Transcribing with HTTP and getting 404 UnknownOperationException

I followed the example in https://docs.aws.amazon.com/transcribe/latest/dg/getting-started-http-websocket.html to get a transcription job information and also followed the POST example in https://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html for signature signing. However, I am getting 404 <UnknownOperationException/>

Because of legacy code, we are using python 2.7 so I cannot use AWS SDK / boto3.

Here is the debug output:

BEGIN REQUEST++++++++++++++++++++++++++++++++++++
Request URL = https://transcribe.us-west-2.amazonaws.com/
data = {"TranscriptionJobName": "test1"}
headers = {'Authorization': 'AWS4-HMAC-SHA256 Credential=********/20220419/us-west-2/transcribe/aws4_request, SignedHeaders=content-type;host;x-amz-content-sha256;x-amz-date;x-amz-target;x-amz-security-token, Signature=*******',
 'Content-Type': 'application/x-amz-json-1.0',
 'X-Amz-Date': '20220419T205700Z',
 'X-Amz-Target': 'com.amazonaws.transcribe.Transcribe.GetTranscriptionJob'}

RESPONSE++++++++++++++++++++++++++++++++++++
Response code: 404

<UnknownOperationException/>

And here is the code:

import sys, os, base64, datetime, hashlib, hmac 
import requests # pip install requests
import pprint

# ************* REQUEST VALUES *************
method = 'POST'
service = 'transcribe'
host = 'transcribe.us-west-2.amazonaws.com'
region = 'us-west-2'
endpoint = 'https://transcribe.us-west-2.amazonaws.com/'
content_type = 'application/x-amz-json-1.0'
amz_target = 'com.amazonaws.transcribe.Transcribe.GetTranscriptionJob'

# params for GetTranscriptionJob
request_parameters1 =  '{'
request_parameters1 +=  '"TranscriptionJobName": "test1"'
request_parameters1 +=  '}'

# params for StartTranscriptionJob
request_parameters2 =  '{'
request_parameters2 +=  '"TranscriptionJobName": "test2 via https",'
request_parameters2 +=  '"IdentifyLanguage": true,'
request_parameters2 +=  '"Media": { "MediaFileUri": "********mp3"},'
request_parameters2 +=  '"Subtitles": true'
request_parameters2 +=  '}'

request_parameters = request_parameters1

def sign(key, msg):
    return hmac.new(key, msg.encode("utf-8"), hashlib.sha256).digest()

def getSignatureKey(key, date_stamp, regionName, serviceName):
    kDate = sign(('AWS4' + key).encode('utf-8'), date_stamp)
    kRegion = sign(kDate, regionName)
    kService = sign(kRegion, serviceName)
    kSigning = sign(kService, 'aws4_request')
    return kSigning

# access_key = os.environ.get('AWS_ACCESS_KEY_ID')
# secret_key = os.environ.get('AWS_SECRET_ACCESS_KEY')
access_key = '**************'
secret_key = '**************'
if access_key is None or secret_key is None:
    print('No access key is available.')
    sys.exit()

# Create a date for headers and the credential string
t = datetime.datetime.utcnow()
amz_date = t.strftime('%Y%m%dT%H%M%SZ')
date_stamp = t.strftime('%Y%m%d') # Date w/o time, used in credential scope


canonical_uri = '/'

canonical_querystring = ''

canonical_headers = 'content-type:' + content_type + '\n' + 'host:' + host + '\n' + 'x-amz-date:' + amz_date + '\n' + 'x-amz-target:' + amz_target + '\n'

signed_headers = 'content-type;host;x-amz-content-sha256;x-amz-date;x-amz-target;x-amz-security-token'

payload_hash = hashlib.sha256(request_parameters.encode('utf-8')).hexdigest()

canonical_request = method + '\n' + canonical_uri + '\n' + canonical_querystring + '\n' + canonical_headers + '\n' + signed_headers + '\n' + payload_hash


algorithm = 'AWS4-HMAC-SHA256'
credential_scope = date_stamp + '/' + region + '/' + service + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' +  amz_date + '\n' +  credential_scope + '\n' +  hashlib.sha256(canonical_request.encode('utf-8')).hexdigest()

signing_key = getSignatureKey(secret_key, date_stamp, region, service)

signature = hmac.new(signing_key, (string_to_sign).encode('utf-8'), hashlib.sha256).hexdigest()


authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' +  'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

headers = {'Content-Type':content_type,
           'X-Amz-Date':amz_date,
           'X-Amz-Target':amz_target,
           'Authorization':authorization_header}


# ************* SEND THE REQUEST *************
print('\nBEGIN REQUEST++++++++++++++++++++++++++++++++++++')
print('Request URL = ' + endpoint)
print('data = ' + request_parameters)
print('headers = ' + pprint.pformat(headers))

r = requests.post(endpoint, data=request_parameters, headers=headers)

print('\nRESPONSE++++++++++++++++++++++++++++++++++++')
print('Response code: %d\n' % r.status_code)
print(r.text)