Quick Answer
The answer is that a TypeError will be raised if the key is a list. This occurs because Python dictionary keys must be immutable, or hashable, types, and a list is mutable and therefore unhashable—when you try to use a list as a key, Python cannot compute a stable hash value, triggering the TypeError at runtime. On the Certified Entry-Level Python Programmer PCEP exam, this concept tests your understanding of hashability and the fundamental rule that only immutable objects like strings, numbers, or tuples can serve as dictionary keys. A common trap is assuming any object can be a key, but the exam often presents a mutable type like a list or set to catch that mistake. For a quick memory tip, think “keys must be fixed, not flexible”—if the object can change (like a list), it cannot be a key.
This PCEP practice question tests your understanding of computer programming and python fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A Python program is designed to process user input and store results in a dictionary. The code uses the statement: my_dict[user_key] = value. Under which condition will this statement raise a TypeError?
If the key is None.
Why wrong: None is hashable and allowed as a key.
If the key already exists and you try to assign a different value.
Why wrong: That updates the value, no error.
If the key does not already exist in the dictionary.
Why wrong: That creates a new key, not an error.
If the key is a list.
Lists are unhashable and cannot be used as dictionary keys.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
If the key is a list.
Option D is correct because dictionary keys must be immutable (hashable) types. A list is mutable and therefore unhashable, so using it as a key in a dictionary assignment raises a TypeError. The statement `my_dict[user_key] = value` will fail at runtime if `user_key` is a list.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
If the key is None.
Why it's wrong here
None is hashable and allowed as a key.
If the key already exists and you try to assign a different value.
Why it's wrong here
That updates the value, no error.
If the key does not already exist in the dictionary.
Why it's wrong here
That creates a new key, not an error.
If the key is a list.
Why this is correct
Lists are unhashable and cannot be used as dictionary keys.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Python Institute often tests the distinction between mutable and immutable types as dictionary keys, trapping candidates who think any object can be a key or that duplicate keys cause errors.
Detailed technical explanation
Python dictionaries rely on hash tables, where each key must be hashable (implement `__hash__()` and be immutable). Lists are mutable and lack a stable hash, so they cannot be used as keys. This is enforced by the `__hash__` method raising a `TypeError` for unhashable types. A real-world scenario is when user input is mistakenly stored as a list (e.g., from `input().split()`) and then used as a dictionary key, causing a runtime crash.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the PCEP exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Computer Programming and Python Fundamentals — This question tests Computer Programming and Python Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: If the key is a list. — Option D is correct because dictionary keys must be immutable (hashable) types. A list is mutable and therefore unhashable, so using it as a key in a dictionary assignment raises a TypeError. The statement `my_dict[user_key] = value` will fail at runtime if `user_key` is a list.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This PCEP practice question is part of Courseiva's free Python Institute certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCEP exam.
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