path: root/_doc/example.ryd

version: 0.2
text: rst
fix_inline_single_backquotes: true
pdf: true
--- |
********
Examples
********

Basic round trip of parsing YAML to Python objects, modifying
and generating YAML:
--- !python |
  import sys
  from ruamel.yaml import YAML

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

  yaml = YAML()
  code = yaml.load(inp)
  code['name']['given'] = 'Bob'

  yaml.dump(code, sys.stdout)

--- !stdout |
Resulting in::
--- |
----

YAML handcrafted anchors and references as well as key merging
are preserved. The merged keys can transparently be accessed
using ``[]`` and ``.get()``:
--- !python |
  from ruamel.yaml import 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
  """

  yaml = YAML()
  data = yaml.load(inp)
  assert data[7]['y'] == 2
--- |

The ``CommentedMap``, which is the ``dict`` like construct one gets when round-trip loading,
supports insertion of a key into a particular position, while optionally adding a comment:
--- !python |
  import sys
  from ruamel.yaml import YAML

  yaml_str = """\
  first_name: Art
  occupation: Architect  # This is an occupation comment
  about: Art Vandelay is a fictional character that George invents...
  """

  yaml = YAML()
  data = yaml.load(yaml_str)
  data.insert(1, 'last name', 'Vandelay', comment="new key")
  yaml.dump(data, sys.stdout)

--- !stdout |
gives::
--- |
Please note that the comment is aligned with that of its neighbour (if available).

The above was inspired by a `question <http://stackoverflow.com/a/36970608/1307905>`_
posted by *demux* on StackOverflow.

----

By default ``ruamel.yaml`` indents with two positions in block style, for
both mappings and sequences. For sequences the indent is counted to the
beginning of the scalar, with the dash taking the first position of the
indented "space".

You can change this default indentation by e.g. using ``yaml.indent()``:


--- !python |

import sys
from ruamel.yaml import YAML

d = dict(a=dict(b=2),c=[3, 4])
yaml = YAML()
yaml.dump(d, sys.stdout)
print('0123456789')
yaml = YAML()
yaml.indent(mapping=4, sequence=6, offset=3)
yaml.dump(d, sys.stdout)
print('0123456789')


--- !stdout |

giving::


--- |

If a block sequence or block mapping is the element of a sequence, the
are, by default, displayed `compact
<http://yaml.org/spec/1.2/spec.html#id2797686>`__ notation. This means
that the dash of the "parent" sequence is on the same line as the
first element resp. first key/value pair of the child collection.

If you want either or both of these (sequence within sequence, mapping
within sequence) to begin on the next line use ``yaml.compact()``:

--- !python |

import sys
from ruamel.yaml import YAML

d = [dict(b=2), [3, 4]]
yaml = YAML()
yaml.dump(d, sys.stdout)
print('='*15)
yaml = YAML()
yaml.compact(seq_seq=False, seq_map=False)
yaml.dump(d, sys.stdout)


--- !stdout |

giving::


--- | 

------

The following program uses three dumps on the same data, resulting in a stream with 
three documents:

--- !python |
import sys
from ruamel.yaml import YAML

data = {1: {1: [{1: 1, 2: 2}, {1: 1, 2: 2}], 2: 2}, 2: 42}

yaml = YAML()
yaml.explicit_start = True
yaml.dump(data, sys.stdout)
yaml.indent(sequence=4, offset=2)
yaml.dump(data, sys.stdout)


def sequence_indent_four(s):
    # this will fail on direclty nested lists: {1; [[2, 3], 4]}
    levels = []
    ret_val = ''
    for line in s.splitlines(True):
        ls = line.lstrip()
        indent = len(line) - len(ls)
        if ls.startswith('- '):
            if not levels or indent > levels[-1]:
                levels.append(indent)
            elif levels:
                if indent < levels[-1]:
                    levels = levels[:-1]
            # same -> do nothing
        else:
            if levels:
                if indent <= levels[-1]:
                    while levels and indent <= levels[-1]:
                        levels = levels[:-1]
        ret_val += '  ' * len(levels) + line
    return ret_val

yaml = YAML()
yaml.explicit_start = True
yaml.dump(data, sys.stdout, transform=sequence_indent_four)

--- !stdout |
gives as output::

--- | 

The transform example, in the last document, was inspired by a
`question posted by *nowox*
<https://stackoverflow.com/q/44388701/1307905>`_ on StackOverflow.

-----

Output of ``dump()`` as a string
++++++++++++++++++++++++++++++++

The single most abused "feature" of the old API is not providing the (second)
stream parameter to one of the ``dump()`` variants, in order to get a monolithic string
representation of the stream back.

Apart from being memory inefficient and slow, quite often people using this did not
realise that ``print(round_trip_dump(dict(a=1, b=2)))`` gets you an extra,
empty, line after ``b: 2``.

The real question is why this functionality, which is seldom really
necessary, is available in the old API (and in PyYAML) in the first place. One
explanation you get by looking at what someone would need to do to make this
available if it weren't there already. Apart from subclassing the ``Serializer``
and providing a new ``dump`` method, which would ten or so lines, another
**hundred** lines, essentially the whole ``dumper.py`` file, would need to be
copied and to make use of this serializer.

The fact is that one should normally be doing ``round_trip_dump(dict(a=1, b=2)),
sys.stdout)`` and do away with 90% of the cases for returning the string, and
that all post-processing YAML, before writing to stream, can be handled by using
the ``transform=`` parameter of dump, being able to handle most of the rest. But
it is also much easier in the new API to provide that YAML output as a string if
you really need to have it (or think you do):

--- !python |
import sys
from ruamel.yaml import YAML
from ruamel.yaml.compat import StringIO

class MyYAML(YAML):
    def dump(self, data, stream=None, **kw):
        inefficient = False
        if stream is None:
            inefficient = True
            stream = StringIO()
        YAML.dump(self, data, stream, **kw)
        if inefficient:
            return stream.getvalue()

yaml = MyYAML()   # or typ='safe'/'unsafe' etc
--- |
with about one tenth of the lines needed for the old interface, you can once more do::
--- !code |
print(yaml.dump(dict(a=1, b=2)))
--- |
instead of::
--- !code |
yaml.dump((dict(a=1, b=2)), sys.stdout)
print()  # or sys.stdout.write('\n')