Posting to a Gist with Github API

There is a Gist client I am maintaining that would do exactly what you need as well as what you asked for in the comments. You can clone it and install it via pip from here.
A brief summary of usage while creating and updating gists as you mentioned –

Create a gist

• Create interactively from an editor like nano, vim or gedit
  • gifc create create.md -d "How to create a gist from cli" -i nano
• Directly enter contents from cli
  • gifc create create.md -d "How to create a gist from cli" -m '''If you want to create a gist from an existing file then you do the following- `./gifc -c create.md -e "How to create a gist from cli" -i file.md`'''
• Take the contents from a file
  • gifc create create.md -d "How to create a gist from cli" -f file.md

Update a gist

• Edit all (or some) files iteratively

  • gifc update ffd2f4a482684f56bf33c8726cc6ae63 -i vi
    You can get the gist id from the get method from earlier

• Change description

  • gifc update ffd2f4a482684f56bf33c8726cc6ae63 -cd "New description"
    You can get the gist id from the get method from earlier

• Edit contents of a file interactively in an editor like nano, vim or gedit

  • gifc update ffd2f4a482684f56bf33c8726cc6ae63 -f file_to_update.md
• Do both
  • gifc update ffd2f4a482684f56bf33c8726cc6ae63 -f file_to_update.md -cd "New description"

If you want to do this for Python3, I’ve updated Bertrand’s previous answer into a function.

"""Interact with Github Gists."""
import urllib
import json


def gist_of_it(token: str, payload: dict, gist_id: str = '', action: str = 'update'):
    """
    Update or create a Github Gist.

    Args:
        token (str): The authorization token to be embededed in the request header.
            Get a Github access token from settings > Developer Settings > Personal access tokens
        payload (dict): A python dict for using in creating or updating the gist with format:

            {
                'description': 'The description for the gist',
                'public': True or False,
                'files': {
                    'the_filename.txt': {'content': 'Whatever the Gist file contents will be.'}
                }
            }

        gist_id (str): The unique identifier of the gist to update. The gist_id can be found at the
            end of your previosuly created Gist url. Not required when creating a new Gist. Defaults to ''.
        action (str, optional): Specify to update and existing Gist or create a new one.
            Values are either `'create'` or `'update'`. Defaults to 'update'.

    Returns:
        str: The json contents of the updated or new Gist page..

    """
    if action not in ['create', 'update']:
        raise ValueError('The action value must be either "create" or "update."')

    access_url: str = 'https://api.github.com/gists'

    # If you want to create a new gist file, do not append the gist_id to the access_url.
    if action == 'update':
        access_url = f'{access_url}/{gist_id}'

    # Create json string to be uploaded and encode it.
    json_data: bytes = json.dumps(payload).encode('utf-8')

    headers: dict = {
        'Content-Type': 'application/json',
        'Authorization': f'Bearer {token}'
    }

    # Post the data.
    req: object = urllib.request.Request(access_url, json_data, headers)

    with urllib.request.urlopen(req) as response:
        gist_page = response.read()  # The updated Gist page.

    return gist_page