Work with YAML files

YAML (YAML Ain’t Markup Language) - another text format for writing data.

YAML is more human-friendly than JSON, so it is often used to describe actions in software. Playbooks in Ansible, for example.

YAML syntax

Like Python, YAML uses indents to specify the structure of document. But YAML can only use spaces and cannot use tabs. Another similarity with Python is that comments start with # and continue until the end of line.

List

A list can be written in one line:

[switchport mode access, switchport access vlan, switchport nonegotiate, spanning-tree portfast, spanning-tree bpduguard enable]

Or every item in the list in separate row:

- switchport mode access
- switchport access vlan
- switchport nonegotiate
- spanning-tree portfast
- spanning-tree bpduguard enable

When a list is written in such a block, each row must start with ``- `` (minus and space) and all lines in the list must be at the same indentation level.

Dictionary

A dictionary can also be written in one line:

{vlan: 100, name: IT}

Or a block:

vlan: 100
name: IT

Strings

Strings in YAML don’t have to be quoted. This is convenient, but sometimes quotes should be used. For example, when a special character (special for YAML) is used in a string.

This line, for example, should be quoted to be correctly understood by YAML:

command: "sh interface | include Queueing strategy:"

Combination of elements

A dictionary with two keys: access and trunk. Values that correspond to these keys - command lists:

access:
- switchport mode access
- switchport access vlan
- switchport nonegotiate
- spanning-tree portfast
- spanning-tree bpduguard enable

trunk:
- switchport trunk encapsulation dot1q
- switchport mode trunk
- switchport trunk native vlan 999
- switchport trunk allowed vlan

List of dictionaries:

- BS: 1550
  IT: 791
  id: 11
  name: Liverpool
  to_id: 1
  to_name: LONDON
- BS: 1510
  IT: 793
  id: 12
  name: Bristol
  to_id: 1
  to_name: LONDON
- BS: 1650
  IT: 892
  id: 14
  name: Coventry
  to_id: 2
  to_name: Manchester

PyYAML module

Python uses a PyYAML module to work with YAML. It is not part of the standard module library, so it needs to be installed:

pip install pyyaml

Work with it is similar to csv and json modules.

Reading from YAML

Converting data from YAML file to Python objects (info.yaml file):

- BS: 1550
  IT: 791
  id: 11
  name: Liverpool
  to_id: 1
  to_name: LONDON
- BS: 1510
  IT: 793
  id: 12
  name: Bristol
  to_id: 1
  to_name: LONDON
- BS: 1650
  IT: 892
  id: 14
  name: Coventry
  to_id: 2
  to_name: Manchester

Reading from YAML (yaml_read.py file):

import yaml
from pprint import pprint

with open('info.yaml') as f:
    templates = yaml.safe_load(f)

pprint(templates)

The result is:

$ python yaml_read.py
[{'BS': 1550,
  'IT': 791,
  'id': 11,
  'name': 'Liverpool',
  'to_id': 1,
  'to_name': 'LONDON'},
 {'BS': 1510,
  'IT': 793,
  'id': 12,
  'name': 'Bristol',
  'to_id': 1,
  'to_name': 'LONDON'},
 {'BS': 1650,
  'IT': 892,
  'id': 14,
  'name': 'Coventry',
  'to_id': 2,
  'to_name': 'Manchester'}]

YAML format is very convenient for storing different parameters, especially if they are filled manually.

Writing to YAML

Write Python objects to YAML (yaml_write.py file):

import yaml

trunk_template = [
    'switchport trunk encapsulation dot1q', 'switchport mode trunk',
    'switchport trunk native vlan 999', 'switchport trunk allowed vlan'
]

access_template = [
    'switchport mode access', 'switchport access vlan',
    'switchport nonegotiate', 'spanning-tree portfast',
    'spanning-tree bpduguard enable'
]

to_yaml = {'trunk': trunk_template, 'access': access_template}

with open('sw_templates.yaml', 'w') as f:
    yaml.dump(to_yaml, f, default_flow_style=False)

with open('sw_templates.yaml') as f:
    print(f.read())

File sw_templates.yaml:

access:
- switchport mode access
- switchport access vlan
- switchport nonegotiate
- spanning-tree portfast
- spanning-tree bpduguard enable
trunk:
- switchport trunk encapsulation dot1q
- switchport mode trunk
- switchport trunk native vlan 999
- switchport trunk allowed vlan