How do I create a CSV file and append values to it?

[tom_l]

Use the file notification service:

File

notify:
  - name: write_to_csv
    platform: file
    filename: my_csv_file.csv

In use:

action:
  - service: notify.write_to_csv
    data:
      message: "some,csv,data,to,write,here"

You can use templates in the message, e.g.

action:
  - service: notify.write_to_csv
    data:
      message: "{{ now(),states('sensor.temperature'),states('sensor.humidity') }}"