YAML Primer

This page provides a basic overview of the YAML syntax as used by QFace in the embedded annotations and the external annotations document.

According to the official YAML website, YAML is “a human friendly data serialization standard for all programming languages”.

YAML Foundation

For QFace every YAML file is a dictionary of values.

@singleton: true
@base: QObject
interface Heater {

A dictionary in YAML is expressed like this

In an external YAML file the key on the root level is the fully qualified name of the symbol

    singleton: true
    base: QObject


A dictionary is a simple key: value pair with a colon followed by a space (the space is mandatory).

key: value
key2: value
    key31: value
    key32: value

A nested dictionary can be achieved by a new level of indentation.

An alternate form for a dictionary is this

key3: { key31: value, key32: value }

In a template the dictionay can be used as attributes of an object

{% if interface.tags.key == 'value' %}YES{% endif %}

To test is a key exists you can use the key in dictionary form

{% if 'key' in interface.tags %}YES{% endif %}


A list is an array of values

- item1
- item2
- item3:
    - item31
    - item32

A nested list can be created by indenting the list and postfixing the parent entry with a colon.

An alternate form is


YAML only knows line comments. A comment starts with a # and ends with line.

# this is the key for the value
key: value

Primitive Types

YAML understands different primitive types.


YAML understands strings either as an identifier or quoted using " or '.

You can use code blocks using the | sign. The block continues until the indentation ends. Or the > folding block, where each new line is replaced with a space.


YAML understands different number formats, here is a short list of the most important ones

# an integer
value: 10

# an hex value
value: 0xFF

# a float
value: 1.01


YAML understand different values as true/false.

positive: yes
positive: true
negative: no
negative: false

Besides these words it understand different writing forms (e.g. YES, Yes, Y). Same applies for the negative version.