Examples

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

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)

Resulting in:

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

with the old API:

from __future__ import print_function

import sys
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'

ruamel.yaml.dump(code, sys.stdout, Dumper=ruamel.yaml.RoundTripDumper)

# the last statement can be done less efficient in time and memory with
# leaving out the end='' would cause a double newline at the end
# print(ruamel.yaml.dump(code, Dumper=ruamel.yaml.RoundTripDumper), end='')

Resulting in

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

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

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:

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)

gives:

first_name: Art
last name: Vandelay    # new key
occupation: Architect  # This is an occupation comment
about: Art Vandelay is a fictional character that George invents...

Please note that the comment is aligned with that of its neighbour (if available).

The above was inspired by a question 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”.

The following program with three dumps:

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)

gives as output:

---
1:
  1:
  - 1: 1
    2: 2
  - 1: 1
    2: 2
  2: 2
2: 42
---
1:
  1:
    - 1: 1
      2: 2
    - 1: 1
      2: 2
  2: 2
2: 42
---
1:
  1:
    - 1: 1
      2: 2
    - 1: 1
      2: 2
  2: 2
2: 42

The transform example was inspired by a question posted by *nowox* 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 realiase that print(round_trip_dump(dict(a=1, b=2))) gets you an extra, empty, line after b: 2.

The real quesiton is why is 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):

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:

print(yaml.dump(dict(a=1, b=2)))

instead of:

yaml.dump((dict(a=1, b=2)), sys.stdout)
print()  # or sys.stdout.write('\n')