Refreshing Spotify access_token - oauth-2.0

Question

I'm trying to use the refresh_token to (obviously) refresh the Spotify access_token I have. The documentation gives this as an example curl statement:
curl -H "Authorization: Basic ZjM4Zj...Y0MzE=" -d grant_type=refresh_token -d refresh_token=NgAagA...NUm_SHo https://accounts.spotify.com/api/token
and here is my Python implemtation:
payload = {
'grant_type': 'refresh_token',
'refresh_token': <REFRESH_TOKEN>
}
r = requests.post('https://accounts.spotify.com/api/token', data=payload,
headers={'Authorization': 'Basic <CLIENT_SECRET>'})
In both cases I get back {"error":"invalid_client"}
I've tried passing my client_id and the client_secret in the post data but I always get back invalid_client. Someone here on SO said I need to pass in code, but code is a single use, very short lived object that has already been consumed/expired.
Any thoughts to what I'm doing wrong?

Answer 1

The only problem I can see is that you write:
headers={'Authorization': 'Basic <CLIENT_SECRET>'}
when it, using your notation, should be:
headers={'Authorization': 'Basic ' + base64.b64encode('<CLIENT_ID>:<CLIENT_SECRET>')}
However, that could be simplifed further since you are using requests. As described here: http://docs.python-requests.org/en/latest/user/authentication/ Just remove headers and replace it with auth instead:
auth=('<CLIENT_ID>', '<CLIENT_SECRET>')
However, I guess that this isn't what is actually wrong in your code either, since you somehow managed to get a refresh token which requires that you authorized successfully once before.