PART 2 • LEVEL 08

🗂️ Dictionaries

Connect meaningful keys to values for fast lookup, record management, counting, grouping and structured real-world data.

⏱ 95–110 minutes📚 8 concepts💻 5 programs🧠 6-question quiz

Start Lesson ↓Jump to Problems

student_record.pyPython 3

student = {
    "name": "Bhavya",
    "score": 92
}
student["grade"] = "A"

print(student["name"])
print(student.get("grade"))
print(len(student))

OUTPUTBhavya
A
3

LEARNING OUTCOMES

By the End of This Level

You will be able to organise and process key–value data confidently.

Create dictionaries with valid, unique keys.

Access, add, update and safely remove entries.

Iterate through keys, values and key–value pairs.

Model nested records and solve counting problems.

KEY → VALUE

Dictionary Fundamentals

A dictionary is a mutable mapping that associates each unique key with one value.

Dictionary literals use braces and colons: {"name": "Bhavya", "score": 92}. Python preserves insertion order, but dictionary access is based on keys rather than positions.

Mapping

A data structure that retrieves a value using its key instead of a numeric sequence index.

Empty dictionary{} or dict()

String keys{"name": "Venu"}

Numeric keys{1: "One"}

Lengthlen({"a": 1}) → 1

Key rule

Keys must be unique and immutable/hashable, such as strings, numbers or tuples. Lists and dictionaries cannot be dictionary keys.

RETRIEVE SAFELY

Accessing Dictionary Values

Square brackets require the key to exist. The get() method can return a safe default when the key is missing.

Expression	Behaviour	Result
`student["name"]`	Direct lookup	Value or `KeyError`
`student.get("name")`	Safe lookup	Value or `None`
`student.get("grade", "NA")`	Lookup with default	Value or `"NA"`
`"score" in student`	Membership test	Checks keys

EXAMPLE 1

Read Required and Optional Fields

student = {"name": "Bhavya", "score": 92}
print("Name =", student["name"])
print("City =", student.get("city", "Not given"))

OUTPUT

Name = Bhavya
City = Not given

MUTABLE MAPPING

Adding, Updating and Removing Entries

Assignment adds a key when it is new and updates the associated value when the key already exists.

1Choose KeyMeaningful identifier

→

2Check EntryNew or existing

→

3Assign ValueAdd or replace

→

4Use MappingUpdated record

EXAMPLE 2

Update Inventory

stock = {"Pen": 10, "Book": 5}
stock["Book"] = 8
stock["Bag"] = 3
removed = stock.pop("Pen")
print(stock)
print("Removed =", removed)

OUTPUT

{'Book': 8, 'Bag': 3}
Removed = 10

DICTIONARY TOOLKIT

Essential Dictionary Methods

Method	Returns / performs	Typical use
`keys()`	View of keys	Process labels or identifiers
`values()`	View of values	Calculate totals or averages
`items()`	Key–value pairs	Unpack both during iteration
`get(key, default)`	Safe value lookup	Avoid missing-key errors
`update(other)`	Merges entries in place	Add or replace several entries
`pop(key)`	Removed value	Delete and use an entry
`setdefault(key, value)`	Existing or inserted value	Initialise grouping/counting data
`copy()`	Shallow dictionary copy	Preserve the original mapping

Dynamic views

keys(), values() and items() produce views that reflect later changes to the dictionary.

VISIT ENTRIES

Dictionary Iteration

Looping directly over a dictionary visits its keys. Use items() when the loop needs both keys and values.

KEYS

Direct loop

for key in data:
    print(key)

Visits every key.

PAIRS

`items()`

for key, value in data.items():
    print(key, value)

Unpacks each pair clearly.

VALUES

`values()`

for value in data.values():
    print(value)

Visits stored values only.

EXAMPLE 3

Display Subject Scores

scores = {"Python": 92, "SQL": 88}
for subject, score in scores.items():
    print(subject, "=", score)

OUTPUT

Python = 92
SQL = 88

STRUCTURED RECORDS

Nested Dictionaries

A dictionary value can itself be a dictionary, allowing related records to be organised hierarchically.

EXAMPLE 4

Store Multiple Students

students = {
    101: {"name": "Bhavya", "score": 92},
    102: {"name": "Venu", "score": 88}
}
print(students[101]["name"])
print(students[102]["score"])

OUTPUT

Bhavya
88

Read from outside to inside

In students[101]["name"], first retrieve the record stored at key 101, then retrieve its "name" value.

