CODEWITHSUNDEEP
dockeryaml
eugene
eugene

Reputation: 41665

what's the double arrow (<<: *django) mean in a docker-compose file?

I see something like 

   celerybeat:
     <<: *django

in https://github.com/pydanny/cookiecutter-django example docker files.

What does it mean? I can't google <<: *

Upvotes: 53

Views: 12007

Answers (1)

Yuankun
Yuankun

Reputation: 7793

<< and * are both YAML keys (you can also think of them as operators). And one more key related to your question is &.

In YAML, you can define anchors and later use them. For example,

foo: &myanchor
  key1: "val1"
  key2: "val2"

bar: *myanchor

In this code snippet, & defines an anchor and names it myanchor.

*myanchor references that anchor. Now both foo and bar have the same keys and values.

<< is called the YAML merge key. You can compare it to class inheritance in OOP (not so accurate, but can help you understand). See below example.

foo: &myanchor
  key1: "val1"
  key2: "val2"

bar:
  << : *myanchor
  key2: "val2-new"
  key3: "val3"

In this code snippet, we merge keys and values from foo to bar but override the key2 to a new value. And we add a new key-value pair to bar.

Now bar has the following value:

{
  "bar": {
     "key1": "val1",
     "key2": "val2-new",
     "key3": "val3"
   }
}

Upvotes: 98

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