Dictionary creation in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dictionary comprehension with condition

What is the output of this Python code?

Python

result = {x: x**2 for x in range(5) if x % 2 == 0}
print(result)

A {0: 0, 2: 4, 4: 16}

B {1: 1, 3: 9}

C {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

D SyntaxError

💡 Hint

Look at the condition after the for loop in the comprehension.

The comprehension includes only even numbers (x % 2 == 0). So keys are 0, 2, 4 with their squares as values.

❓ Predict Output

intermediate

Output of dictionary from zip function

What does this code print?

Python

keys = ['a', 'b', 'c']
values = [1, 2, 3]
result = dict(zip(keys, values))
print(result)

A {'a': 1, 'b': 2, 'c': 3}

B {1: 'a', 2: 'b', 3: 'c'}

C TypeError

D {('a', 1), ('b', 2), ('c', 3)}

💡 Hint

zip pairs elements from two lists into tuples.

dict(zip(keys, values)) creates a dictionary pairing each key with its corresponding value.

❓ Predict Output

advanced

Output of nested dictionary comprehension

What is the output of this code?

Python

result = {x: {y: x*y for y in range(3)} for x in range(2)}
print(result)

A SyntaxError

B {0: {0: 0, 1: 1, 2: 2}, 1: {0: 0, 1: 1, 2: 2}}

C {0: {0: 0, 1: 0, 2: 0}, 1: {0: 1, 1: 1, 2: 1}}

D {0: {0: 0, 1: 0, 2: 0}, 1: {0: 0, 1: 1, 2: 2}}

💡 Hint

The inner dictionary multiplies x by y for y in 0 to 2.

For x=0, all products are 0. For x=1, products are 0,1,2. So nested dicts reflect these values.

❓ Predict Output

advanced

Output of dictionary creation with duplicate keys

What is the output of this code?

Python

result = {x % 3: x for x in range(6)}
print(result)

A {0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5}

B {0: 0, 1: 1, 2: 2}

C {0: 3, 1: 4, 2: 5}

D KeyError

💡 Hint

Keys repeat because of modulo operation; later values overwrite earlier ones.

Keys are 0,1,2 repeated twice. The last value for each key is kept in the dictionary.

🧠 Conceptual

expert

Error raised by invalid dictionary creation syntax

What error does this code raise?

Python

result = {x: x*2 if x > 2 for x in range(5)}

A TypeError

B SyntaxError

C KeyError

D No error, outputs {3: 6, 4: 8}

💡 Hint

Check the placement of the if condition in the dictionary comprehension.

The syntax is invalid because the if condition is inside the value expression without else, causing SyntaxError.

Practice

1. Which of the following is the correct way to create an empty dictionary in Python?

easy

A. my_dict = ()

B. my_dict = []

C. my_dict = {}

D. my_dict = ''

Solution

1. Step 1: Understand dictionary syntax

   Dictionaries in Python are created using curly braces {}.

2. Step 2: Identify the empty dictionary

   An empty dictionary is represented as {}, not square brackets, parentheses, or quotes.

3. Final Answer:

   my_dict = {} -> Option C

4. Quick Check:

   Empty dictionary = {} [OK]

Hint: Use curly braces {} for dictionaries, brackets [] for lists [OK]

Common Mistakes:

• Using [] which creates a list, not a dictionary
• Using () which creates a tuple, not a dictionary
• Using '' which creates an empty string

2. Which of the following is the correct syntax to create a dictionary with keys 'a' and 'b' and values 1 and 2 respectively?

easy

A. my_dict = ['a':1, 'b':2]

B. my_dict = {'a'=1, 'b'=2}

C. my_dict = ('a':1, 'b':2)

D. my_dict = {'a':1, 'b':2}

Solution

1. Step 1: Check dictionary key-value pair syntax

   Dictionary pairs use colon : between key and value inside curly braces.

2. Step 2: Identify correct syntax

   my_dict = {'a':1, 'b':2} uses curly braces and colons correctly. Options A and C use wrong brackets, and D uses equals sign which is invalid.

3. Final Answer:

   my_dict = {'a':1, 'b':2} -> Option D

4. Quick Check:

   Dictionary pairs use : inside {} [OK]

Hint: Use colons : between keys and values inside {} [OK]

Common Mistakes:

• Using square brackets [] instead of curly braces {}
• Using parentheses () instead of curly braces {}
• Using equals sign = instead of colon :

3. What will be the output of the following code?

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[2])

medium

A. 'two'

B. KeyError

C. 2

D. 'one'

Solution

1. Step 1: Understand dictionary key lookup

   Accessing my_dict[2] retrieves the value for key 2.

2. Step 2: Find value for key 2

   Key 2 maps to the string 'two' in the dictionary.

3. Final Answer:

   'two' -> Option A

4. Quick Check:

   my_dict[2] = 'two' [OK]

Hint: Dictionary[key] returns the value for that key [OK]

Common Mistakes:

• Confusing keys and values
• Expecting the key itself as output
• Mistaking KeyError when key exists

4. The following code throws an error. What is the problem?

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[4])

medium

A. Key 4 does not exist in the dictionary

B. Syntax error in dictionary creation

C. Values must be integers, not strings

D. Dictionary keys must be strings

Solution

1. Step 1: Check dictionary keys

   The dictionary has keys 1, 2, and 3 only.

2. Step 2: Accessing a missing key

   Trying to access my_dict[4] causes a KeyError because key 4 is not present.

3. Final Answer:

   Key 4 does not exist in the dictionary -> Option A

4. Quick Check:

   Accessing missing key causes KeyError [OK]

Hint: Check if key exists before accessing dictionary [OK]

Common Mistakes:

• Assuming all keys exist
• Confusing syntax error with runtime error
• Thinking keys must be strings

5. You want to create a dictionary from two lists: keys = ['name', 'age', 'city'] and values = ['Alice', 30, 'NY']. Which code correctly creates this dictionary?

hard

A. my_dict = {keys: values}

B. my_dict = {keys[i]: values[i] for i in range(len(keys))}

C. my_dict = dict(keys, values)

D. my_dict = dict(zip(values, keys))

Solution

1. Step 1: Understand dictionary creation from two lists

   We need to pair each key with its corresponding value by index.

2. Step 2: Analyze options

   my_dict = {keys[i]: values[i] for i in range(len(keys))} uses dictionary comprehension with index to pair keys and values correctly. my_dict = dict(keys, values) is invalid syntax. my_dict = {keys: values} creates a dictionary with one key (the list) which is invalid. my_dict = dict(zip(values, keys)) reverses keys and values.

3. Final Answer:

   my_dict = {keys[i]: values[i] for i in range(len(keys))} -> Option B

4. Quick Check:

   Use dict comprehension with index to pair keys and values [OK]

Hint: Use dict comprehension with index or zip() to pair keys and values [OK]

Common Mistakes:

• Using dict() with two lists directly
• Swapping keys and values in zip()
• Using list as a key