Is it possible to limit Flask POST data size on a per-route basis?

You'll need to check this for the specific route itself; you can always test the content length; request.content_length is either None or an integer value:

cl = request.content_lengthif cl is not None and cl > 3 * 1024 * 1024:    abort(413)

Do this before accessing form or file data on the request.

You can make this into a decorator for your views:

from functools import wrapsfrom flask import request, abortdef limit_content_length(max_length):    def decorator(f):        @wraps(f)        def wrapper(*args, **kwargs):            cl = request.content_length            if cl is not None and cl > max_length:                abort(413)            return f(*args, **kwargs)        return wrapper    return decorator

then use this as:

@app.route('/...')@limit_content_length(3 * 1024 * 1024)def your_view():    # ...

This is essentially what Flask does; when you try to access request data, the Content-Length header is checked first before attempting to parse the request body. With the decorator or manual check you just made the same test, but a little earlier in the view lifecycle.