structured-templates

This package implements a simple templating engine for structured data, such as YAML or JSON. It is designed to be used in conjunction with any structured data format, such as JSON, YAML or TOML. It provides control elements and variable substition, all based on evaluating keys and strings.

Example

apiVersion: v1
kind: Secret
data:
  password: ${{ b64encode(secrets.password) }}
  if(secrets.username):
    username: ${{ b64encode(secrets.username) }}
  for(key, value in secrets.additional.items()):
    ${{ key }}: ${{ b64encode(value) }}

This example demonstrates how to use the templating engine to generate a Kubernetes Secret object. The if(...) and for(...) control elements are used to conditionally include keys and loop over a dictionary, respectively. The ${{ ... }} syntax is used to evaluate expressions and substitute the result into the template.

Usage

Install the package from PyPI:

pip install structured-templates

The TemplateEngine is the main class that is used to evaluate templates.

from structured_templates import TemplateEngine

engine = TemplateEngine()
assert engine.evaluate({"key": "${{ 1 + 1 }}"}) == {"key": 2}

Specification

Value substitution

Value substitution is done by evaluating an expression and substituting the result into the template. The expression can be any valid Python expression, and the result type is inserted as-is into the template. If the result is a string, the value substition can be prefixed and/or suffixed with text.

key: prefix${{ expression }}suffix
10integers: ${{ list(range(10)) }}

Attempting to substitute a non-string value into a string will raise an error.

Control elements

if(<expr>)

Conditionally include keys in the parent object or items in a list. The value of this control block can only be a scalar value if the parent object is a list.

items:
 - item1
 - if(condition): item2
 - if(condition):
   - item3
   - item4

May result in

items:
  - item1
  - item2
  - item3
  - item4

for(<names> in <expr>)

Iterate over the elements of an iterable. The value for this field is evaluated in the new scope. If the value is an object, the results of every loop are merged into a single object. If the value is a list, the results are concatenated. The value of this control block cannot be a scalar value.

items:
  for(idx in range(2)):
    ${{ str(idx) }}: Number ${idx}

Results in

items:
  '0': Number 0
  '1': Number 1    

macro(<expr>)

Define a macro that can be used in the template.

macro(inc(x)): ${{ x + 1 }}
key: ${{ inc(1) }}

Results in

key: 2