YAML in Python

Internal

• Python Code Examples

PyYAML

https://pyyaml.org/

https://pyyaml.org/wiki/PyYAMLDocumentation

https://pypi.org/project/PyYAML/

PyYAML provides YAML serialization/deserialization in Python.

Installation

pip install pyyaml

requirements.txt:

pyyaml == 5.3.1

To install, see the "Installation" section from https://pyyaml.org/wiki/PyYAMLDocumentation.

Concepts

PyYAML core model is centered on constructors, representers and tags.

Constructor

https://matthewpburruss.com/post/yaml/#defining-pyyaml-constructors-going-from-yaml-to-python

A constructor allows you to take a serialized YAML node and return a class instance.

Representer

A representer is user-defined function that intercepts, as part of the YAML serialization process, data object instances to be serialized, optionally processes them, and then messages the Dumper instance received as argument to create the proper serialized representation for the given data object instance, as a Node instance. The instance thus created is returned as result of the representer function, contributing to the serialization result. The representer gets the Dumper instance as a first argument, and the data object as the second.

This is an implementation of a representer that uses the literal block scalar representation for multi-line strings, and the default plain flow scalar representation for everything else.

def custom_scalar_representer(dumper: Dumper, data: str):
    # for multi-line strings, use the literal block scalar representation, use the default otherwise
    if data and '\n' in data:
        return dumper.represent_scalar('tag:yaml.org,2002:str', data, '|')
    else:
        return dumper.represent_scalar('tag:yaml.org,2002:str', data)

In this case, tag:yaml.org,2002:str is the tag name for string nodes.

The representers are registered with add_representer(). Representers can be added for specific types (such as str or int):

yaml.add_representer(str, custom_scalar_representer)

Tag

The tag uses the special character ! preceding the tag name to label a YAML node. A tag helps PyYAML to know which constructor or representer to call.

Deserialize YAML

import yaml

with open('some-file.yaml', 'rt') as f:
  content = f.read()
data = yaml.load(content, Loader=yaml.Loader)

Serialize YAML

Serializing YAML with PyYAML

import yaml

# with the default flow style, the document is rendered in a non-indented manner 
print (yaml.dump(data, default_flow_style=False))

Safely Navigate a Complex Data Structure

Suggestions on how to safely recursively navigate a complex data structure:

Safely Navigate a Complex Data Structure

Representers

• https://github.com/yaml/pyyaml/issues/98

Configure yaml.dump() to render blank instead of null

PyYAML dump() uses Representer() to represent None. By default, the representer dump() is configured with represents None as "null". To change that:

1. Define a method that "represents" None

def representer_for_none(self, _):
    return self.represent_scalar('tag:yaml.org,2002:null', '')

2. Add it to the module:

import yaml

[...]

yaml.add_representer(type(None), represent_none)

This will render:

d = {
    'a': None,
    'b': 'c'
}

as:

a:
b: c

what is the lifecycle of the method registered to the module?

Configure yaml.dump() to render | multi-lines

def literal_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|')

def default_presenter(dumper, data):
     return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='')

try:
   yaml.add_representer(str, literal_presenter)
   yaml_text = yaml.dump(config)
   return yaml_text
finally:
    yaml.add_representer(str, default_presenter)