CLASSIC PATTERN

Frequency Counting

A frequency dictionary uses each observed item as a key and its occurrence count as the value.

EXAMPLE 5

Count Characters

text = "banana"
frequency = {}

for character in text:
    frequency[character] = frequency.get(character, 0) + 1

print(frequency)

OUTPUT

{'b': 1, 'a': 3, 'n': 2}

SOLVING WITH MAPPINGS

Common Dictionary Patterns

🔍

Fast Lookup

Map IDs, names or codes directly to associated information.

🔢

Counting

Map each observed value to its frequency.

🗃️

Grouping

Map categories to lists of related values.

🔗

Record Modelling

Represent named fields without relying on numeric positions.

Requirement	Suitable dictionary design
Find a student using roll number	`{roll_number: student_record}`
Count words	`{word: frequency}`
Group students by branch	`{branch: [students]}`
Store configuration	`{setting: value}`

DEBUGGING CORNER

Common Dictionary Mistakes

Missing Key

data["city"]

Direct access raises KeyError when the key is absent; use get() when absence is normal.

Membership

92 in student

The in operator checks dictionary keys, not values.

Duplicate Key

{"a": 1, "a": 2}

The later value replaces the earlier value for the same key.

Changing Size

del data[key]

Do not change dictionary size while iterating over its live view; iterate over a copied list of keys when removal is required.

Debug mappings:1. Print available keys2. Confirm exact key type3. Use get() for optional data4. Inspect nested levels5. Verify update order

QUICK REVISION

📌 Dictionaries — Quick Revision

Review these rules before attempting the quiz and programming problems.

A dictionary stores key–value pairs.

Keys are unique and must be hashable.

Dictionaries are mutable and preserve insertion order.

Square brackets raise KeyError for a missing key.

get() supports safe lookup and defaults.

Assignment adds a new key or updates an existing key.

items() provides key–value pairs.

Direct dictionary iteration visits keys.

Nested dictionaries model structured records.

Frequency dictionaries map each item to its count.

KNOWLEDGE CHECK

🧠 Level 8 Quiz

Choose one answer for each question.

1. Which symbol separates a dictionary key from its value?

Comma Colon Semicolon

2. Which statement about dictionary keys is correct?

Keys must be unique and hashable Keys must always be integers Duplicate keys store every value

3. What does `data.get("x", 0)` return when `"x"` is absent?

KeyError None 0

4. Which method provides key–value pairs?

values() items() keys()

5. What does `"name" in student` check?

Whether "name" is a key Whether "name" is a value Whether the dictionary is empty

6. Which operation merges entries into the same dictionary?

append() add() update()

PRACTICE

🎯 5 Programming Problems — Dictionaries

Follow the CodeBhavya pattern: understand the task, inspect the sample, use the editor, run, check, then compare with the official program.

📈 Dictionaries Practice Progress

Solved0 / 5

Completed with Solution0

Total Score0 / 500

Completion0%

A problem counts as Solved when its output passes before the official solution is opened.

1. Create a Student Record

Read a name and score, store them using meaningful keys and display both values.

Sample input: Bhavya, 92Expected output: Name = Bhavya and Score = 92

💡 Create keys named "name" and "score".

Official Python Program

student = {"name": input(), "score": int(input())}
print("Name =", student["name"])
print("Score =", student["score"])

2. Update Inventory

Read an existing item and its new quantity, then update and display the complete stock mapping.

Sample input: Book, 8Expected output: Stock = {'Pen': 10, 'Book': 8}

Official Python Program

stock = {"Pen": 10, "Book": 5}
item = input()
quantity = int(input())
stock[item] = quantity
print("Stock =", stock)

3. Safe Product Lookup

Read a product name and display its price, or Not found when it is absent.

Sample input: PencilExpected output: Price = Not found

💡 Pass "Not found" as the second argument to get().

Official Python Program

prices = {"Pen": 10, "Book": 50}
item = input()
print("Price =", prices.get(item, "Not found"))

4. Character Frequency

Read text and count every character using a frequency dictionary.

Sample input: bananaExpected output: Frequency = {'b': 1, 'a': 3, 'n': 2}

💡 For each character, assign frequency.get(character, 0) + 1.

