Aws Sigv4 Request Signature Does Not Match

I am getting Invalid Request with below error message when trying to create a thing-type using Rest API. I am not using aws-sdk rather python rest client.

I followed the steps given here to sign the request using sigv4. The example given there is working for listing the Thing-Types. So the credentials I am using, are correct. Error Message :

Response code: 403

{"message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details."}

The code I am using :

# See: 
# This version makes a POST request and passes request parameters
# in the body (payload) of the request. Auth information is passed in
# an Authorization header.
import sys, os, base64, datetime, hashlib, hmac
import requests  # pip install requests

# ************* REQUEST VALUES *************
method = 'POST'
service = 'execute-api'
api = '
request_parameter='{"thingTypeProperties":{"searchableAttributes":  ["gas"],"thingTypeDescription":"Devices that reports gas level readings"}}'
host = api.split('/')[2] if api.count('/') >= 2 else ''
region = api.split('.')[1] if api.count('.') >= 1 else ''
endpoint = api.split('?')[0] if api.count('?') >= 0 else api
#request_parameters = api.split('?')[1] if api.__contains__('?') else ''


# Request parameters for CreateTable--passed in a JSON block.
request_parameters = payload
print(host + ' $ ' + region + ' $ ' + endpoint + ' $ ' + request_parameters)
# Key derivation functions. See:
# 
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

# Read AWS access key from env. variables or configuration file. Best practice is NOT
# to embed credentials in code.
access_key = 'XXXXXXXXXXXXX'
secret_key = 'XXXXXXXXXXXXXX'   
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

# ************* TASK 1: CREATE A CANONICAL REQUEST *************
# 

# Step 1 is to define the verb (GET, POST, etc.)--already done.

# Step 2: Create canonical URI--the part of the URI from domain to query
# string (use '/' if no path)
canonical_uri = '/'

## Step 3: Create the canonical query string. In this example, request
# parameters are passed in the body of the request and the query string
# is blank.
canonical_querystring = ''

# Step 4: Create the canonical headers. Header names must be trimmed
# and lowercase, and sorted in code point order from low to high.
# Note that there is a trailing \n.
canonical_headers = 'content-type:' + content_type + '\n' + 'host:' + host + '\n' + 'x-amz-date:' + amz_date + '\n'

# Step 5: Create the list of signed headers. This lists the headers
# in the canonical_headers list, delimited with ";" and in alpha order.
# Note: The request can include any headers; canonical_headers and
# signed_headers include those that you want to be included in the
# hash of the request. "Host" and "x-amz-date" are always required.
# For DynamoDB, content-type and x-amz-target are also required.
signed_headers = 'content-type;host;x-amz-date'

# Step 6: Create payload hash. In this example, the payload (body of
# the request) contains the request parameters.
payload_hash = hashlib.sha256(request_parameters.encode('utf-8')).hexdigest()

# Step 7: Combine elements to create create canonical request
canonical_request = method + '\n' + canonical_uri + '\n' + canonical_querystring + '\n' + canonical_headers + '\n' + signed_headers + '\n' + payload_hash

# ************* TASK 2: CREATE THE STRING TO SIGN*************
# Match the algorithm to the hashing algorithm you use, either SHA-1 or
# SHA-256 (recommended)
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()

# ************* TASK 3: CALCULATE THE SIGNATURE *************
# Create the signing key using the function defined above.
signing_key = getSignatureKey(secret_key, date_stamp, region, service)

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

# ************* TASK 4: ADD SIGNING INFORMATION TO THE REQUEST *************
# Put the signature information in a header named Authorization.
authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' + 'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

# # Python note: The 'host' header is added automatically by the Python 'requests' library.
headers = {'Content-Type': content_type,
           'X-Amz-Date': amz_date,
           'Authorization': authorization_header}

# ************* SEND THE REQUEST *************
print('\nBEGIN REQUEST++++++++++++++++++++++++++++++++++++')

print( 'Request URL = ' + endpoint)


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

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