[google-app-engine] Error parsing yaml file: mapping values are not allowed here

I want to upload an app to Google App Engine:

I get this

Error parsing yaml file:
mapping values are not allowed here
  in "/home/antonio/Desktop/ATI/climate-change/app.yaml", line 2, column 8 

When running

./appcfg.py update /home/antonio/Desktop/ATI/climate-change

with this app.yaml file:

application:climate-change
version: 1
runtime: python27
api_version: 1
threadsafe: true

handlers:
- url: /.*
  script: helloworld.app

line 2, column 8 corresponds to the version line. What is wrong here? Btw, I'm using Ubuntu 12.04 here.

Another cause is wrong indentation which means trying to create the wrong objects. I've just fixed one in a Kubernetes Ingress definition:

Wrong

- path: / 
    backend: 
      serviceName: <service_name> 
      servicePort: <port> 

Correct

- path: /
  backend:
    serviceName: <service_name>
    servicePort: <port>

Maybe this will help someone else, but I've seen this error when the RHS of the mapping contains a colon without enclosing quotes, such as:

someKey: another key: Change to make today: work out more

should be

someKey: another key: "Change to make today: work out more"

I've seen this error in a similar situation to mentioned in Joe's answer:

description: Too high 5xx responses rate: {{ .Value }} > 0.05

We have a colon in description value. So, the problem is in missing quotes around description value. It can be resolved by adding quotes:

description: 'Too high 5xx responses rate: {{ .Value }} > 0.05'

Incorrect:

people:
  empId: 123
  empName: John
    empDept: IT

Correct:

people:
  emp:
    id: 123
    name: John
    dept: IT

My issue was a missing set of quotes;

Foo: bar 'baz'

should be

Foo: "bar 'baz'"

Or, if spacing is not the problem, it might want the parent directory name rather than the file name.

Not $ dev_appserver helloapp.py
But $ dev_appserver hello/

For example:

Johns-Mac:hello john$ dev_appserver.py helloworld.py
Traceback (most recent call last):
  File "/usr/local/bin/dev_appserver.py", line 82, in <module>
    _run_file(__file__, globals())
...
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/yaml_listener.py", line 212, in _GenerateEventParameters
    raise yaml_errors.EventListenerYAMLError(e)
google.appengine.api.yaml_errors.EventListenerYAMLError: mapping values are not allowed here
  in "helloworld.py", line 3, column 39

Versus

Johns-Mac:hello john$ cd ..
Johns-Mac:fbm john$ dev_appserver.py hello/
INFO     2014-09-15 11:44:27,828 api_server.py:171] Starting API server at: http://localhost:61049
INFO     2014-09-15 11:44:27,831 dispatcher.py:183] Starting module "default" running at: http://localhost:8080