YAML Syntax

From Ever changing code


YAML syntax styles
Block style Flow style
Human frendly, less compact An extension of JSON, foldinf long lines, tags and anchors
host: host-1
datacenter:        #mapping
  location: Poland #key-value mapping indentention
  cabinet: 11
roles:             #list
  - web
  - dns
host: "host-1"
datacentrer: { location:
  Poland , cabinet: 11 }
roles: [ web , dns ] 
# {} kv mapping
# [] list (array)


  • Printable Unicode
  • Unsupported symbols
    • C0/C1 blocks
      • Exceptions: Tab, Line Feed, Carrage Return, Delete, Next line
    • Surrogates
  • Encoding: UTF-8, 16, 32
  • To be JSON compatibile must be UTF-32


Mappings are also known as:

  • assosiative arrays
  • hash tables
  • kv pairs
  • collection (YAML specific term, that groups things indented at the same level)
Mappings, the colon-space combination that marks it as a mapping
Block style Flow style
host: host-1
  location: Poland
  cabinet: 11
# cabinet: 12   #the same keys are not allowed
host: host-1
datacentre: { location: Poland, cabinet: 11 }

Sequences (list)

Also known as:

  • Lists, dictionary, arrays or collections
  • Denoted with a dash - and space
  • Can be combined with mappings:
    • mapping of sequences
    • sequence of mapping
    • list of maps


host: host-1     # sequence/list of mappings
  - datacenter:
    location: Poland
    cabinet: 11
roles:           # sequence
  - web
  - dns
  - ""  #blank lines are not allowed, thus double-quotes are required

Flow style sequence

roles: [ web, dns, "" ]

Examples of lists and maps

Sequences are lists

- Item 1
- Item 2

Mappings are key-value pairs

key1: value1
key2: value2

A sequence of mappings:

- key1: value1
  key2: value2

A mapping of mappings:

  key1: value1
  key2: value2


Scalar is a string, number of boolean with whitespace permitted.

host: host-1    
  location: "Poland\n" # double-quotes allow escape sequences, "\n" - will be read as new-line
  cabinet: '11'        # change a number into string single or double-quotes
  - web
  - dns
  - ""  #blank lines are not allowed, thus double-quotes are required
comments: | # pipe (multiline scalar) it preserves newlines
  Comment 1 note of the required indent
  Comment 2
comments_other: > # right chevron (folded scalar) does not preserve new lines, but helps with code readibility
                  # converts newlines /spacing into single space.
  Other_comment is that indent is still needed
  Another comment.
<tab><tab> item-1 this preserves new lines to create a list
<tab><tab> item-2

Block Scalars - control multiline string

Check ^^ link above.


YAML allows for multiple directive/documents in one file. The document/directive starts with --- but it's often optional for single directive files. Ansible requires it but not Salt. The delimiters are reuired for multi-directive files.

host: host-1
  location: Poland
host: host-2
  Location: Spain
... # (optinal) mark end of directive/collection without cloasing the data stream


# Valid comment1 - must be a space in between '# comment
host: host-1 # Inline: valid comment2, comments in the middle are not allowed
datacenter:  # Inline comment3, all inline comments come at the end of line
             # <- any empty lines are read as comments too 
  location: Poland
host: host-2
  Location: Spain
... #(optimal) mark end of directive/collection without closing the data stream


Tags provide us with three functions:

  • ability to assign a universal resource indicator (URI)
  • the ability to assign local tags to that indicator, using eg. ! WAW
  • change how the YAML parser reads certain scalars when processing the YAML, using eg.!!str (!!) indicator to change the data type of a scalar, used to change a data type
  • a custom URI (Universal Resource Indicator). These can be used with PyYAML or other YAML APIs by exposing resources
  • to set local tags - reference only within local directive/file


# assign a URI header
%TAG ! tag:hostsdata:warsaw:  # syntax: '%TAG ! tag:reference:path'
                              # can use also prefix eg '!foo' but '!' is enough

Local tags

# Warsaw DataCentre
%TAG ! tag:hostsdata:warsaw:   # assign a URI header by creating a local-tag-reference-path
host: warsaw-1
  location: !WAW Warsaw        # create a local-tag-reference (assign local tags to URI)
  cab: !!str 11                # changed number to string

# Reference 'location by calling the tag:

Default data types:

  • seq - Sequence
  • map - Map
  • str - String
  • int - Integer
  • float - Floating-point decimal
  • null - Null
  • binary - Binary code
  • omap - Ordered map
  • set - Unordered set


Anchors allow to reuse data across a YAML file.

  • & define an anchor by prefix to assign some data a name, you can reuse the name to reassign new values and used itm in different place of the file
  • use the * to reference data with that name to call it repeatedly throughout the file
host: waw-1
  location: &WAW Warsaw # set an anchor
  cabinet: !!str 11
roles: &ROLES
# &ROLES it's also accepted in newline, as long there is a space after a ':'
  - web
  - db
host: krk-2
  location: *WAW  # reference the anchor, this pull also the comment
  cabinet: !!str 12
roles: *ROLES  # when the file is parsed, data will be pulled from
               # &ROLES anchor

Override example

master_cipher: &master_cipher
  alias: "default.master"
  plain: "password1"
  passhash: "aaaaaaaaaaaa=="
slave_cipher: &slave_cipher
  alias: "default.slave"
  plain: "password2"
  passhash: "bbbbbbbbbbbb=="
  - << : *master_cipher              # '<<' part of syntax, points where data structure will be inserted (anchored)
  - << : *master_cipher              # 'master_cipher' data structure will be expanded here, where
    alias: "default.master.common"   #   'alias' value will get overriden
  - << : *slave_cipher               # 'slave_cipher' data structure will be expanded here
    alias: "default.slave.common"    #   'alias' value will get overriden


  yes: 49.7 # this is boolean as it's YAML recognized world, fix by "yes"
  nah: 50.1