YAML Tags

YAML Tags are simple identifiers used to represent information about the type of native data structures.

YAML tags can be used in three ways:

to set a custom URI (universal resource indicator), which custom URI is used for reference.
to set local tags. The scope of local tag is only within the YAML file.
to set the data type of a variable.

In YAML, custom URIs must be defined before the beginning of the document i.e. before the three dashes (---).

This can be done using the following syntax:

%TAG ! tag:prefix

where:

For example:

%TAG ! tag:custom-uri
---
server: Linux-Server
datacenter:
    location: new-york
    zones: 12
    points: 34
roles:
    - backup

note

%TAG is never used in practice.

Instead, everyone uses the two special tag handles:

The primary tag handle is !, which by default expands to !. So !bar just resolves to !bar, a local tag, specific to the document and not expected to be unique.
The secondary tag handle is !!, which by default expands to tag:yaml.org,2002:, the prefix YAML uses for its own built-in types. So !!bar resolves to tag:yaml.org,2002:bar, and the tag for a string would more commonly be written as !!str

For declaring a local tag in YAML add the exclamation mark (!) and prefix to the value in the key-value pair.

name: ! local-tag-name value

note

Local tags are not URIs, and they are not expected to be globally unique.

For example:

%TAG ! tag:custom-uri
---
server: Linux-Server
datacenter:
    location: ! LOC new-york # local tag
    zones: 12
    points: 34
roles:
    - backup

You can also use a tag to change the reading functionality of the YAML parser in order to specify a datatype.

This is done by typing two exclamation marks !! followed by the name of the datatype.

For example, we want to specify the zip as a string:

house:
  road: "main road"
  city: "Big City"
  zip: !!str 12345
  floor: 4

note

This can be done with any datatype. See other type in YAML Datatypes Chapter

YAML Tags​