ceremcem ceremcem - 1 month ago 20
YAML Question

More complex inheritance in YAML?

YAML has inheritance. The most clear example I have ever seen is here: http://blog.101ideas.cz/posts/dry-your-yaml-files.html

I need something more complex: I need to override object's object's property. Here is an example:

database: &default
server:
ip: 192.168.1.5
port: 2000
db_name: test
user:
name: root
password: root

# database foo differs from default by only its port and user password
foo_database:
<<: *default
server:
port: 2001
db_name: foo
user:
password: foo_root


I want to get this result:

foo_database.server.ip -> 192.168.1.5
foo_database.server.port -> 2001
foo_database.db_name -> foo
foo_database.user.name -> root
foo_database.user.password -> foo_root


But if you declare like this, you will get these properties incorrect (according to expected values):

foo_database.server.ip -> will be None
foo_database.user.name -> will be None


because new "server" object has only "port" property and it overrides whole old "server" object.

How do I get the kind of inheritance which I want to achieve?

Answer

Unfortunately, you can't get the kind of "inheritance" you want to achieve because YAML's "inheritance" is more like a form of "merging hashes".

Expanding out your configuration at the point you use the *default alias, you have:

foo_database:
  server:
    ip: 192.168.1.5
    port: 2000
  db_name: test
  user: 
    name: root
    password: root

If you use hashes with the same keys afterwards, they will completely overwrite the hashes declared earlier, leaving you with (excuse the formatting):

foo_database:

  server:
    ip: 192.168.1.5
    port: 2000
  db_name: test
  user: 
   name: root
   password: root  

  server:
    port: 2001
  db_name: foo
  user:
    password: foo_root

So, in your case, it would seem that since the config is not exactly the same, DRYing up your configuration using anchors and aliases probably isn't the right approach.

More references on this issue below:

Edit

If you really wanted to, I think you could reconfigure your YAML as below to get exactly what you want, but in your case, I would say the extra obfuscation isn't worth it:

server_defaults: &server_defaults
  ip: 192.168.1.5
  port: 2000

user_defaults: &user_defaults
  name: root
  password: root

database: &default
  server:
    <<: *server_defaults
  db_name: test
  user: 
    <<: *user_defaults

foo_database:
  <<: *default
  server:
    <<: *server_defaults
    port: 2001
  db_name: foo
  user:
    <<: *user_defaults
    password: foo_root
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