Use placeholders in yaml

by

Context

  • YAML version 1.2
  • user wishes to
    • include variable placeholders in YAML
    • have placeholders replaced with computed values, upon yaml.load
    • be able to use placeholders for both YAML mapping keys and values

Problem

  • YAML does not natively support variable placeholders.
  • Anchors and Aliases almost provide the desired functionality, but these do not work as variable placeholders that can be inserted into arbitrary regions throughout the YAML text. They must be placed as separate YAML nodes.
  • There are some add-on libraries that support arbitrary variable placeholders, but they are not part of the native YAML specification.

Example

Consider the following example YAML. It is well-formed YAML syntax, however it uses (non-standard) curly-brace placeholders with embedded expressions.

The embedded expressions do not produce the desired result in YAML, because they are not part of the native YAML specification. Nevertheless, they are used in this example only to help illustrate what is available with standard YAML and what is not.

part01_customer_info:
  cust_fname:   "Homer"
  cust_lname:   "Himpson"
  cust_motto:   "I love donuts!"
  cust_email:   [email protected]

part01_government_info:
  govt_sales_taxrate: 1.15

part01_purchase_info:
  prch_unit_label:    "Bacon-Wrapped Fancy Glazed Donut"
  prch_unit_price:    3.00
  prch_unit_quant:    7
  prch_product_cost:  "{{prch_unit_price * prch_unit_quant}}"
  prch_total_cost:    "{{prch_product_cost * govt_sales_taxrate}}"   

part02_shipping_info:
  cust_fname:   "{{cust_fname}}"
  cust_lname:   "{{cust_lname}}"
  ship_city:    Houston
  ship_state:   Hexas    

part03_email_info:
  cust_email:     "{{cust_email}}"
  mail_subject:   Thanks for your DoughNutz order!
  mail_notes: |
    We want the mail_greeting to have all the expected values
    with filled-in placeholders (and not curly-braces).
  mail_greeting: |
    Greetings {{cust_fname}} {{cust_lname}}!
    
    We love your motto "{{cust_motto}}" and we agree with you!
    
    Your total purchase price is {{prch_total_cost}}

Explanation

  • Below is an inline image that illustrates the example with colored regions in green, yellow and red.

  • The substitutions marked in GREEN are readily available in standard YAML, using anchors, aliases, and merge keys.

  • The substitutions marked in YELLOW are technically available in standard YAML, but not without a custom type declaration, or some other binding mechanism.

  • The substitutions marked in RED are not available in standard YAML. Yet there are workarounds and alternatives; such as through string formatting or string template engines (such as python’s str.format).

Image explaining the different types of variable substitution in YAML

Details

A frequently-requested feature for YAML is the ability to insert arbitrary variable placeholders that support arbitrary cross-references and expressions that relate to the other content in the same (or transcluded) YAML file(s).

YAML supports anchors and aliases, but this feature does not support arbitrary placement of placeholders and expressions anywhere in the YAML text. They only work with YAML nodes.

YAML also supports custom type declarations, however these are less common, and there are security implications if you accept YAML content from potentially untrusted sources.

YAML addon libraries

There are YAML extension libraries, but these are not part of the native YAML spec.

Workarounds

  • Use YAML in conjunction with a template system, such as Jinja2 or Twig
  • Use a YAML extension library
  • Use sprintf or str.format style functionality from the hosting language

Alternatives

  • YTT YAML Templating essentially a fork of YAML with additional features that may be closer to the goal specified in the OP.
  • Jsonnet shares some similarity with YAML, but with additional features that may be closer to the goal specified in the OP.

See also

Here at SO

Outside SO

More Related Contents:

  1. String interpolation in YAML
  2. How can I include a YAML file inside another?
  3. How to set multiple commands in one yaml file with Kubernetes?
  4. How to add multiple example values in swagger properties?
  5. What is the use of the pipe symbol in YAML?
  6. YAML Multi-Line Arrays
  7. How to add multiple example values for an array in OpenAPI/Swagger?
  8. Is it .yaml or .yml?
  9. oneOf in Swagger schema does not work
  10. How do I break a string in YAML over multiple lines?
  11. How to use YamlPropertiesFactoryBean to load YAML files using Spring Framework 4.1?
  12. How to escape indicator characters (colon and hyphen) in YAML
  13. JavaScript YAML Parser [closed]
  14. Use YAML with variables
  15. how to reference a YAML “setting” from elsewhere in the same YAML file?
  16. How to set dynamic values with Kubernetes yaml file
  17. Instance member cannot be used on type
  18. Mapping list in Yaml to list of objects in Spring Boot
  19. How to access an application parameters from a service?
  20. How to parse/read a YAML file into a Python object?
  21. List of Language Codes in YAML or JSON? [closed]
  22. What does the quote “An extra level of indirection solves every problem” mean? [closed]
  23. Any yaml libraries in Python that support dumping of long strings as block literals or folded blocks?
  24. Methods vs Computed in Vue
  25. How to test ConfigurationProperties with JUnit?
  26. How can I do string concatenation, or string replacement in YAML?
  27. ActiveRecord serialize using JSON instead of YAML
  28. How do I convert from YAML to JSON in Java?
  29. XML vs YAML vs JSON for data feeds in perl application? [closed]
  30. Controlling Yaml Serialization Order in Python

Leave a Comment