How to represent maps in YAML

Introduction

Maps are data structures used to store key-value pairs. There are two ways to represent maps in YAML:

  • Mapping nodes

  • Mapping scalars

Mapping nodes are used to represent complex data structures, such as hashes or dictionaries. In a mapping node, each key must be unique and each key can have only one value. The order of the keys is not important.

Mapping scalars are used to represent simple data structures, such as arrays or tuples. In a mapping scalar, each key must be unique but each key can have multiple values. The order of the keys is important.

Syntax

map:
  - key1: value1
  - key2: value2
  - key3: value3

In YAML, maps are represented using the colon (:) character. We can also use quotation marks (" or ') to enclose the keys and values if we need to.

Example

foo: bar
baz: qux

We can also nest maps within maps:

foo:
  nested_map:
    key: value

As we can see, the indentation is important in YAML. The nested map must be indented further than the parent map.

Representing maps in YAML

The following code snippet shows how to represent employee data using maps in YAML:

---
name: John Smith
age: 25
address:
  street: 123 Main Street
  city: New York
  state: NY
  zip: 10001
...

In the example above, the first key is name and the value is John Smith. The second key is age and the value is 25. The third key is address and the value is a nested map with four keys and four values. The fourth key is zip and the value is 10001.

Creating maps in YAML using Python

In Python, a YAML map is a dictionary. It's a collection of key-value pairs, where the keys are unique strings and the values can be anything.

To create a YAML map in Python, we first need to install the PyYAML module. Once we have PyYAML installed, we can create a YAML map:

import yaml
from yaml import BaseLoader
my_map = yaml.load("""
key1: value1 
key2: value2 
key3: value3
""", BaseLoader)
print(my_map)
print(my_map['key1']) # Outputs value1
print(my_map['key2']) # Outputs value2 
print(my_map['key3']) # Outputs value3

The code above will output the following dictionary:

{'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}

The keys in a YAML map must be unique. We can access the values in a YAML map using square brackets:

my_map['key1']

If we try to access a key that doesn't exist, we get an error.

Conclusion

To represent a map in YAML, we can use either mapping nodes or mapping scalars. In most cases, it is recommended to use mapping nodes. However, if we need to preserve the order of the keys, then we should use mapping scalars. We also learned how to create and parse YAML maps in Python.

Note: If you want to learn more about YAML, take the course Learn YAML from Scratch.

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