YAML in Python

Wed, 09 Dec 2020 00:00:00 +0000

YAML

YAML = YAML Ain’t Markup Language
Human-readable data-serialization language
It is commonly used for configuration files, but it is also used in data storage (e.g. debugging output) or transmission (e.g. document headers).
Filename extension: .yaml

Use YAML in Python

PyYAML is a YAML parser and emitter for Python.

$ pip install pyyaml

Read YAML

Let’s say we have a YAML file called person_info.yaml:

name: "Ecko"
age: 20
hobby:
 - "Basketball"
 - "gym"
 - "coding":
 language:
 - Python
 - Java
 editor:
 - vscode
 - JupyterLab
university: "KIT"
student: True

We use yaml.safe_load() to read a YAML file:

with open("test.yaml", "r") as f:
 person_info = yaml.safe_load(f)

person_info

{
 'age': 20,
 'hobby': [
 'Basketball',
 'gym',
 {'coding':
 {
 'editor': ['vscode', 'JupyterLab'],
 'language': ['Python', 'Java']
 }
 }
 ],
 'name': 'Ecko',
 'student': True,
 'university': 'KIT'
}

The YAML file will be read to a dict, and we can access keys and values. For example,

print(person_info["name"])

Ecko

Read multiple YAMLs

Multiple YAML documents are read with yaml.safe_load_all().

Example: persons.yaml

---
name: "Ecko"
age: 20
hobby:
 - "Basketball"
 - "gym"
 - "coding":
 language:
 - Python
 - Java
 editor:
 - vscode
 - JupyterLab
university: "KIT"
student: True
---
name: "Tan"
age: 21
hobby:
 - football
 - reading
university: "TUM"
student: True

We read this YAML file and print name and age of each person:

with open("persons.yaml") as stream:
 persons = yaml.safe_load_all(stream)

 for person in persons:
 name = person["name"]
 age = person["age"]
 print(f"Name: {name}, Age: {age}")

Name: Ecko, Age: 20
Name: Tan, Age: 21

Write YAML

To serialize a Python object into a YAML stream, use yaml.safe_dump().

In order to format the output YAML, you can use keyword arguments of safe_dump(). Some useful arguments are:

default_flow_style : indicates if a collection is block or flow. The possible values are None, True, False.
indent: sets the preferred indentation
explicit_start: if True, adds an explicit start using ---
explicit_end: if True, adds an explicit end using ---

For example

person_info = {
 "name": "Ecko",
 "age": 20,
 "hobby": [
 "Basketball",
 {
 "coding": {
 "language": ["Python", "Java", "C#"],
 "start_from": 2014
 }
 }
 ]
}

with open("person.yaml", "w") as f:
 yaml.safe_dump(person_info, f, default_flow_style=False, indent=2)

person.yaml will look like following:

age: 20
hobby:
- Basketball
- coding:
 language:
 - Python
 - Java
 - C#
 start_from: 2014
name: Ecko

Write multiple YAML files

Use yaml.safe_dump_all()

Reference

Python YAML tutorial: short and quick tutorial
Python的PyYAML模块详解
PyYAML Documentation: official documentation of PyYAML. However, it is out-of-date and maybe the most messy and the worst documentation I’ve ever read. 👎

YAML | Haobin Tan