helper.py

from flask import request, url_for

def url_for_here(**changed_args):
    args = request.args_args.copy()
    args.update(request.view_args)
    args.update(changed_args)
    return url_for(request.endpoint, **args)

app.jinja_env.globals['url_for_here'] = url_for_here

template.html

<a href="{{ url_for_here(page=42) }}">page 42</a>