Issue
I have 4 files:
flask_blog.py
from flask import Flask, render_template
app = Flask(__name__)
@app.route("/")
@app.route("/home")
def home_page():
return render_template("home.html")
@app.route("/about")
def about():
return render_template("about.html", title = "About")
layout.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
{% if title %}
<title>Flask Blog - {{ title }}</title>
{% else %}
<title>Flask Blog</title>
{% endif %}
</head>
<body>
{% block content %}{% endblock content %}
</body>
</html>
home.html
{% extends "layout.html" %}
{% block content %}{% endblock content %}
about.html
{% extends "layout.html" %}
{% block content %}{% endblock content %}
On my about page, the title is "Flask Blog - About", and in my home page, the title is "Flask blog". Now, this is happening due to the if/else statements in the layout.html file. The if/else statements are:
{% if title %}
<title>Flask Blog - {{ title }}</title>
{% else %}
<title>Flask Blog</title>
{% endif %}
Now, the value of the 'title' variable must be 0 or None, since we are going to the else statement for the home page. Now, my question is, we didn't assign a value to the 'title' variable beforehand. Better yet, we haven't even created the 'title' variable in the flask_blog.py file. How are we not getting a NameError? Shouldn't we get NameError: name 'title' is not defined?
Solution
The templating language used by default in flask is Jinja. Jinja is not Python, and as such it works a little differently sometimes.
When a variable is not defined, if statements in Jinja template evaluate to False. As a result it doesn't throw an exception but instead it just goes to the next else block.
More specifically, an undefined variable becomes of type Undefined, which has it's own documentation page: https://jinja.palletsprojects.com/en/3.1.x/api/#undefined-types
This is useful in a templating language, because it makes it easier to re-use complex templates without having to specify every parameter every time you render it. If a parameter (and corresponding if block) is not relevant for a call to render_template, you can just omit it and don't worry about it.
Answered By - jaapz
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