Getting an oauth request token from etrade in Pyth

I'm trying to get an oauth request token from the etrade api (sandbox) in Python with this thing:

import requests
from oauthlib.oauth1 import Client

consumer_key = 'foo'     # actual key used
consumer_secret = 'bar'  # actual secret used
request_url = 'https://etwssandbox.etrade.com/oauth/sandbox/request_token'

client = Client(consumer_key, client_secret = consumer_secret)
uri, headers, body = client.sign(request_url)
add_params = ', realm="", oauth_token= "", oauth_callback="oob"'
headers['Authorization'] += add_params

r = requests.get(url = uri, headers = headers)
print(r.text) # abbreviated resp: " . . . .auth_problem=consumer_key_rejected,oauth_problem_advice=The oauth_consumer_key foo can be used only in SANDBOX environment . . .

The header generated is:

{'Authorization': 'OAuth oauth_nonce="99985873301258063061424248905", oauth_timestamp="1424248905", oauth_version="1.0", oauth_signature_method="HMAC-SHA1", oauth_consumer_key="foo", oauth_signature="A7ZY91UyZz6NfSGmMA5YWGnVM%2FQ%3D", realm="", oauth_token= "", oauth_callback="oob"'}

I have also tried the url: 'https://etwssandbox.etrade.com/oauth/sandbox/rest/request_token'

And I have tried the header without the add_params (it seems to need the blank oauth_token?)

Note: Confusingly, the response periodically comes back: "Http/1.1 400 Bad Request" with exactly the same url/header.

Any idea what I'm doing wrong?

A helpful person at etrade clarified for the doc-challenged that all oauth api requests (whether you are working in the sandbox or not) need to be sent to the main api url: 'https://etws.etrade.com/oauth/{api}'.

It is only after authenticating a session that the sandbox urls should be used: 'https://etwssandbox.etrade.com/{non-oauth-module}/sandbox/rest/{api}'.

In case others are having problems authenticating a session with etrade in python3, this works in the sandbox at least:

from rauth import OAuth1Service
import webbrowser 

def getSession():
    # Create a session
    # Use actual consumer secret and key in place of 'foo' and 'bar'
    service = OAuth1Service(
              name = 'etrade',
              consumer_key = 'foo',
              consumer_secret = 'bar',
              request_token_url = 'https://etws.etrade.com/oauth/request_token',
              access_token_url = 'https://etws.etrade.com/oauth/access_token',
              authorize_url = 'https://us.etrade.com/e/t/etws/authorize?key={}&token={}',
              base_url = 'https://etws.etrade.com')

    # Get request token and secret    
    oauth_token, oauth_token_secret = service.get_request_token(params = 
                                  {'oauth_callback': 'oob', 
                                   'format': 'json'})

    auth_url = service.authorize_url.format(consumer_key, oauth_token)

    # Get verifier (direct input in console, still working on callback)
    webbrowser.open(auth_url)
    verifier = input('Please input the verifier: ')

    return service.get_auth_session(oauth_token, oauth_token_secret, params = {'oauth_verifier': verifier})

# Create a session
session = getSession()

# After authenticating a session, use sandbox urls
url = 'https://etwssandbox.etrade.com/accounts/sandbox/rest/accountlist.json'

resp = session.get(url, params = {'format': 'json'})

print(resp)