YAML Syntax

YAML is a simple and powerful language. It is designed to make the language easy to read.

Basic Syntax Rules

Characters are case-sensitive.
Indentation is used for denoting structure.
Only spaces can be used for indentation, but tab characters are not allowed.
The specific number of spaces in the indentation is unimportant as long as parallel elements have the same left justification.
Comments begin with the number sign (#).

Data Types Supported by YAML

Object: A set of key-value pairs, which is also known as maps, hashes, or dictionaries.
Array: A group of values arranged in sequence, which is also known as sequence or list.
Scalar: A single and irreducible value, which is the minimum data unit.

Object

An object is a group of key-value pairs. For key: value, the colon (:) must be followed by a space or newline character. The valid expression is as follows:

animal: pets
plant:
  tree

You can also write multiple key-value pairs into an inline object.

hash: {name: Steve, foo: bar}

However, an error occurs in the following scenario:

foo: somebody said I should put a colon here: so I did
windows_drive: c:

To resolve the issue, you can enclose values in single quotation marks (' ') as follows:

foo: 'somebody said I should put a colon here: so I did'
windows_drive: 'c:'

Array

An array is represented by a hyphen (-) and space. The valid expression is as follows:

animal:
- Cat
- Dog
- Goldfish

You can also use the inline representation.

animal: [Cat, Dog, Goldfish]

Objects and arrays can be used in combination to form a composite structure.

languages:
 - Ruby
 - Perl
 - Python
websites: 
 YAML: yaml.org
 Ruby: ruby-lang.org
 Python: python.org
 Perl: use.perl.org

Scalar

Scalars include strings, Boolean values, integers, floats, null, time, and dates.

String:
By default, a string is not enclosed in quotation marks.
```
str:This_is_a_line
```
If a string contains spaces or special characters, the string needs to be enclosed in quotation marks.
```
str: 'content: a string'
```
Both single and double quotation marks can be used. The difference between them is that the former can identify escape characters while the latter cannot convert special characters.
```
s1: 'content:\n a string'
s2: "content:\n a string"
```
If there is a single quotation mark between two single quotation marks, ensure that two consecutive single quotation marks are used to achieve conversion.
```
str: 'labor''s day'
```
Strings can be written into multiple lines. The lines except the first line must be indented with one space. The newline character will be converted to a space.
```
str: This_is
 a_multi_line
```
Integer:
```
int_value: 314
```
Float:
```
float_value: 3.14
```
Null:
```
parent: ~
```
Time
The time is in the ISO8601 format.
```
iso8601: 2018-12-14t21:59:43.10-05:00
```
Date:
The date is in the compound ISO8601 format: year-month-day.
```
date: 1976-07-31
```

Special Symbols

Three hyphens (---) indicate the start of a YAML file. Three periods (...) indicate the end of a YAML file.

---
# A list of delicious fruits
- Apple
- Orange
- Strawberry
- Mango
...

You can use two exclamation marks (!!) to forcibly convert an integer, a float, or a Boolean value.

strbool: !!str true
strint: !!str 10

For a string occupying multiple lines, you can use a literal block scalar (|) to preserve newlines or folded block scalar (>) to fold newlines. The two symbols are often used in the character strings in YAML files.

this: |
 Foo
 Bar
that: >
 Foo
 Bar

The corresponding objects are as follows:

{ this: 'Foo\nBar\n', that: 'Foo Bar\n' }

It is recommended that you use "|" to meet the requirements of most scenarios.

Comment

YAML supports comments. This is an advantage of YAML compared with JSON.

Comments in YAML files begin with the number sign (#), as shown in the following:

languages:
 - Ruby                # Ruby programming language
 - Go                  # Go programming language
 - Python              # Python programming language