[Python] Spotify API {'error': 'invalid_client'} Authorization Code Flow [400]

This is one of my many attempts at making a POST request to https://accounts.spotify.com/api/token.

Scope was set to 'playlist-modify-public, playlist-modify-private'.

I'm using Python 3.7, Django 2.1.3.

No matter what I do, response_data returns {'error': 'invalid_client'}

I've tried many things, including passing the client_id/client_secret inside the body of the request as per the official Spotify documentation for this particular request... to no avail.

Please help!

def callback(request):

auth_token = request.GET.get('code') # from the URL after user has clicked accept
code_payload = {
'grant_type': 'authorization_code',
'code': str(auth_token),
'redirect_uri': REDIRECT_URI,
}

auth_str = '{}:{}'.format(CLIENT_ID, CLIENT_SECRET)
b64_auth_str = base64.b64encode(auth_str.encode()).decode()

headers = {
'Content-Type': 'application/x-www-form-urlencoded',
'Authorization': 'Basic {}'.format(b64_auth_str)
}

post_request = requests.post(SPOTIFY_TOKEN_URL, data=code_payload, headers=headers)

response_data = json.loads(post_request.text)
# ==> {'error': 'invalid_client'}

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