Control structures (if, for) in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this Flask template render?

Given this Flask Jinja2 template snippet, what will be the rendered HTML output if items = ['apple', 'banana', 'cherry']?

Flask

{% raw %}
<ul>
  {% for item in items %}
    {% if item == 'banana' %}
      <li><strong>{{ item }}</strong></li>
    {% else %}
      <li>{{ item }}</li>
    {% endif %}
  {% endfor %}
</ul>
{% endraw %}

A <ul><li>apple</li><li><strong>banana</strong></li><li>cherry</li></ul>

B <ul><li><strong>apple</strong></li><li>banana</li><li><strong>cherry</strong></li></ul>

C <ul><li><strong>apple</strong></li><li><strong>banana</strong></li><li><strong>cherry</strong></li></ul>

D <ul><li>apple</li><li>banana</li><li>cherry</li></ul>

💡 Hint

Look at the if condition inside the for loop and which item it highlights.

The template loops over each item. Only when the item is 'banana' it wraps it in tags. Others are plain list items.

📝 Syntax

intermediate

Identify the syntax error in this Flask template code

Which option correctly fixes the syntax error in this Flask Jinja2 template snippet?

{% raw %}

  {% for user in users %}
    {% if user.active %}
      
{{ user.name }}
    {% endif %}
  {% endfor %}

{% endraw %}

Assume users is a list of user objects with an active boolean attribute.

Flask

{% raw %}
<ul>
  {% for user in users %}
    {% if user.active %}
      <li>{{ user.name }}</li>
    {% endif %}
  {% endfor %}
</ul>
{% endraw %}

A No change needed; the code is correct as is.

B Add a colon after {% if user.active %} like {% if user.active: %}.

C Replace {% endif %} with {% endfor %} to close the for loop.

D Replace {% for user in users %} with {% for user of users %}.

💡 Hint

Check Jinja2 syntax for control structures carefully.

Jinja2 uses {% endif %} to close if blocks and {% endfor %} to close for loops. The given code correctly closes both blocks. No colon is needed in Jinja2 tags. The 'for ... of' syntax is invalid in Jinja2.

❓ state_output

advanced

What is the output count of list items rendered?

Given this Flask Jinja2 template snippet and numbers = [1, 2, 3, 4, 5], how many <li> elements will be rendered?

{% raw %}

  {% for n in numbers %}
    {% if n % 2 == 0 %}
      
{{ n }}
    {% endif %}
  {% endfor %}

{% endraw %}

Flask

{% raw %}
<ul>
  {% for n in numbers %}
    {% if n % 2 == 0 %}
      <li>{{ n }}</li>
    {% endif %}
  {% endfor %}
</ul>
{% endraw %}

A 3

B 2

C 5

D 0

💡 Hint

Count how many numbers in the list are even.

The template renders a list item only if the number is even. From 1 to 5, the even numbers are 2 and 4, so 2 list items are rendered.

🔧 Debug

advanced

Why does this Flask template raise an error?

Consider this Flask Jinja2 template snippet:

{% raw %}

  {% for item in items %}
    {% if item.is_active %}
      
{{ item.name }}
    {% endif %}
  {% endfor %}

{% endraw %}

If items is a list of dictionaries without the key is_active, what error will occur?

Flask

{% raw %}
<ul>
  {% for item in items %}
    {% if item.is_active %}
      <li>{{ item.name }}</li>
    {% endif %}
  {% endfor %}
</ul>
{% endraw %}

A TypeError: 'list' object is not callable

B KeyError: 'is_active'

C UndefinedError: 'dict object' has no attribute 'is_active'

D No error; renders empty list items.

💡 Hint

Check how Jinja2 accesses dictionary keys vs attributes.

Jinja2 treats dot notation as attribute access. Since dictionaries don't have attributes, it raises UndefinedError when trying to access a missing attribute. KeyError is not raised because dot notation does not access keys directly.

🧠 Conceptual

expert

Which option best describes Jinja2's control flow evaluation order?

In Flask's Jinja2 templates, when rendering nested control structures like for loops and if statements, which statement is true about the evaluation order?

A Jinja2 compiles the entire template and renders all <code>if</code> blocks before any <code>for</code> loops.

B The <code>if</code> statements run independently of <code>for</code> loops and do not affect iteration.

C All <code>if</code> conditions are evaluated first for all items, then the <code>for</code> loop renders the filtered results.

D The outer <code>for</code> loop runs first, then for each iteration the inner <code>if</code> condition is evaluated before rendering content.

💡 Hint

Think about how nested loops and conditions work in normal programming.

Jinja2 processes templates top to bottom. For nested loops and conditions, the outer loop runs first. For each item, the inner if condition is checked before rendering that item. This is standard control flow behavior.