Control structures (if, for) in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to loop over the list and print each item.

Flask

items = ['apple', 'banana', 'cherry']
for [1] in items:
    print([1])

Drag options to blanks, or click blank then click option'

Ai

Bitem

Cfruit

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not consistent inside the loop.

Forgetting to use the loop variable in the print statement.

The variable item is used to represent each element in the list during the loop.

2fill in blank

medium

Complete the code to check if a user is logged in and show a welcome message.

Flask

if [1] == True:
    print('Welcome back!')
else:
    print('Please log in.')

Drag options to blanks, or click blank then click option'

Alogged_in

Buser_logged

Clogin_status

Dis_logged

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not defined or inconsistent.

Forgetting to compare the variable to True explicitly.

The variable logged_in is commonly used to check if the user is logged in.

3fill in blank

hard

Fix the error in the for loop to correctly iterate over the range of numbers.

Flask

for num in range([1]):
    print(num)

Drag options to blanks, or click blank then click option'

Arange(10)

B1, 10

C0, 10

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Passing multiple arguments without unpacking.

Using range(10) inside the range function.

The range function needs a single integer to define the stop value. Using 10 iterates from 0 to 9.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps words to their lengths for words longer than 3 letters.

Flask

lengths = {word: [1] for word in words if len(word) [2] 3}

Drag options to blanks, or click blank then click option'

Alen(word)

B>

C<

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using the word itself as the value instead of its length.

Using the wrong comparison operator.

The dictionary maps each word to its length using len(word). The condition filters words longer than 3 letters using >.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase words to their lengths for words longer than 4 letters.

Flask

result = { [1]: [2] for w in words if len(w) [3] 4 }

Drag options to blanks, or click blank then click option'

Aw.upper()

Blen(w)

C>

Dw

Attempts:

3 left

💡 Hint

Common Mistakes

Using the original word as the key instead of uppercase.

Using the wrong comparison operator or value.

The key is the uppercase word using w.upper(). The value is the length len(w). The condition filters words longer than 4 letters using >.

Practice

1. What does the {% if %} statement do in a Flask template?

easy

A. It repeats HTML for each item in a list.

B. It imports another template file.

C. It defines a function in the template.

D. It shows HTML only when a condition is true.

Solution

1. Step 1: Understand the purpose of {% if %}

   The {% if %} statement checks a condition and controls whether some HTML is shown.

2. Step 2: Compare with other options

   {% for %} repeats HTML, functions and imports are not done with {% if %}.

3. Final Answer:

   It shows HTML only when a condition is true. -> Option D

4. Quick Check:

   {% if %} controls conditional display [OK]

Hint: Remember: if = condition, for = repeat [OK]

Common Mistakes:

• Confusing if with for loop
• Thinking if defines functions
• Assuming if imports templates

2. Which of the following is the correct syntax to start a for loop in a Flask template?

easy

A. {% for item in list %}

B. {% loop item in list %}

C. {% foreach item in list %}

D. {% iterate item over list %}

Solution

1. Step 1: Recall Flask template for loop syntax

   The correct syntax uses {% for variable in iterable %}.

2. Step 2: Check options for exact match

   Only {% for item in list %} matches the correct syntax exactly.

3. Final Answer:

   {% for item in list %} -> Option A

4. Quick Check:

   Flask uses {% for variable in iterable %} [OK]

Hint: For loops use 'for variable in list' syntax [OK]

Common Mistakes:

• Using 'loop' or 'foreach' instead of 'for'
• Using 'over' instead of 'in'
• Missing curly braces or percent signs

3. Given this Flask template snippet:

{% for color in ['red', 'green', 'blue'] %}
  
{{ color }}
{% endfor %}

What will be the rendered HTML output?

medium

A. red green blue

B. colorcolorcolor

C. redgreenblue

D. SyntaxError

Solution

1. Step 1: Understand the for loop iteration

   The loop runs over the list ['red', 'green', 'blue'], assigning each to 'color'.

2. Step 2: Render each item inside <p> tags

   Each iteration outputs <p> followed by the color name and </p>.

3. Final Answer:

   <p>red</p><p>green</p><p>blue</p> -> Option C

4. Quick Check:

   Loop prints each color in <p> tags [OK]

Hint: For loops output each item wrapped in HTML [OK]

Common Mistakes:

• Printing the variable name instead of its value
• Missing endfor tag causing syntax error
• Expecting plain text without tags

4. Identify the error in this Flask template code:

{% for user in users %}
  
{{ user.name }}
{% endfor %}

Assuming users is an empty list, what happens?

medium

A. SyntaxError because endfor is missing.

B. No output is shown because the loop does not run.

C. Runtime error because user.name is undefined.

D. Outputs one empty <p></p> tag.

Solution

1. Step 1: Check loop behavior with empty list

   When users is empty, the for loop body does not execute at all.

2. Step 2: Confirm no output or errors occur

   No iterations means no <p> tags output, and no errors happen.

3. Final Answer:

   No output is shown because the loop does not run. -> Option B

4. Quick Check:

   Empty list means zero loop runs, zero output [OK]

Hint: Empty lists skip for loops silently [OK]

Common Mistakes:

• Expecting an error when list is empty
• Thinking endfor is missing
• Assuming output of empty tags

5. You want to display a message "No items found" if a list items is empty, otherwise list each item inside <li> tags within a <ul></ul>. Which Flask template code correctly does this?

hard

A. {% if items %}

{% for item in items %}
{{ item }}
{% endfor %}

{% else %}
No items found
{% endif %}

B. {% for item in items %}
{{ item }}
{% else %}
No items found
{% endfor %}

C. {% if items.length > 0 %}

{% for item in items %}
{{ item }}
{% endfor %}

{% endif %}
No items found

D. {% if items is empty %}
No items found
{% else %}

{% for item in items %}
{{ item }}
{% endfor %}

{% endif %}

Solution

1. Step 1: Use if to check if list has items

   {% if items %}
   {% for item in items %}
   • {{ item }}
   {% endfor %}
   {% else %}

   No items found

   {% endif %} uses {% if items %} which is true if list is not empty.

2. Step 2: Use else to show message when empty

   {% if items %}
   {% for item in items %}
   • {{ item }}
   {% endfor %}
   {% else %}

   No items found

   {% endif %} correctly uses {% else %} to show "No items found" if empty.

3. Final Answer:

   Uses `{% if items %}` with `<ul>` and `{% else %}` for empty case -> Option A

4. Quick Check:

   Use if-else with for loop for empty list check [OK]

Hint: Use if-else around for loop to handle empty lists [OK]

Common Mistakes:

• Using else inside for loop incorrectly
• Checking length with .length (not valid in Jinja)
• Using 'is empty' which is not valid syntax