Removing dictionary entries in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you remove a key-value pair from a dictionary using the del statement?

Use del dict[key] to remove the entry with the specified key from the dictionary.

beginner

What does the pop() method do in a dictionary?

The pop(key) method removes the key-value pair with the given key and returns its value. If the key is not found, it raises a KeyError unless a default value is provided.

intermediate

How can you safely remove a key from a dictionary without causing an error if the key does not exist?

Use pop(key, default_value) where default_value is returned if the key is missing, preventing an error.

intermediate

What is the difference between popitem() and pop() in dictionaries?

popitem() removes and returns the last inserted key-value pair as a tuple, while pop(key) removes the pair with the specified key.

advanced

Can you remove multiple entries from a dictionary at once? If yes, how?

Yes, by using a loop to remove keys one by one or by creating a new dictionary excluding unwanted keys.

Which statement removes the key 'age' from the dictionary person?

A del person['age']

B person.remove('age')

C person.popitem('age')

D person.delete('age')

The del statement with the key removes that entry from the dictionary.

What does person.pop('name') return?

A The value associated with 'name' and removes it from the dictionary

B Removes 'name' but returns None

C Removes the last item in the dictionary

D Raises an error if 'name' exists

pop(key) returns the value for the key and removes the key-value pair.

How can you avoid an error when removing a key that might not exist?

A Use <code>popitem()</code>

B Use <code>del dict[key]</code> without checking

C Use <code>remove(key)</code>

D Use <code>pop(key, default)</code>

Providing a default value to pop() prevents errors if the key is missing.

What does popitem() do?

A Removes all items

B Removes a specified key

C Removes and returns the last inserted key-value pair

D Returns the value of a key without removing it

popitem() removes and returns the last inserted pair as a tuple.

Which method is NOT used to remove entries from a dictionary?

A pop()

B remove()

C del

D popitem()

remove() is not a dictionary method; it is used with lists.

Explain three ways to remove entries from a Python dictionary and when to use each.

Think about whether you want the removed value or just to delete.

How can you safely remove a key from a dictionary without causing an error if the key does not exist?

Consider methods that provide a fallback value.

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