Removing dictionary entries in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after removing a key?

Consider the dictionary d = {'a': 1, 'b': 2, 'c': 3}. What is the output after running d.pop('b') and then printing d?

Python

d = {'a': 1, 'b': 2, 'c': 3}
d.pop('b')
print(d)

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

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

C KeyError

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

💡 Hint

The pop method removes the key and returns its value.

The pop method removes the specified key from the dictionary and returns its value. After popping 'b', the dictionary no longer contains 'b'.

❓ Predict Output

intermediate

What happens when deleting a non-existent key?

Given d = {'x': 10, 'y': 20}, what happens when you run del d['z']?

Python

d = {'x': 10, 'y': 20}
del d['z']

A None

B {'x': 10, 'y': 20}

C KeyError

D SyntaxError

💡 Hint

Deleting a key that does not exist causes an error.

Using del on a key that is not in the dictionary raises a KeyError because the key cannot be found.

❓ Predict Output

advanced

What is the output after filtering dictionary entries?

What is the output of this code?

d = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
d = {k: v for k, v in d.items() if v % 2 == 0}
print(d)

Python

d = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
d = {k: v for k, v in d.items() if v % 2 == 0}
print(d)

A {'b': 2, 'd': 4}

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

C {'a': 1, 'b': 2, 'c': 3, 'd': 4}

D TypeError

💡 Hint

Only keep items where the value is even.

The dictionary comprehension keeps only items where the value is divisible by 2 (even numbers). So keys 'b' and 'd' remain.

❓ Predict Output

advanced

What is the output after using popitem()?

Given d = {'x': 1, 'y': 2, 'z': 3}, what does d.popitem() return and what is the dictionary afterwards?

Python

d = {'x': 1, 'y': 2, 'z': 3}
popped = d.popitem()
print(popped)
print(d)

A

('y', 2)
{'x': 1, 'z': 3}

B

('x', 1)
{'y': 2, 'z': 3}

C

KeyError
{'x': 1, 'y': 2, 'z': 3}

D

('z', 3)
{'x': 1, 'y': 2}

💡 Hint

popitem removes and returns the last inserted item in Python 3.7+.

In Python 3.7 and later, dictionaries keep insertion order. popitem removes the last inserted key-value pair, which is ('z', 3).

🧠 Conceptual

expert

Which option causes a runtime error when removing dictionary entries?

Which of the following code snippets will cause a runtime error when trying to remove entries from a dictionary?

A

d = {'a': 1, 'b': 2}
keys = list(d.keys())
for key in keys:
    del d[key]

B

d = {'a': 1, 'b': 2}
for key in d:
    del d[key]

C

d = {'a': 1, 'b': 2}
d.pop('a')

D

d = {'a': 1, 'b': 2}
d.pop('c', None)

💡 Hint

Modifying a dictionary while iterating over it can cause errors.

Deleting keys from a dictionary while iterating over it directly causes a RuntimeError due to dictionary size change during iteration.

Practice

1. What does the pop(key) method do when used on a Python dictionary?

easy

A. Removes the key and returns its value

B. Removes the key without returning its value

C. Returns the value without removing the key

D. Deletes the entire dictionary

Solution

1. Step 1: Understand the pop method

   The pop(key) method removes the specified key from the dictionary and returns its value.

2. Step 2: Compare with other methods

   Unlike del dict[key], pop returns the removed value.

3. Final Answer:

   Removes the key and returns its value -> Option A

4. Quick Check:

   pop(key) = remove key + return value [OK]

Hint: pop returns value and removes key from dict [OK]

Common Mistakes:

• Thinking pop only returns value without removal
• Confusing pop with del which doesn't return value
• Assuming pop deletes entire dictionary

2. Which of the following is the correct syntax to remove a key 'age' from dictionary person without returning its value?

easy

A. person.delete('age')

B. person.pop('age')

C. person.remove('age')

D. del person['age']

Solution

1. Step 1: Identify syntax for deletion without return

   To remove a key without returning its value, use del dict[key].

2. Step 2: Check options

   del person['age'] uses del person['age'], which is correct syntax. Options C and D are invalid methods.

3. Final Answer:

   del person['age'] -> Option D

4. Quick Check:

   del dict[key] removes key without return [OK]

Hint: Use del dict[key] to remove key without return [OK]

Common Mistakes:

• Using pop when no return is needed
• Using non-existent methods like remove or delete
• Wrong syntax like person.delete('age')

3. What is the output of this code?

data = {'a': 1, 'b': 2, 'c': 3}
value = data.pop('b')
print(value, data)

medium

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

B. None {'a': 1, 'c': 3}

C. 2 {'a': 1, 'c': 3}

D. KeyError

Solution

1. Step 1: Use pop to remove key 'b'

   The pop('b') removes key 'b' and returns its value 2.

2. Step 2: Print returned value and updated dictionary

   After removal, dictionary is {'a': 1, 'c': 3}. Printing value and dict shows: 2 {'a': 1, 'c': 3}.

3. Final Answer:

   2 {'a': 1, 'c': 3} -> Option C

4. Quick Check:

   pop returns value and removes key [OK]

Hint: pop returns value and removes key from dict [OK]

Common Mistakes:

• Expecting original dict unchanged
• Thinking pop returns None
• Confusing pop with del which returns nothing

4. What error will this code produce?

info = {'name': 'Alice', 'age': 30}
info.pop('gender')

medium

A. None

B. KeyError

C. Returns 'gender'

D. SyntaxError

Solution

1. Step 1: Understand pop behavior without default

   Calling pop on a missing key without a default raises a KeyError.

2. Step 2: Check code

   Since 'gender' is not in info, info.pop('gender') raises KeyError.

3. Final Answer:

   KeyError -> Option B

4. Quick Check:

   pop missing key without default = KeyError [OK]

Hint: pop missing key without default causes KeyError [OK]

Common Mistakes:

• Expecting None instead of error
• Thinking pop returns the key name
• Confusing with pop(key, default)

5. Given items = {'apple': 5, 'banana': 0, 'cherry': 7}, which code removes all entries with value 0 or less safely without errors?

hard

A. for k in list(items):
if items[k] <= 0:
items.pop(k)

B. for k in items:
if items[k] <= 0:
del items[k]

C. items.pop('banana')

D. del items['banana']

Solution

1. Step 1: Avoid modifying dict while iterating

   Directly deleting keys while iterating causes runtime errors. Using list(items) copies keys safely.

2. Step 2: Use pop to remove keys with value ≤ 0

   Loop over copied keys, check value, and use pop to remove safely.

3. Final Answer:

   for k in list(items): if items[k] <= 0: items.pop(k) -> Option A

4. Quick Check:

   Iterate on list copy + pop keys safely [OK]

Hint: Iterate on list(items) to safely remove keys while looping [OK]

Common Mistakes:

• Deleting keys directly while iterating causes error
• Using pop without copying keys list
• Removing only one key instead of all matching