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Quiz: Dictionaries

Test your understanding of Python dictionaries, key-value access, iteration, and dictionary comprehensions with these questions. Click "Show Answer" to reveal the correct answer and explanation.

1. How do you create a dictionary in Python with two entries?

  1. d = ["name": "Alice", "age": 12]
  2. d = ("name": "Alice", "age": 12)
  3. d = {"name": "Alice", "age": 12}
  4. d = dict[name="Alice", age=12]
Show Answer

The correct answer is C. Dictionaries are created with curly braces {}, with each entry written as key: value and entries separated by commas. Keys must be unique within a dictionary. Values can be any type — strings, numbers, lists, or even other dictionaries.

Concept Tested: Creating Dictionaries

2. Given grades = {"Alice": 95, "Bob": 87}, what does grades["Alice"] return?

  1. "Alice"
  2. 0 (the index of "Alice")
  3. 87
  4. 95
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The correct answer is D. Dictionary lookup with square brackets uses the key to retrieve its associated value. grades["Alice"] returns 95. Unlike lists (which use integer indexes starting at 0), dictionaries use the keys you defined — in this case, the string "Alice" maps to the integer 95.

Concept Tested: Accessing Values by Key

3. What happens when you access a dictionary key that does not exist using square brackets?

  1. Python returns None automatically
  2. Python creates the key with a default value of 0
  3. Python raises a KeyError
  4. Python skips the line and continues
Show Answer

The correct answer is C. If you use d[key] with a key that is not in the dictionary, Python raises a KeyError. This crashes your program unless you catch the error. To safely access a key that might not exist, use d.get(key) which returns None (or a default you specify) instead of crashing.

Concept Tested: KeyError

4. What does grades.get("Dave", 0) return if "Dave" is not in the dictionary?

  1. A KeyError exception
  2. None
  3. "Dave"
  4. 0
Show Answer

The correct answer is D. dict.get(key, default) returns the value for key if it exists, otherwise returns the default value. With grades.get("Dave", 0), if "Dave" is not a key, the method returns 0. Without a default, get() returns None. This is much safer than using d[key] when you are unsure if the key exists.

Concept Tested: Safe Access with get()

5. How do you add a new key "Eve" with value 88 to the dictionary grades?

  1. grades.add("Eve", 88)
  2. grades.append({"Eve": 88})
  3. grades["Eve"] = 88
  4. grades.insert("Eve", 88)
Show Answer

The correct answer is C. Adding a new entry to a dictionary uses the same syntax as updating an existing one: d[key] = value. If the key already exists, this updates its value. If the key is new, this creates a new entry. Dictionaries do not have add(), append(), or insert() methods.

Concept Tested: Adding and Updating Entries

6. Which method returns all key-value pairs as tuples for iterating?

  1. d.keys()
  2. d.values()
  3. d.pairs()
  4. d.items()
Show Answer

The correct answer is D. d.items() returns a view of all (key, value) pairs as tuples. You can iterate over them with for key, value in d.items():. d.keys() gives only the keys, d.values() gives only the values, and d.pairs() is not a valid dictionary method.

Concept Tested: Keys, Values, and Items

7. How do you check whether the key "Bob" is in a dictionary without risking a KeyError?

  1. grades.contains("Bob")
  2. "Bob" in grades
  3. grades.find("Bob")
  4. grades.has_key("Bob")
Show Answer

The correct answer is B. The in operator checks for key membership in a dictionary and returns True or False. "Bob" in grades is the standard Pythonic way to test if a key exists. has_key() was removed in Python 3, contains() is not a dictionary method, and find() is a string method.

Concept Tested: Key Membership with in

8. What does a nested dictionary allow you to represent?

  1. A dictionary where all values must be the same type
  2. Structured data where a value is itself a dictionary, enabling multi-level lookups
  3. A dictionary that automatically sorts its keys in alphabetical order
  4. A dictionary that can only be read, not modified
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The correct answer is B. A nested dictionary is one where values are themselves dictionaries. This allows you to represent structured data with multiple levels — for example, a contacts list where each contact has a phone and email. Access is chained: contacts["Alice"]["phone"] first gets Alice's sub-dictionary, then looks up "phone" in it.

Concept Tested: Nested Dictionaries

9. What does this dictionary comprehension produce?

1
squares = {n: n * n for n in range(1, 4)}
  1. [1, 4, 9]
  2. {1, 4, 9} (a set)
  3. {1: 1, 2: 4, 3: 9}
  4. {0: 0, 1: 1, 2: 4}
Show Answer

The correct answer is C. A dictionary comprehension uses the form {key_expr: value_expr for item in iterable}. Here, n becomes the key and n * n becomes the value. range(1, 4) produces 1, 2, 3 — so the result is {1: 1, 2: 4, 3: 9}. The curly braces with a colon pattern distinguish it from a set comprehension.

Concept Tested: Dictionary Comprehensions

10. What does d.pop("Bob") do if "Bob" exists in the dictionary?

  1. Removes "Bob" from the dictionary and returns its value
  2. Removes "Bob" from the dictionary and returns None
  3. Removes the last item added to the dictionary
  4. Returns "Bob"'s value but leaves it in the dictionary
Show Answer

The correct answer is A. d.pop(key) removes the entry with the given key from the dictionary and returns its value. If grades = {"Bob": 87}, then grades.pop("Bob") returns 87 and grades becomes {}. If the key does not exist, pop() raises a KeyError unless you provide a default: d.pop("Bob", None).

Concept Tested: Removing Entries