Very simple Python sample to generate a refresh token for the YouTube API. Note that this will will also create a file called generate_token.py-oauth that contains this information.

generate_token.py

#!/usr/bin/python

import httplib2
import os
import sys

from apiclient.discovery import build
from oauth2client.file import Storage
from oauth2client.client import flow_from_clientsecrets
from oauth2client.tools import run


# CLIENT_SECRETS_FILE, name of a file containing the OAuth 2.0 information for
# this application, including client_id and client_secret. You can acquire an
# ID/secret pair from the API Access tab on the Google APIs Console
#   http://code.google.com/apis/console#access
# For more information about using OAuth2 to access Google APIs, please visit:
#   https://developers.google.com/accounts/docs/OAuth2
# For more information about the client_secrets.json file format, please visit:
#   https://developers.google.com/api-client-library/python/guide/aaa_client_secrets
# Please ensure that you have enabled the YouTube Data API for your project.
CLIENT_SECRETS_FILE = "client_secrets.json"

# Helpful message to display if the CLIENT_SECRETS_FILE is missing.
MISSING_CLIENT_SECRETS_MESSAGE = """
WARNING: Please configure OAuth 2.0

To make this sample run you will need to populate the client_secrets.json file
found at:

   %s

with information from the APIs Console
https://code.google.com/apis/console#access

For more information about the client_secrets.json file format, please visit:
https://developers.google.com/api-client-library/python/guide/aaa_client_secrets
""" % os.path.abspath(os.path.join(os.path.dirname(__file__),
                                   CLIENT_SECRETS_FILE))

# A limited OAuth 2 access scope that allows for uploading files, but not other
# types of account access.
YOUTUBE_READONLY_SCOPE = "https://www.googleapis.com/auth/youtube.readonly"
YOUTUBE_API_SERVICE_NAME = "youtube"
YOUTUBE_API_VERSION = "v3"

flow = flow_from_clientsecrets(CLIENT_SECRETS_FILE,
  message=MISSING_CLIENT_SECRETS_MESSAGE,
  scope=YOUTUBE_READONLY_SCOPE)

storage = Storage("%s-oauth2.json" % sys.argv[0])
credentials = storage.get()

if credentials is None or credentials.invalid:
  credentials = run(flow, storage)

print "Access Token: %s" % credentials.access_token
print "Refresh Token: %s" % credentials.refresh_token

the token generated with this code is invalid and returning a invalid credential error.