App Engine uses the app.yaml file to route incoming requests to the appropriate handlers. It’s important to write proper YAML code in this file, otherwise your application may behave erratically or not at all.
One common problem with YAML files is failing to properly separate key:value pairs. The YAML specification requires a colon ( : ) and one space character between the key and the associated value. Here’s an example of a properly formatted YAML key:value pair:
Now here’s an example of a broken app.yaml file:
Notice the error? The threadsafe property has a colon, but no space separating the key ( threadsafe) and the value ( true ). Here’s a screenshot of appcfg refusing to upload this broken file:
If you receive this error, make sure that all of your YAML properties are separated by a colon and a space. One space is enough, don’t use tabs or multiple spaces.