In the official Quickstart, it’s recommended to use __name__ when using a single module:

  1. … If you are using a single module (as in this example), you should use __name__ because depending on if it’s started as
    application or imported as module the name will be different
    ('__main__' versus the actual import name). …

However, in their API document, hardcoding is recommended when my application is a package:

So it’s important what you provide there. If you are using a single
module, __name__ is always the correct value. If you however are
using a package, it’s usually recommended to hardcode the name of your
package there.

I can understand why it’s better to hardcode the name of my package, but why not hardcoding the name of a single module? Or, in other words, what information can Flask get when it receives a __main__ as its first parameter? I can’t see how this can make it easier for Flask to find the resources…

__name__ is just a convenient way to get the import name of the place the app is defined. Flask uses the import name to know where to look up resources, templates, static files, instance folder, etc. When using a package, if you define your app in then the __name__ will still point at the “correct” place relative to where the resources are. However, if you define it elsewhere, such as mypackage/, then using __name__ would tell Flask to look for resources relative to instead of mypackage.

Using __name__ isn’t orthogonal to “hardcoding”, it’s just a shortcut to using the name of the package. And there’s also no reason to say that the name should be the base package, it’s entirely up to your project structure.