------------------------------------
 
-One can specify a Python version in the ``environment.yml`` file of a repository.
+One can specify a Python version in the ``environment.yml`` file of a repository
+or ``runtime.txt`` file if using ``requirements.txt`` instead of ``environment.yml``.
 
 What versions of Python (or R or Julia...) are supported?
 ---------------------------------------------------------
@@ -19,8 +20,8 @@ Python
 
 Repo2docker officially supports the following versions of Python (specified in environment.yml or runtime.txt):
 
-- 3.7 (added in 0.7)
-- 3.6 (default)
+- 3.7 (added in 0.7, default in 0.8)
+- 3.6 (default in 0.7 and earlier)
 - 3.5
 
 Additional versions may work, as long as the

docs/source/howto/languages.rst

@@ -16,30 +16,41 @@ you use one of the following configuration files:
 * ``requirements.txt``
 * ``environment.yml``
 
-Note that by default, the environment will have **Python 3** installed.
+.. note::
+
+  By default, the environment will have |default_python|.
+
+.. versionchanged:: 0.8
+
+  Upgraded default Python from 3.6 to 3.7.
+
 
 Specifying a version of Python
 ------------------------------
 
 To specify a specific version of Python, you have two options:
 
-* Use ``runtime.txt``. Include a line that specifies the Python version in
-  this file. This line takes the following form::
-
-    python=X.X
-
-  For example,::
-
-    python=2.7
-* Use ``environment.yml``. The Anaconda distribution also lets you define
-  the Python environment within ``environment.yml``. To do so, add ``python=X.X``
-  to your dependencies section, like so::
+* Use :ref:`environment.yml <environment.yml>`. Conda environments let you define
+  the Python version in ``environment.yml``.
+  To do so, add ``python=X.X`` to your dependencies section, like so::
 
     name: python 2.7
     dependencies:
       - python=2.7
       - numpy
 
+* Use :ref:`runtime.txt <runtime.txt>` with :ref:`requirements.txt <requirements.txt>`.
+  If you are using ``requirements.txt`` instead of ``environment.yml``,
+  you can specify the Python runtime version in a separate file called ``runtime.txt``.
+  This file contains a single line of the following form::
+
+    python-X.X
+
+  For example::
+
+    python-3.6
+
+
 The R Language
 ==============