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YAML

YAML (YAML Ain't Markup Language) is an indentation-based file format for data serialization. A JSON (JavaScript Object Notation) doc can be easily converted to a YAML doc, since JSON is a subset of YAML.

NOTE: Only spaces is allowed for indentation, not tabs.

YAML Syntax

Dictionary

YAMLJSON
# This is a comment
number: 3.14159
bool: true
string: 'hello'
another string: bye bye
dict:
  name: Simon
  weight: 66
# the style below is called flow style, whereas the one above block style
another-dict: {name: Simon, weight: 66}
 
{
  "number": 3.14159,
  "bool": true,
  "string": "hello",
  "another string": "bye bye",
  "dict": {
    "name": "Simon",
    "weight": 66
  },
  "another-dict": {
    "name": "Simon",
    "weight": 66
  }
}

List

YAMLJSON
list:
  - apple
  - pear
  - orange
another_list: [apple, pear, orange]
'list of dict':
  - name: Simon
    weight: 66
  - surname: Zhao
 
{
  "list": [
    "apple",
    "pear",
    "orange"
  ],
  "another_list": [
    "apple",
    "pear",
    "orange"
  ],
  "list-of-dict": [
    {
      "name": "Simon",
      "weight": 66
    },
    "surname": "Zhao"
  ]
}

Multiple-lines value

YAMLJSON
code: |
  if args.version:
      print(version)
 
{
  "code": "if args.version:\n    print(version)\n"
}

Multiple documents in a single YAML file

YAMLJSON
---
name: Simon
---
name: Zhao
 
{"name": "Simon"}
{"name": "Zhao"}

Use YAML in Python

The pyyaml package can be used to deal with YAML file:

pip install pyyaml

To read YAML file:

import yaml

# read YAML string
doc = """
a: 1
b:
  c: 3
  d: 4
"""
print(yaml.load(doc))

# read YAML file containing a single YAML document
with open('xxx.yaml') as f:
    print(yaml.load(f))

# read YAML file containing multiple YAML documents
with open('yyy.yaml') as f:
    for doc in yaml.load_all(f)
        print(doc)

# read YAML file and keep the order of keys as well.
# the returned object is an OrderedDict
import yamlordereddictloader  # pip install yamlordereddictloader
with open('xxx.yaml') as f:
    print(yaml.load(f, Loader=yamlordereddictloader.Loader))

To convert Python data structure to YAML format:

import yaml

one = {'name': 'Simon', 'weight': 66}
two = [one, one]

# dump one object
print(yaml.dump(one))

# dump multiple objects
print(yaml.dump_all(two, , explicit_start=True))

Reference