Python YAML

Library for adding Python code in YAML processing

https://github.com/gwww/pyaml

Experimental - This library exists to try out ideas that enhance and make simpler the management and creation of YAML files. In particular, large YAML files, often seen when using Home Assistant Lovelace front-end.

Requirements

• Python 3.6 (or higher)

Description

Usage documentation is below. More examples beyond what is here is in the example directory or in the unit tests in test/test_pyaml.py.

This lib is distinguished from other templating languages in that indentation, crucial in YAML, is preserved on include, eval, and exec

This uses python's eval and exec functions. Google about security concerns around the use of those. Since this software is not accessing "unaudited" code the security risk of using eval and exec is viewed as low. Never accept/use Python code without inspecting the code.

Installation

    $ pip install pyaml-processor

Overview

pyaml reads a YAML file and runs the tagged code inside the YAML file. It supports three processing tags: eval to run code, exec to load code, and include to include other files in the context of the current file. All three processors are aware of YAML indenting requirements.

Eval

eval is triggered in a YAML file using the tags @% to open an eval and %@ to close an eval. Anything in between the two tags is passed to the Python eval function for processing. Whatever is returned from the eval is inserted into the YAML stream. The starting character position of the opening tag is used as the indent level prepended to everything returned.

For the examples in this section assume that the following Python code is in the module resources.py and that file contains the following:

from random import randrange

_PATH = "/local/cards/"

def resources(module, module_type):
    version = f"?v={randrange(1000000)}"
    # This works to, the lib can handle lists, dicts, etc as return values:
    # return [{'url': f"{_PATH}/{module}{version}", "type": module_type}]
    return f"url: {_PATH}/{module}{version}\ntype: {module_type}"

Example 1:

@+ from resources import resources +@
resources:
  - @% resources("layout-card", "module") %@
  - @% resources("card-mod", "module") %@

Processing with pyaml results in:

resources:
  - url: /local/cards//layout-card?v=238120
    type: module
  - url: /local/cards//card-mod?v=885753
    type: module

Notice that the indentation is preserved from the position on the line where the eval was invoked.

Note that the space around the start and end tags is optional.

Exec

exec is triggered in a YAML file using the tags @% to open an eval and %@ to close an exec. Anything in between the two tags is passed to the Python exec function for processing. Whatever is returned from the exec is NOT inserted into the YAML stream. The code inside the exec tags is dedented meaning common leading whitespace on each line is removed.

Example 2:

@+
def markdown_card(label):
    return \
f"""type: markdown
style: |
  ha-card {{background: purple}}
content: |
  ## {label}"""
+@

title: My awesome Lovelace config
views:
  - title: Home
    cards:
      - @%markdown_card("Kitchen")%@
      - @%markdown_card("Living room")%@

Processing with pyaml results in:

title: My awesome Lovelace config
views:
  - title: Home
    cards:
      - type: markdown
        style: |
          ha-card {background: purple}
        content: |
          ## Kitchen
      - type: markdown
        style: |
          ha-card {background: purple}
        content: |
          ## Living room

Note: any type of Python code may exist between the tags, however, it is likely more maintainable to put code, such the code in the example above, into it's own Python module.

Include

Includes the contents of the file into the YAML stream. The included file may contain eval and exec blocks. Include is trigged using the same open and closing tag of @@.

The advantage of using pyaml include over the include processing from PyYAML is that pyaml preserves indentation.

For example if example3_include.yaml contains:

- zoo: tiger
- moo: cow

And the following YAML file:

big_pets:
  @@include some_file.yaml@@

Processing with pyaml results in:

big_pets:
  - zoo: tiger
  - moo: cow

Running

There are two programs available to try out the library. In the example directory there is a Python script called simple. This takes a file name as a single parameter and writes the converted output to standard out. The input file is a YAML file. While in the example directory you could, for instance, type ./simple example1.yaml to see the output of the first example in this README.

The second program is called pyaml is in the bin directory. It's a slightly more featured. Run it with --help for additional details.

Development

This project uses poetry for development dependencies. Installation instructions are on their website.

To get started developing:

git clone https://github.com/gwww/pyaml.git
cd pyaml
poetry install
poetry shell # Or activate the created virtual environment
pytest # to ensure everything installed properly

There is a Makefile in the root directory as well. The make command followed by one of the targets in the Makefile can be used. If you don't have or wish to use make the Makefile serves as examples of common commands that can be run.