Refering to a directory in a Flask app doesn't work unless the path is absolute

In Python (and most languages), where the code resides in a package is different than what the working directory is when running a program. All relative paths are relative to the current working directory, not the code file it's written in. So you would use the relative path nltk_data/ even from a blueprint, or you would use the absolute path and leave no ambiguity.

The root_path attribute on an app (or blueprint) refers to the package directory for the app (or blueprint). Join your relative path to that to get the absolute path.

resource_path = os.path.join(app.root_path, 'enltk_data')

There's probably no reason to be appending this folder every time you call a view. I'm not familiar with nltk specifically, but there's probably a way to structure this so you set up the data path once when you create your app.

project    /    app    /    blueprint
                       /    data

                            ^ join with root_path to get here
                ^ app.root_path always points here, no matter where cwd is
^ current working directory