YTE - A YAML template engine with Python expressions

YTE is a template engine for YAML format that utilizes the YAML structure in combination with Python expressions for enabling to dynamically build YAML documents.

Syntax

The key idea of YTE is to rely on the YAML structure to enable conditionals, loops and other arbitrary Python expressions to dynamically render YAML files. Python expressions are thereby declared by prepending them with a ? anywhere in the YAML. Any such value will be automatically evaluated by YTE, yielding plain YAML as a result. Importantly, YTE templates are still valid YAML files (for YAML, the ? expressions are just strings).

Examples

Conditionals

Template

?if True:
  foo: 1
?elif False:
  bar: 2
?else:
  bar: 1

Rendered

foo: 1

Template

?if True:
  - a
  - b

Rendered

- a
- b

Template

- foo
- bar
- ?if True:
    baz
  ?else:
    bar

Rendered

- foo
- bar
- baz

Loops

Template

?for i in range(2):
  '?f"key:{i}"': 1  # When expressions in keys or values contain colons, they need to be additionally quoted.
  ?if i == 1:
      foo: true

Rendered

"key:0": 1
"key:1": 1
foo: true

Custom definitions

Template

  # The special keyword __definitions__ allows to define custom statements.
  # It can be used anywhere in the YAML, also repeatedly and inside of ifs or loops.
  __definitions__:
    - from itertools import product
    - |
      def squared(value):
          return value ** 2

  ?for item in product([1, 2], ["a", "b"]):
    - ?f"{item}"

  ?if True:
    - ?squared(2)

Rendered

- 1-a
- 1-b
- 2-a
- 2-b
- 4

Usage

Command line interface

YTE comes with a command line interface. To render any YTE template, just issue

yte < the-template.yaml > the-rendered-version.yaml

Python API

Alternatively, you can invoke YTE via its Python API:

from yte import process_yaml

# set some variables as a Python dictionary
variables = ...

# render a string and obtain the result as a Python dict
result = process_yaml("""
?for i in range(10):
  - ?f"item-{i}"
""", variables=variables)

# render a file and obtain the result as a Python dict
with open("the-template.yaml", "r") as template:
    result = process_yaml(template, variables=variables)

# render a file and write the result as valid YAML
with open("the-template.yaml", "r") as template, open("the-rendered-version.yaml", "w") as outfile:
    result = process_yaml(template, outfile=outfile, variables=variables)

Comparison with other engines

Lots of template engines are available, for example the famous generic jinja2. The reasons to generate a YAML specific engine are

1. The YAML syntax can be exploited to simplify template expression syntax, and make it feel less foreign (i.e. fewer special characters for control flow needed) while increasing human readability.
2. Whitespace handling (which is important with YAML since it has a semantic there) becomes unnecessary (e.g. with jinja2, some tuning is required to obtain proper YAML rendering).

Of course, YTE is not the first YAML specific template engine. Others include

• Yglu
• Emrichen

The main difference between YTE and these two is that YTE extends YAML with plain Python syntax instead of introducing another specialized language. Of course, the choice is also a matter of taste.