Emrichen – Template engine for YAML & JSON

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.

Installation

Python 3.7+ required.

pip install emrichen

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

pip install -e .

Supported tags

Tag	Arguments	Example	Description
!All	An iterable	!All [true, false]	Returns true iff all the items of the iterable argument are truthy.
!Any	An iterable	!Any [true, false]	Returns true iff at least one of the items of the iterable argument is truthy.
!Base64	The value to encode	!Base64 foobar	Encodes the value (or a string representation thereof) into base64.
!Compose	value: The value to apply tags on tags: A list of tag names to apply, latest first	!Base64,Var foo	Used internally to implement tag composition. Usually not used in the spelt-out form. See Tag composition below.
!Concat	A list of lists	!Concat [[1, 2], [3, 4]]	Concatenates lists.
!Debug	Anything, really	!Debug,Var foo	Enriches its argument, outputs it to stderr and returns it. Useful to check the value of some expression deep in a big template, perhaps even one that doesn’t even fully render.
!Defaults	A dict of variable definitions	See examples/defaults/	Defines default values for variables. These will be overridden by any other variable source. NOTE: !Defaults must appear in a document of its own in the template file (ie. separated by ---). The document containing !Defaults will be erased from the output.
!Error	Error message	!Error "Must define either foo or bar, not both"	If the !Error tag is present in the template after resolving all conditionals, it will cause the template rendering to exit with error emitting the specified error message.
!Exists	JSONPath expression	!Exists foo	Returns true if the JSONPath expression returns one or more matches, false otherwise.
!Filter	test, over	See tests/test_cond.py	Takes in a list and only returns elements that pass a predicate.
!Format	Format string	!Format "{foo} {bar!d}"	Interpolate strings using Python format strings. JSONPath supported in variable lookup (eg. {people[0].first_name} will do the right thing). NOTE: When the format string starts with {, you need to quote it in order to avoid being interpreted as a YAML object.
!Group	Accepts the same arguments as !Loop, except template is optional (default identity), plus the following: by: (required) An expression used to determine the key for the current value result_as: (optional, string) When evaluating by, the enriched template is available under this name.	TBD	Makes a dict out of a list. Keys are determined by by. Items with the same key are grouped in a list.
!If	test, then, else	See tests/test_cond.py	Returns one of two values based on a condition.
!Include	Path to a template to include	!Include ../foo.yml	Renders the requested template at this location. Both absolute and relative paths work.
!IncludeBase64	Path to a binary file	!IncludeBase64 ../foo.pdf	Loads the given binary file and returns the contents encoded as Base64.
!IncludeBinary	Path to a binary file	!IncludeBinary ../foo.pdf	Loads the given binary file and returns the contents as bytes. This is practically only useful for hashing.
!IncludeGlob	A string (or a list thereof) of glob patterns of templates to include	!IncludeGlob bits/**.in.yml	Expands the glob patterns and renders all templates into a list. YAML files that contain more than one document will have all of those templates rendered into the same flat list. Expansion results are lexicographically sorted. As with Python’s glob.glob(), use a double star (**) for recursion.
!IncludeText	Path to an UTF-8 text file	!IncludeText ../foo.toml	Loads the given UTF-8 text file and returns the contents as a string.
!Index	Accepts the same arguments as !Loop, except template is optional (default identity), plus the following: by: (required) An expression used to determine the key for the current value result_as: (optional, string) When evaluating by, the enriched template is available under this name. duplicates: (optional, default error) error, warn(ing) or ignore duplicate values.	TBD	Makes a dict out of a list. Keys are determined by by.
!IsBoolean	Data to typecheck.	!IsBoolean ...	Returns True if the value enriched is of the given type, False otherwise.
!IsDict	Data to typecheck.	!IsDict ...	Returns True if the value enriched is of the given type, False otherwise.
!IsInteger	Data to typecheck.	!IsInteger ...	Returns True if the value enriched is of the given type, False otherwise.
!IsList	Data to typecheck.	!IsList ...	Returns True if the value enriched is of the given type, False otherwise.
!IsNone	Data to typecheck.	!IsNone ...	Returns True if the value enriched is None (null) or Void, False otherwise.
!IsNumber	Data to typecheck.	!IsNumber ...	Returns True if the value enriched is of the given type, False otherwise.
!IsString	Data to typecheck.	!IsString ...	Returns True if the value enriched is of the given type, False otherwise.
!Join	items: (required) A list of items to be joined together. separator: (optional, default space) The separator to place between the items. OR a list of items to be joined together with a space as the separator.	!Join [foo, bar] !Join { items: [foo, bar], separator: ', ' }	Joins a list of items together with a separator. The result is always a string.
!Lookup	JSONPath expression	!Lookup people[0].first_name	Performs a JSONPath lookup returning the first match. If there is no match, an error is raised.
!LookupAll	JSONPath expression	!LookupAll people[*].first_name	Performs a JSONPath lookup returning all matches as a list. If no matches are found, the empty list [] is returned.
!Loop	over: (required) The data to iterate over (a literal list or dict, or !Var) as: (optional, default item) The variable name given to the current value index_as: (optional) The variable name given to the loop index. If over is a list, this is a numeric index starting from 0. If over is a dict, this is the dict key. index_start: (optional, default 0) First index, for eg. 1-based indexing. previous_as: (optional) The variable name given to the previous value. On the first iteration of the loop, the previous value is null. Added in 0.2.0 template: (required) The template to process for each iteration of the loop. as_documents: (optional) Whether to “unfold” the output of this loop into separate YAML documents when writing YAML. Only has an effect at the top level of a template.	See examples/loop/.	Loops over a list or dict and renders a template for each iteration. The output is always a list.
!MD5	Data to hash	!MD5 'some data to hash'	Hashes the given data using the MD5 algorithm. If the data is not binary, it is converted to UTF-8 bytes.
!Merge	A list of dicts	!Merge [{a: 5}, {b: 6}]	Merges objects. For overlapping keys the last one takes precedence.
!Not	a value	!Not !Var foo	Logically negates the given value (in Python semantics).
!Op	a, op, b	See tests/test_cond.py	Performs binary operators. Especially useful with !If to implement greater-than etc.
!SHA1	Data to hash	!SHA1 'some data to hash'	Hashes the given data using the SHA1 algorithm. If the data is not binary, it is converted to UTF-8 bytes.
!SHA256	Data to hash	!SHA256 'some data to hash'	Hashes the given data using the SHA256 algorithm. If the data is not binary, it is converted to UTF-8 bytes.
!URLEncode	A string to encode OR url: The URL to combine query parameters into query: An object of query string parameters to add OR a string of query string parameters	!URLEncode "foo+bar" !URLEncode { url: "https://example.com/", query: { foo: bar } }	Encodes strings for safe inclusion in a URL, or combines query string parameters into a URL.
!Var	Variable name	!Var image_name	Replaced with the value of the variable.
!Void	Anything or nothing	foo: !Void	The dict key, list item or YAML document that resolves to !Void is removed from the output.
!With	vars: A dict of variable definitions. template: The template to process with the variables defined.	See examples/with/.	Binds local variables that are only visible within template. Useful for giving a name for common sub-expressions.

Tags in JSON

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.

Tag composition

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

Custom tags

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.

CLI

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

Examples

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

Python API

TODO

emrichen(template, *variable_sources, **override_variables)

Context(*variable_sources, **override_variables)

Template(template_source)

License

The MIT License (MIT)

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

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copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

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