yamlpp-lang 0.3.3

YAMLpp: a macro language for dynamic YAML preprocessing

YAML File Preprocessor (YAMLpp)

Problem

YAML is an excellent file format but it is essentially static. Sometimes, the content of a YAML file must change according to circumstances (typically when the environment changes or when you have different configuratons for test or production, etc.).

Manually maintaining different versions can be time-consuming and error-prone.

Introducing YAMLpp

What if we had a way to generate a new YAML file (or more than one) according to a single pattern?

The purpose of YAML Preprocessor (YAMLpp) is to help programmers prepare YAML files from a template, with rules that produce the YAML tree according to source data. It extends standard YAML with constructs for variable declaration, conditionals, iteration, functions, importing and exporting YAML files, and importing Python modules.

YAMLpp is a macro language, since it manipulates the YAML tree on which it resides.

Here is a simple example:

YAMLpp:

.frame:
  name: "Alice"

message: "Hello, {{ name }}!"

Output:

message: "Hello, Alice!"

General principles

The language is composed of constructs, which are denoted keys starting with a dot (.), such as .frame, .if, .switch, etc.

The YAMLpp preprocessor uses these constructs modify the tree, and the constructs disappear.

The result is pure YAML.

YAMLpp obeys the rules of YAML syntax:

• It provides declarative constructs without breaking YAML syntax.
• It allows modular, reusable, and expressive constructs that create YAML files

🚀 Quickstart

Installation

pip install yamlpp-lang

Command-line usage

yamlpp input.yaml -o output.yaml

• input.yaml → your YAML file with YAMLpp directives
• output.yaml → the fully expanded YAML after preprocessing

To consult the help:

yamlpp --help

Python API

from yamlpp import Interpreter

FILENAME = 'my_file.yaml'
i = Interpreter()

# the initial (source) tree:
initial_tree = i.load(FILENAME)

# the destination (target) tree (pure YAML):
tree = i.render()

# print the resulting YAML
print(i.yaml)

🔧 YAMLpp Constructs (Quick Reference)

Construct	Purpose	Minimal Example
.frame	Define local variables valid for siblings and descendants.	.frame: name: "Alice" message: "Hello {{ name }}"
.do	Execute a sequence or map of instructions.	.do: - step: "Init" - step: "Run"
.foreach	Iterate over values with a loop body.	.frame: items: [1,2] .foreach: .values: [x, items] .do: - val: "{{ x }}"
.switch	Branch to a different node based on an expression and cases.	.switch: .expr: "{{ color }}" .cases: red: {msg: "Stop"} .default: {msg: "?"}
.if	Conditional node creation with then and else.	.if: .cond: "{{ x>0 }}" .then: {res: "Pos"} .else: {res: "Neg"}
.load	Insert and preprocess another YAMLpp (or YAML) file.	.import_module: "other.yaml"
.function	Define a reusable block with arguments and a body.	.function: .name: "greet" .args: ["n"] .do: - msg: "Hi {{ n }}"
.call	Invoke a previously defined function with arguments.	.call: .name: "greet" .args: ["Bob"]
.import_module	Import a Python module exposing functions, filters, and variables.	.module: "module.py"
.export	Export a portion of the tree into an external file.	.export: .filename: "out.yaml" .do: - foo: "bar"