Handling Multiple Query Strings for Jinja2 Templates with Flask

from urllib.parse import urlencode
def qs_active(existing_qs, filter, by):
    """Active when identical key/value in existing query string."""
    qs_set = {(filter, by)}
    # Not active if either are empty.
    if not existing_qs or not qs_set:
        return False
    # See if the intersection of sets is the same.
    existing_qs_set = set(existing_qs.items())
    return existing_qs_set.intersection(qs_set) == qs_set

def qs_toggler(existing_qs, filter, by):
    """Resolve filter against an existing query string."""
    qs = {filter: by}
    # Don't change the currently rendering existing query string!
    rtn_qs = existing_qs.copy()
    # Test for identical key and value in existing query string.
    if qs_active(existing_qs, filter, by):
        # Remove so that buttons toggle their own value on and off.
        rtn_qs.pop(filter)
    else:
        # Update or add the query string.
        rtn_qs.update(qs)
    return urlencode(rtn_qs)

import unittest
from psalms.jinja_filters import qs_active, qs_toggler


class TestMp3Model(unittest.TestCase):
    def test_jinja_filters_qs_toggler(self):
        TEST_QS = {"a": "1", "b": "2"}
        # Adds a new qs
        self.assertEqual("a=1&b=2&c=3", qs_toggler(TEST_QS.copy(), "c", "3"))
        # Changes an existing qs
        self.assertEqual("a=1&b=1", qs_toggler(TEST_QS.copy(), "b", "1"))
        # Removes a qs of the same value.
        self.assertEqual("b=2", qs_toggler(TEST_QS.copy(), "a", "1"))
        # Handle empty existing qs
        self.assertEqual("a=1", qs_toggler({}, "a", "1"))

    def test_jinja_filters_qs_active(self):
        TEST_QS = {"a": "1", "b": "2"}
        self.assertTrue(qs_active(TEST_QS, "a", "1"))
        self.assertTrue(qs_active(TEST_QS, "b", "2"))
        self.assertFalse(qs_active(TEST_QS, "a", "2"))
        self.assertFalse(qs_active(TEST_QS, "b", "1"))
        self.assertFalse(qs_active(TEST_QS, "c", "3"))
        # Handles empty query strings
        self.assertFalse(qs_active({}, "a", "1"))
        self.assertFalse(qs_active({}, "", ""))
        self.assertFalse(qs_active({"a": "1"}, "", ""))

import flask # You should already have this!

# You should already have something like this for your views!
from myapp.views import Main, Music

# You'll add this to your current solution.
from myapp.jinja_filters import qs_active, qs_toggler

# You should already have this!
app = flask.Flask(__name__)

# You'll add this to your current solution
app.jinja_env.filters["qs_active"] = qs_active
app.jinja_env.filters["qs_toggler"] = qs_toggler

# You should already have some other stuff as well, like this:
app.add_url_rule(
    "/music/", view_func=Music.as_view("music"), methods=["GET"]
)

class Music(flask.views.MethodView):
    @login_required
    def get(self):
        # Existing query string as a dictionary
        existing_qs_as_dict = request.args.to_dict()

        # use the existing query string to filter the db.
        # You'll do this your way!
        sidebar, results = navigator(existing_qs_as_dict)    

        # Send existing query string back into the template
        return flask.render_template(
            "music.html",
            sidebar=sidebar,
            results=results,
            existing_qs=existing_qs_as_dict,
        )

{% macro filter_music_button(filter, by) %}
  <a href="/music/?{{ existing_qs|qs_toggler(filter, by) }}">
    <button{% if existing_qs|qs_active(filter, by) %} class="active" {% endif %}>
      {{ by }}
    </button>
  </a>
{% endmacro %}

{% from "macros.html" import filter_music_button with context %}

{% block menu %}
  {% for filter, bys in sidebar.items() %}
    <h3>
      {{ filter }}
    </h3>
    <p>
      {% for by in bys %}
        {{ filter_music_button(filter, by) }}
      {% endfor %}
    </p>
  {% endfor %}
{% endblock %}

<h3>
  year
</h3>
<p>
  <a href="/music/?year=1979&genre=rock">
    <button>1979</button>
  </a>
  <a href="/music/?genre=rock">
    <button class="active">1980</button>
  </a>
  <a href="/music/?year=1981&genre=rock">
    <button>1981</button>
  </a>
</p><h3>
  genre
</h3>
<p>
  <a href="/music/?year=1980&genre=jazz">
    <button>jazz</button>
  </a>
  <a href="/music/?year=1980&genre=pop">
    <button>pop</button>
  </a>
  <a href="/music/?year=1980">
    <button class="active">rock</button>
  </a>
</p>