Installation

Install the PyYAML package via pip:

python3 -m pip install pyyaml

Loading a YAML File

The safest way to load a YAML document is with safe_load():

import yaml
data = yaml.safe_load(open('./data.yaml'))

YAML Syntax Essentials

• Case-sensitive – True and true are different.
• Indentation – Spaces matter only for alignment; same-level eleemnts must be left-aligned.
• Comments – Lines starting with # are comments.

Lists

A list is indicated by a dash followed by a space:

-
  - 10
  - 10
-
  - 20
  - 30

The above YAML produces the Python list: [[10, 10], [20, 30]].

Nested lists example:

-
  - [1, 2]
  - [2, 3]

Results in: [[[1, 2], [2, 3]]].

Dictionaries

Key-value pairs are written as key: value. Nested dictionaries use deeper indentation:

key:
  child1: value1
  child2: value2

Yields: {'key': {'child1': 'value1', 'child2': 'value2'}}.

Nested Structures (List of Dictionaries)

key:
  - id: 1
    name: lijia
    price: 100
  - id: 2
    name: litian
    price: 200

Parsed as: {'key': [{'id': 1, 'name': 'lijia', 'price': 100}, {'id': 2, 'name': 'litian', 'price': 200}]}.

Mixed Types

name:
  - lijia
  - litian
  - lijiajia
age:
  lijia: 30
  litian: 28
  lijiajia: 33

Produces: {'name': ['lijia', 'litian', 'lijiajia'], 'age': {'lijia': 30, 'litian': 28, 'lijiajia': 33}}.

Practical Example: Parameterzied Tests with pytest

Use YAML to store test data and load it directly via yaml.safe_load():

import pytest
import yaml

# data.yaml content example:
# - ['3+5', 8]
# - ['2+5', 7]
# - ['7+5', 30]

@pytest.mark.parametrize('expression,expected', yaml.safe_load(open('./data.yaml')))
def test_eval_expression(expression, expected):
    assert eval(expression) == expected

Verifying YAML Data Outside pytest

To inspect what the YAML file actual contains, run a simple script:

import yaml

with open('./data.yaml') as file:
    parsed = yaml.safe_load(file)
    print(parsed)