App Engine; File referenced by handler not found: main.py - google-app-engine

Question

First post here at stack overflow. Please forgive my posting errors.
I have spent a lot of time at this. I started with the 500 server error.
This long is stating python not found. My app is JS, CSS, and HTML only. (at this point) I have included the yaml, because I cant rule out for myself if I have errors there through my research.
Pointers are greatly appreciated.
Thanks.
My app.yaml:
application: application
version: secureable
runtime: python27
api_version: 1
threadsafe: false
handlers:
- url: /(.*\.(gif|png|jpg|ico|js|css))
static_files: \1
upload: (.*\.(gif|png|jpg|ico|js|css))
- url: /robots.txt
static_files: robots.txt
upload: robots.txt
- url: .*
script: main.py
inbound_services:
- mail
The error:
httpRequest: {
status: 500
0: {
logMessage: "File referenced by handler not found: main.py"
severity: "WARNING"
time: "2017-09-24T21:12:30.191830Z"
}
]
megaCycles: "2"
method: "GET"
requestId: resource: "/index.html"
startTime: "2017-09-24T21:12:30.138333Z"
status: 500
traceId: "618d060203d57aea2bfddc905e350698"
urlMapEntry: "main.py"
userAgent: "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0"
versionId: "secureable"
}
receiveTimestamp: "2017-09-24T21:12:30.926277443Z"
resource: {
labels: {
module_id: "default"
project_id: "Application"
version_id: "secureable"
zone: "us9"
}
type: "gae_app"
}
severity: "WARNING"
timestamp: "2017-09-24T21:12:30.138333Z"
}

Answer 1

If your app is only HTML, CSS, and JS, you can remove the catch-all pointer to the Python script all together and instead use an app.yaml format like the one shown in the Hosting a Static Website on App Engine tutorial:
runtime: python27
api_version: 1
threadsafe: true
handlers:
- url: /
static_files: www/index.html
upload: www/index.html
- url: /(.*)
static_files: www/\1
upload: www/(.*)
Later if you want to add server-side logic with a Python module, you can add in a handler with a script associated with it. When you take that step, you use an import style pointer in the form of [script_name].[var_pointing_to_wsgi_application_in_script]. So if you have main.py and within that a variable called application that is set to your WSGI application, then you would use script: main.application.
Commonly a WSGI application is either webapp2 (example) or Flask (example).

Answer 2

Your script: main.py statement in the handlers section of the app.yaml file is wrong, it should be script: main.app.
From the script row in the Handlers element table (sadly not properly formatted, including the quote from the page source to make it readable):
script
A script: directive must be a python import path, for example,
package.module.app that points to a WSGI application. The last
component of a script: directive using a Python module path is
the name of a global variable in the module: that variable must be a
WSGI app, and is usually called app by convention.