Emrichen takes in templates written in YAML or JSON, processes tags that do things like variable substitution, and outputs YAML or JSON.

What makes Emrichen better for generating YAML or JSON than a text-based template system is that it works within YAML (or JSON).

Ever tried substituting a list or dict into a YAML document just to run into indentation issues? Horrible! Handling quotation marks and double backslash escapes? Nope!

In Emrichen, variables are typed in the familiar JSON types, making these a non-issue. Emrichen is a pragmatic and powerful way to generate YAML and JSON.

Consider the following template that produces a minimal Kubernetes deployment:

!Defaults
tag: latest
image: !Format "nginx:{tag}"
replicas: 3
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: !Var replicas
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: !Var image
        ports:
        - containerPort: 80

This small template already demonstrates three of Emrichen's powerful tags: !Defaults that provides default values for variables; !Var that performs simple variable substitution; and !Format that performs string formatting.

Put it in a file, say, nginx.in.yaml (we use .in.yaml to denote templates) and render it using this command:

emrichen nginx.in.yaml

Prefer JSON output?

emrichen --output-format json nginx.in.yaml

Wanna change the tag?

emrichen --define tag=1-alpine nginx.in.yaml

Note how image is evaluated lazily – you need not override image just to change tag!

See below for a table of supported tags. There's a lot of them. If you need one that's not there yet, please shoot us an issue or PR.

Python 3.7+ required.

pip install emrichen

If you have cloned Emrichen from GitHub (for development):

pip install -e .

JSON doesn't have a native tag construct. Instead, use an object with a single key that is the name of the tag (including the bang, eg. !Var). For example:

{
    "foo": {
        "!Var": "foo"
    }
}

Limitations of the JSON support:

• Object keys starting with ! are not supported.
• A template rendered as JSON may only contain a single document.
  • JSON templates always have a single document only.
  • YAML templates may only contain a single non-!Void, non-!Defaults document.
• As !Defaults must appear in a document of its own, it's not supported in JSON templates. Use a var file instead.

Due to YAML, you can't do !Base64 !Var foo. We provide a convenient workaround: !Base64,Var foo.

It is possible to implement your own custom tags in Python. Tags are classes that inherit from the emrichen.tags.base.BaseTag class and implement the enrich(self, context: emrichen.context.Context) method. For examples of tag implementations, see the built-in tags under emrichen/tags/ or the example at tests/custom_tags/.

Place the implementation in a Python module that is on the PYTHONPATH. Same directory as the template is usually fine.

All subclasses of BaseTag are automatically registered as long as the module containing them is imported. If you are using the CLI, add -m your_tags to the command line where your_tags corresponds to your_tags.py. If you are using Emrichen programmatically, be sure to import your custom tags (import your_tags) before instantiating a template.

There is an example of custom tags that you can try out as follows (provided you have cloned Emrichen from GitHub and installed it using pip install -e .). The example is not installed when installing Emrichen from PyPI (pip install emrichen).

emrichen -m tests.custom_tags examples/custom_tags.yml

The example defines a !KubeEnv custom tag that turns a dictionary of key—value mappings into a list of environment variables in the format expected by Kubernetes.

usage: emrichen [-h] [--template-format {yaml,json}] [--var-file VAR_FILE]
usage: emrichen [-h] [--template-format {yaml,json}] [--var-file VAR_FILE]
                [--define VAR=VALUE] [--output-file OUTPUT_FILE]
                [--output-format {yaml,json,pprint}] [--include-env]
                [template_file]

A YAML to YAML preprocessor.

positional arguments:
  template_file         The YAML template to process. If unspecified, the
                        template is read from stdin.

optional arguments:
  -h, --help            show this help message and exit
  --template-format {yaml,json}
                        Template format. If unspecified, attempted to be
                        autodetected from the input filename (but defaults to
                        YAML).
  --var-file VAR_FILE, -f VAR_FILE
                        A YAML file containing an object whose top-level keys
                        will be defined as variables. May be specified
                        multiple times.
  --define VAR=VALUE, -D VAR=VALUE
                        Defines a single variable. May be specified multiple
                        times.
  --output-file OUTPUT_FILE, -o OUTPUT_FILE
                        Output file. If unspecified, the template output is
                        written into stdout.
  --output-format {yaml,json,pprint}
                        Output format. If unspecified, attempted to be
                        autodetected from the output filename (but defaults to
                        YAML).
  --include-env, -e     Expose process environment variables to the template.

Variable precedence: -D > -e > -f > !Defaults

cd examples/kubernetes
emrichen -f vars.yml -D tag=build-256 deployment.in.yml

TODO

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Copyright © 2018–2022 Santtu Pajukanta
Copyright © 2018–2022 Aarni Koskela

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