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

Question

I downloaded nltk data into the data directory in my Flask app. The views reside in a blueprint in another directory on the same level as the data directory. In the view I'm trying to set the path to the data, but it doesn't work.

nltk.data.path.append('../nltk_data/')

This doesn't work. If I use the whole path, it does work.

nltk.data.path.append('/home/username/myapp/app/nltk_data/')

Why does the first form not work? How can I refer to the location of the data correctly?

Answer 1

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')

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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.

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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