Using 'is (not) defined' for safety in Jinja2

j2_defined.py

from flask import *
import random
app = Flask(__name__)
@app.route("/")
def main():
if random.randint(1, 2) == 1:
return render_template("routing.html", promotion="concert")
else:
return render_template("routing.html")
@app.route("/beibs")
def belieber():
return render_template("routing.html", singer="beiber")
@app.route("/tswift")
def swifty():
return render_template("routing.html", singer="tswift")
app.run(debug=True)
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

templates\routing.html

<p>Welcome to ticket central, try <a href="/">re-routing the page</a> a few times.</p>
{% if promotion is defined %}
<p>Concert promotion: <a href="/tswift">Tay Tay</a> or <a href="/beibs">Beiber</a></p>
{% elif promotion is not defined and singer is not defined %}
<p>No promotions.</p>
{% else %}
<p>Tickets have been issued.</p>
{% endif %}
{% if singer is defined %}
{% if singer == "beiber" %}
<p>Have fun at the Justin Beiber concert!</p>
{% else %}
<p>Have fun at the Taylor Swift concert!</p>
{% endif %}
{% endif %}
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX