python_cloud_function_get_signed_url.py

# Copyright 2019 Google LLC.
# SPDX-License-Identifier: Apache-2.0
# This snippet shows you how to use Blob.generate_signed_url() from within compute engine / cloud functions
# as described here: https://cloud.google.com/functions/docs/writing/http#uploading_files_via_cloud_storage
# (without needing access to a private key)
# Note: as described in that page, you need to run your function with a service account
# with the permission roles/iam.serviceAccountTokenCreator

import os, google.auth
from google.auth.transport import requests
from google.auth import compute_engine
from datetime import datetime, timedelta
from google.cloud import storage

auth_request = requests.Request()
credentials, project = google.auth.default()
storage_client = storage.Client(project, credentials)
data_bucket = storage_client.lookup_bucket(os.getenv("BUCKET_NAME"))
signed_blob_path = data_bucket.blob("FILENAME")
expires_at_ms = datetime.now() + timedelta(minutes=30)
# This next line is the trick!
signing_credentials = compute_engine.IDTokenCredentials(auth_request, "", service_account_email=credentials.service_account_email)
signed_url = signed_blob_path.generate_signed_url(expires_at_ms, credentials=signing_credentials, version="v4")

If all of the above did not work for you, this worked for me:
https://stackoverflow.com/a/78647388/9713831