Flask-SQLAlchemy pagination error: Could not build url for endpoint with values ['page']

Question

I am stumped regarding what is causing an error I am experiencing while trying to get pagination to work properly using Flask-SQLAlchemy. Here is the view for the relevant page:

@main.route('/collections/<int:page>', defaults={"page": 1})
def all_collections(page):
    users = User.users_with_bottles().paginate(page=page, per_page=1, error_out=False)

    if users is None:
        return redirect(url_for('main.collection',
                                name=current_user.username))

    total_collections = Bottle.total_collections()

    return render_template('gallery.html',
                           users=users,
                           total_num=total_collections,
                           title='Browse Collections')

And here is the part of gallery.html which seems to cause the issue:

{% for page in users.iter_pages() %}
    {% if page %}
        # the below line seems to be the issue
        <a href="{{ url_for('main.all_collections', page=page) }}">{{ page }}</a>
    {% else %}
        ...
    {% endif %}
{% endfor %}

As far as I can tell, there are pages which should work for linking, as if I do:

{% if page %}
    {{ page }}
{% endif %}

It prints 1 2 3. Removing the url_for part in gallery.html causes http://127.0.0.1:5000/collections/1 to load normally. I'm unsure about the cause of this issue because I have a number of other main. views with dynamic URLs which work fine, so I don't know what I am doing wrong in this instance.

Below is the traceback for this issue:

Traceback (most recent call last):
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 2464, in __call__
return self.wsgi_app(environ, start_response)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 2450, in wsgi_app
response = self.handle_exception(e)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 1867, in handle_exception
reraise(exc_type, exc_value, tb)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 1952, in full_dispatch_request
rv = self.handle_user_exception(e)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 1821, in handle_user_exception
reraise(exc_type, exc_value, tb)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 1950, in full_dispatch_request
rv = self.dispatch_request()
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 1936, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\app\main\views.py", line 46, in all_collections
return render_template('gallery.html',
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\templating.py", line 137, in render_template
return _render(
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\templating.py", line 120, in _render
rv = template.render(context)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\jinja2\environment.py", line 1090, in render
self.environment.handle_exception()
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\jinja2\environment.py", line 832, in handle_exception
reraise(*rewrite_traceback_stack(source=source))
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\jinja2\_compat.py", line 28, in reraise
raise value.with_traceback(tb)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\app\templates\gallery.html", line 1, in top-level template code
{% extends 'base.html' %}
  File "C:\Users\Me\PycharmProjects\flask-sake-project\app\templates\base.html", line 6, in top-level template code
{% block content %}{% endblock %}
  File "C:\Users\Me\PycharmProjects\flask-sake-project\app\templates\gallery.html", line 39, in block "content"
<a href="{{ url_for('main.all_collections', page=page) }}">{{ page }}</a>
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\helpers.py", line 370, in url_for
return appctx.app.handle_url_build_error(error, endpoint, values)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\app.py", line 2216, in handle_url_build_error
reraise(exc_type, exc_value, tb)
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\flask\helpers.py", line 357, in url_for
rv = url_adapter.build(
  File "C:\Users\Me\PycharmProjects\flask-sake-project\venv\Lib\site-packages\werkzeug\routing.py", line 2179, in build
raise BuildError(endpoint, values, method, self)
werkzeug.routing.BuildError: Could not build url for endpoint 'main.all_collections' with values ['page'].

Edit: I will also note that all my searches for a solution to this error have not really helped, hence why I asked this question. I only found one other Stack Overflow post with someone experiencing the same issue as me and it did not have an accepted answer (and few replies generally).

Answer 1

I think what's going wrong how you're handling the default values, you've currently got:

@main.route('/collections/<int:page>', defaults={"page": 1})
    def all_collections(page):
        ...

Then you are passing in a page integer as well url_for('...', page=x) so flask is ending up with two values, and getting grumpy. What you want to do is something like:

@main.route('/collections/<int:page>')
@main.route('/collections/', defaults={'page': 1}  # if no page is asked for-- return page 1.
    def all_collections(page):
        ...

So now /collections/ will use the default value of 1 but if you specify a page you trigger the other route option, and nothing gets confused.