Template engine for YAML & JSON
Project description
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 3.5+ required. Python 2 is not and will not be supported.
pip3 install emrichen
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. |
!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. |
!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. |
!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. template : (required) The template to process for each iteration of the loop. |
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 'Törkylempijävongahdus' | 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 'Törkylempijävongahdus' | 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 'Törkylempijävongahdus' | 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. |
!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
.
CLI
usage: emrichen [-h] [--template-format {yaml,json}] [--var-file VAR_FILE]
[--define VAR=VALUE] [--output-file OUTPUT_FILE]
[--output-format {yaml,json}] [--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}
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 © 2018–2019 Santtu Pajukanta
Copyright © 2018–2019 Aarni Koskela
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
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