What are the common ways to import private data into Google Colaboratory notebooks? Is it possible to import a non-public Google sheet? You can't read from system files. The introductory docs link to a guide on using BigQuery, but that seems a bit... much.
An official example notebook demonstrating local file upload/download and integration with Drive and sheets is available here: https://colab.research.google.com/notebooks/io.ipynb
The simplest way to share files is to mount your Google Drive.
To do this, run the following in a code cell:
from google.colab import drive
drive.mount('/content/drive')
It will ask you to visit a link to ALLOW "Google Files Stream" to access your drive. After that a long alphanumeric auth code will be shown that needs to be entered in your Colab's notebook.
Afterward, your Drive files will be mounted and you can browse them with the file browser in the side panel.
Here's a full example notebook