
ruamel.yaml
===========

Starting with 0.10.7 the package has been reorganised and the
command line utility is in its own package ``ruamel.yaml.cmd`` (so
installing ``ruamel.yaml`` doesn't pull in possibly irrelevant modules
only used in the command line utility)

``ruamel.yaml`` is a YAML package for Python. It is a derivative
of Kirill Simonov's `PyYAML 3.11 <https://bitbucket.org/xi/pyyaml>`_
which supports YAML1.1

Major differences with PyYAML 3.11:

- integrated Python 2 and 3 sources, running on Python 2.6, 2.7 (CPython,
  PyPy), 3.3 and 3.4.
- round trip mode that **includes comments** (block mode, key ordering kept)
- support for simple lists as mapping keys by transforming these to tuples
- ``!!omap`` generates ordereddict (C) on Python 2, collections.OrderedDict
  on Python 3, and ``!!omap`` is generated for these types.
- some `YAML 1.2 <http://yaml.org/spec/1.2/spec.html>`_ enhancements
  (``0o`` octal prefix, ``\/`` escape)
- pep8 compliance
- tox and py.test based testing
- Tests whether the C yaml library is installed as well as the header
  files. That library  doesn't generate CommentTokens, so it cannot be used to
  do round trip editing on comments. It can be used to speed up normal
  processing (so you don't need to install ``ruamel.yaml`` and ``PyYaml``).
  See the section *Optional requirements*.
- Basic support for multiline strings with preserved newlines and
  chomping ( '``|``', '``|+``', '``|-``' ). As this subclasses the string type
  the information is lost on reassignment. (This might be changed
  in the future so that the preservation/folding/chomping is part of the
  parent container, like comments).
- RoundTrip preservation of flow style sequences ( 'a: b, c, d') (based
  on request and test by Anthony Sottile)
- anchors names that are hand-crafted (not of the form``idNNN``) are preserved
- `merges <http://yaml.org/type/merge.html>`_ in dictionaries are preserved
- adding/replacing comments on block-style sequences and mappings
  with smart column positioning
- collection objects (when read in via RoundTripParser) have an ``lc``
  property that contains line and column info ``lc.line`` and ``lc.col``.
  Individual positions for mappings and sequences can also be retrieved
  (``lc.key('a')``, ``lc.value('a')`` resp. ``lc.item(3)``)
- preservation of whitelines after block scalars. Contributed by Sam Thursfield.

Round trip including comments
=============================

The major motivation for this fork is the round-trip capability for
comments. The integration of the sources was just an initial step to
make this easier.

adding/replacing comments
-------------------------

Starting with version 0.8, you can add/replace comments on block style
collections (mappings/sequences resuting in Python dict/list). The basic
for for this is::

  from __future__ import print_function

  import ruamel.yaml

  inp = """\
  abc:
    - a     # comment 1
  xyz:
    a: 1    # comment 2
    b: 2
    c: 3
    d: 4
    e: 5
    f: 6 # comment 3
  """

  data = ruamel.yaml.load(inp, ruamel.yaml.RoundTripLoader)
  data['abc'].append('b')
  data['abc'].yaml_add_eol_comment('comment 4', 1)  # takes column of comment 1
  data['xyz'].yaml_add_eol_comment('comment 5', 'c')  # takes column of comment 2
  data['xyz'].yaml_add_eol_comment('comment 6', 'e')  # takes column of comment 3
  data['xyz'].yaml_add_eol_comment('comment 7', 'd', column=20)

  print(ruamel.yaml.dump(data, Dumper=ruamel.yaml.RoundTripDumper), end='')

.. example code add_comment.py

Resulting in::

  abc:
  - a       # comment 1
  - b       # comment 4
  xyz:
    a: 1    # comment 2
    b: 2
    c: 3    # comment 5
    d: 4              # comment 7
    e: 5 # comment 6
    f: 6 # comment 3


.. example output add_comment.py


If the comment doesn't start with '#', this will be added. The key is
the element index for list, the actual key for dictionaries. As can be seen
from the example, the column to choose for a comment is derived
from the previous, next or preceding comment column (picking the first one
found).

