PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions

Exhibit

d = {'a':1, 'b':2}
for k, v in d.items():
    if k == 'a':
        del d[k]
print('done')

Refer to the exhibit. What happens when this code is executed?

⚠ Common exam trap

The PCEP exam often tests the misconception that deleting dictionary keys during iteration is safe or only causes a KeyError, when in fact Python explicitly raises a RuntimeError to prevent undefined behavior.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

RuntimeError: dictionary changed size during iteration

Modifying a dictionary (adding or deleting keys) while iterating over it with a for loop raises a RuntimeError in Python. The code attempts to delete keys from the dictionary `d` during iteration, which changes the dictionary's size and triggers the error before 'done' is printed.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Prints 'done' with no error
Why it's wrong here
Deleting an item during iteration raises RuntimeError.
✓
RuntimeError: dictionary changed size during iteration
Why this is correct
Correct: The dictionary size changed during iteration.
✗
KeyError: 'a'
Why it's wrong here
KeyError would occur if accessing a missing key, but here we are deleting an existing key.
✗
Nothing; the code runs silently
Why it's wrong here
The code raises an exception.

Go deeper

PCEP Practice Question: Functions, Tuples, Dictionaries and Exceptions

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question