The following error is rather annoying when dealing with Google Cloud functions.

ERROR: (gcloud.functions.deploy) OperationError: code=3, message=Function failed on loading user code. This is likely due to a bug in the user code. Error message: Code in file main.py can't be loaded.
Detailed stack trace:
Traceback (most recent call last):
  File "/env/local/lib/python3.7/site-packages/google/cloud/functions/worker_v2.py", line 359, in check_or_load_user_function
    _function_handler.load_user_function()
  File "/env/local/lib/python3.7/site-packages/google/cloud/functions/worker_v2.py", line 236, in load_user_function
    spec.loader.exec_module(main_module)
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/user_code/main.py", line 4, in <module>
    from google.cloud import secretmanager
ImportError: cannot import name 'secretmanager' from 'google.cloud' (unknown location)

The solution to that is to place a requirements.txt in your project with:

google-cloud-secret-manager==2.0.0

Further read: https://dev.to/googlecloud/using-secrets-in-google-cloud-functions-5aem