Config file formats
===================

There are only a few configuration file formats that are easily
readable and editable: JSON, INI/ConfigParser, YAML (XML is to cluttered
to be called easily readable).

Unfortunately `JSON <http://www.json.org/>`_ doesn't support comments,
and although there are some solutions with pre-processed filtering of
comments, there are no libraries that support round trip updating of
such commented files.

INI files support comments, and the excellent `ConfigObj
<http://www.voidspace.org.uk/python/configobj.html>`_ library by Foord
and Larosa even supports round trip editing with comment preservation,
nesting of sections and limited lists (within a value). Retrieval of
particular value format is explicit (and extensible).

YAML has basic mapping and sequence structures as well as support for
ordered mappings and sets. It supports scalars various types
including dates and datetimes (missing in JSON).
YAML has comments, but these are normally thrown away.

Block structured YAML is a clean and very human readable
format. By extending the Python YAML parser to support round trip
preservation of comments, it makes YAML a very good choice for
configuration files that are human readable and editable while at
the same time interpretable and modifiable by a program.

Extending
=========

There are normally six files involved when extending the roundtrip
capabilities: the reader, parser, composer and constructor to go from YAML to
Python and the resolver, representer, serializer and emitter to go the other
way.

Extending involves keeping extra data around for the next process step,
eventuallly resulting in a different Python object (subclass or alternative),
that should behave like the original, but on the way from Python to YAML
generates the original (or at least something much closer).

Smartening
==========

When you use round-tripping, then the complex data you get are
already subclasses of the built-in types. So you can patch
in extra methods or override existing ones. Some methods are already
included and you can do::

    yaml_str = """\
    a:
    - b:
      c: 42
    - d:
        f: 196
      e:
        g: 3.14
    """


    data = yaml.load(yaml_str, Loader=yaml.RoundTripLoader)

    assert data.mlget(['a', 1, 'd', 'f'], list_ok=True) == 196


Examples
========

Basic round trip of parsing YAML to Python objects, modifying
and generating YAML::

  from __future__ import print_function

  import ruamel.yaml

  inp = """\
  # example
  name:
    # details
    family: Smith   # very common
    given: Alice    # one of the siblings
  """

  code = ruamel.yaml.load(inp, ruamel.yaml.RoundTripLoader)
  code['name']['given'] = 'Bob'

  print(ruamel.yaml.dump(code, Dumper=ruamel.yaml.RoundTripDumper), end='')

.. example code small.py

Resulting in ::

  # example
  name:
    # details
    family: Smith   # very common
    given: Bob      # one of the siblings


.. example output small.py


YAML handcrafted anchors and references as well as key merging
is preserved. The merged keys can transparently be accessed
using ``[]`` and ``.get()``::

  import ruamel.yaml

  inp = """\
  - &CENTER {x: 1, y: 2}
  - &LEFT {x: 0, y: 2}
  - &BIG {r: 10}
  - &SMALL {r: 1}
  # All the following maps are equal:
  # Explicit keys
  - x: 1
    y: 2
    r: 10
    label: center/big
  # Merge one map
  - <<: *CENTER
    r: 10
    label: center/big
  # Merge multiple maps
  - <<: [*CENTER, *BIG]
    label: center/big
  # Override
  - <<: [*BIG, *LEFT, *SMALL]
    x: 1
    label: center/big
  """

  data = ruamel.yaml.load(inp, ruamel.yaml.RoundTripLoader)
  assert data[7]['y'] == 2


.. example code anchor_merge.py


Optional requirements
=====================

If you have the C yaml library and headers installed, as well as
the header files for your Python executables then you can use the
non-roundtrip but faster C loader and emitter.

On Debian systems you should use::

    sudo apt-get install libyaml-dev python-dev python3-dev

you can leave out ``python3-dev`` if you don't use python3

For CentOS (7) based systems you should do::

   sudo yum install libyaml-devel python-devel

Testing
=======

Testing is done using `tox <https://pypi.python.org/pypi/tox>`_, which
uses `virtualenv <https://pypi.python.org/pypi/virtualenv>`_ and
`pytest <http://pytest.org/latest/>`_.
