Recently I have been developing a web application in python in order to keep track of customer data in an update service. I used the wonderful Google App Engine framework and it does really a great job at providing a powerful and easy to use development environment.
However, I have been pulling my hair about the App Engine launcher that gets installed when you install the SDK on windows: while running my application from a local command line, the graphical user interface insisted on showing my application (added through the "Add existing application..." menu) in red, indicating an error. But which error ?
Today I found out why ! This is all because of an Unicode Byte Order Mark (BOM) in the app.yaml ! I was using the great Notepad++ to edit this essential file for the web application but was not aware that it was encoding it as utf-8 with BOM (which is an invisible mark at the beginning of the file).
Removing this BOM (in Notepad++'s Encoding menu, see image below) worked and now the launcher sees my application when I want to test it locally! Yay... I hope this tip is useful to someone.