Question 71 markMCQSection A

$Which of the following statements is true about dictionaries in Python ?$

Q: Which of the following statements is true about dictionaries in Python ?

**Option B — 'A dictionary cannot have two elements with same key'** is the true statement, because dictionary keys must always be unique; if a key is repeated during creation, the later value simply overwrites the earlier one.

Review of Python Basics (Class XI Recap)

Dictionaries — Key Properties

Options

(A)

A dictionary is an example of sequence datatype.

(B)

A dictionary cannot have two elements with same key.

(C)

A dictionary cannot have two elements with same value.

(D)

The key and value of an element cannot be the same.

Official answer

Option B — 'A dictionary cannot have two elements with same key' is the true statement, because dictionary keys must always be unique; if a key is repeated during creation, the later value simply overwrites the earlier one.

dictionary

mapping type

unique keys

key-value pair

hashable keys

Python data structures

Marking scheme

1. $1 mark: for correctly selecting option B.$

Hint

$Dictionary KEYS must be unique; values and key=value combinations have no such restriction.$

Quick oral answer

$A dictionary cannot have two elements with the same key because keys must be unique; if repeated during creation, the last value overwrites the earlier one, but values and even key=value pairs can repeat freely.$

Common mistakes

1. $Believing a dictionary is a sequence type because it can be looped over — it is a mapping type, accessed by key not position.$
2. $Assuming values in a dictionary must also be unique, like keys.$

Analysis and explanation

This question checks the fundamental rules that govern Python dictionaries as a mapping (not sequence) data type. Concept A dictionary is a mapping type that stores data as key-value pairs; keys must be unique and hashable (immutable), but values can repeat freely. Why option B is correct If you try to define two entries with the same key, e.g., {'a':1, 'a':2}, Python does not raise an error — it silently keeps only the LAST value, proving that duplicate keys cannot coexist as separate elements. Why other options are wrong Option A — a dictionary is a mapping type, not a sequence type (lists, tuples, strings are sequence types with positional/index-based access); dictionaries are accessed by key, not position. Option C — values in a dictionary CAN repeat, e.g., {'a':1, 'b':1} is perfectly valid, since only keys must be unique. Option D — a key and its value CAN be identical, e.g., {'a':'a'} or {5:5} is valid Python. Exam trap Students often assume dictionaries are sequence types because they can be iterated over, but 'sequence' in Python specifically means ordered, index-accessible types like list/tuple/string.

Interesting facts

$Since Python 3.7, dictionaries officially guarantee insertion order is preserved — but this does not make them a 'sequence' type in the formal sense used for indexing/slicing.$

$Dictionary keys must be hashable (immutable), which is why lists cannot be used as dictionary keys but tuples can.$

Question details

Q7 of 48 questions

Board: CBSE
Class: Class 12
Subject: Computer Science
Year: 2026
Section: Section A
Marks: 1
Type: MCQ
Chapter: Review of Python Basics (Class XI Recap)
Topic: $Dictionaries — Key Properties$
Bloom's level: understand
Syllabus topic: $Unit 1: Computational Thinking and Programming - 2 — Dictionaries: Keys, Values, and Uniqueness$
Author: CBSE
Reviewed by: Padhantu Academic Team

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Frequently asked questions

$What happens if I define a dictionary with a repeated key?$

$Python does not raise an error; it silently retains only the last key-value pair for that key, discarding the earlier one(s).$

$Can dictionary values be duplicated?$

$Yes, dictionary values have no uniqueness restriction — multiple keys can map to the exact same value.$