GoCardless bad request error

gistfile1.py

# when get_gc_partner_authorization_url() is called, returns a URL
# when that URL is visited in the browser, error occurs
# error is: "invalid client_id or redirect_uri for sandbox environment"

gocardless.environment = settings.GOCARDLESS_ENVIRONMENT

gocardless.set_details(
	app_id=settings.GOCARDLESS_MERCHANT_APP_ID, # taken from https://dashboard-sandbox.gocardless.com/developer/api-keys
    app_secret=settings.GOCARDLESS_MERCHANT_APP_SECRET, # taken from https://dashboard-sandbox.gocardless.com/developer/api-keys
    access_token=settings.GOCARDLESS_MERCHANT_ACCESS_TOKEN, # taken from https://dashboard-sandbox.gocardless.com/developer/api-keys
    merchant_id=settings.GOCARDLESS_MERCHANT_ID # taken from https://dashboard-sandbox.gocardless.com/developer/api-keys
    )

def get_gc_partner_authorization_url(prefill_merchant_details):
	"""
	passes merchant details to GoCardless and returns a URL, where a merchant can authorise with GoCardless
	info: https://developer.gocardless.com/python/#merchant-account-authorization
	"""

	client = gocardless.Client(
		settings.GOCARDLESS_PARTNER_APP_ID, # taken from https://partner-sandbox.gocardless.com/developer/api-keys
		settings.GOCARDLESS_PARTNER_APP_SECRET # taken from https://partner-sandbox.gocardless.com/developer/api-keys
	)

	merchant_details = {
		"user": {
                # some processing happens here to set up merchant_details var from prefill_merchant_details...
		}
	}	

	redirect_uri = settings.BASE_URL + '/api/v1/gocardless/authorize-merchant'

	url = client.new_merchant_url(
	  redirect_uri=redirect_uri,
	  merchant=merchant_details
	)

	return url

Turns out error was caused by not setting the redirect_uri in GC sandbox settings.