Official Python Program

text = input()
frequency = {}
for character in text:
    frequency[character] = frequency.get(character, 0) + 1
print("Frequency =", frequency)

5. Merge Subject Scores

Merge two subject-score dictionaries and display the resulting mapping.

First: {"Python": 90, "C": 85}Second: {"Java": 88, "SQL": 92}Expected output: Scores = {'Python': 90, 'C': 85, 'Java': 88, 'SQL': 92}

💡 Call update() on the first dictionary with the second dictionary.

Official Python Program

scores = {"Python": 90, "C": 85}
more_scores = {"Java": 88, "SQL": 92}
scores.update(more_scores)
print("Scores =", scores)

KEY TAKEAWAY

🎯 Think in Keys and Values

1Choose Key

Use a meaningful unique identifier.

→

2Store Value

Associate the required information.

→

3Retrieve Safely

Select brackets or get().

→

4Process Entries

Iterate, count, group or merge.

Dictionaries make programs clearer when values have meaningful labels: design reliable keys, use safe lookup where appropriate and select iteration views deliberately.

INTERACTIVE LEARNING

🎬 Dictionary Mutation — Visual Flow

Watch one inventory dictionary change through lookup, update and insertion.

PROGRAM TRACING

🔎 Program Tracing — Frequency Counter

Trace the characters in aba and observe how the dictionary count changes.

INTERVIEW PREPARATION

🎤 Dictionaries — Interview Questions

Answer aloud before opening each explanation.

What is the difference between bracket lookup and get()?

Bracket lookup raises KeyError when a key is absent. get() returns None or a supplied default instead.

What types can be dictionary keys?

How do keys(), values() and items() differ?

keys() provides keys, values() provides stored values and items() provides key–value pairs suitable for unpacking.

What happens when a dictionary literal repeats a key?

What is the average time complexity of dictionary lookup?

Lookup, insertion and deletion are O(1) on average because dictionaries use a hash table.

When is a nested dictionary useful?

EXTRA TIPS

💡 Dictionaries — Extra Tips

01
Use descriptive keys such as "student_name" instead of unclear abbreviations.
02
Use get() when a missing key is an expected situation.
03
Use items() to avoid repeatedly looking up a value during iteration.
04
Use setdefault() carefully for grouping, or prefer clearer explicit logic while learning.
05
Copy a dictionary before experimental updates when the original data must remain unchanged.
06
For predictable reports, sort keys before displaying them when insertion order is not meaningful.

EXTRA PRACTICE

✍️ Dictionaries — Extra Practice Questions

Count the frequency of every word in a sentence.
Find the key associated with the highest numeric value.
Invert a dictionary whose values are unique.
Group student names by branch.
Merge two dictionaries and add values for common keys.
Remove all entries whose values are below a threshold.
Build a dictionary mapping numbers from 1 to n to their squares.
Check whether two dictionaries contain identical key–value pairs.
Create a nested dictionary for semester-wise subject marks.
Find the first non-repeating character using frequency counting.

LEVEL 8 CHECKPOINT

Dictionaries Complete

When you can design keys, choose safe lookup, iterate through entries and solve all five dictionary programs without help, mark this level complete and continue.

Continue to Level 9 →

🗂️ Dictionaries

By the End of This Level

Dictionary Fundamentals

Accessing Dictionary Values

Read Required and Optional Fields

Adding, Updating and Removing Entries

Update Inventory

Essential Dictionary Methods

Dictionary Iteration

Direct loop

items()

values()

Display Subject Scores

Nested Dictionaries

Store Multiple Students

Frequency Counting

Count Characters

Common Dictionary Patterns

Fast Lookup

Counting

Grouping

Record Modelling

Common Dictionary Mistakes

📌 Dictionaries — Quick Revision

🧠 Level 8 Quiz

1. Which symbol separates a dictionary key from its value?

2. Which statement about dictionary keys is correct?

3. What does data.get("x", 0) return when "x" is absent?

4. Which method provides key–value pairs?

5. What does "name" in student check?

6. Which operation merges entries into the same dictionary?

🎯 5 Programming Problems — Dictionaries

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

🎯 Think in Keys and Values

🎬 Dictionary Mutation — Visual Flow

🗂️ How Dictionary Entries Change

1. Create the Original Mapping

🔎 Program Tracing — Frequency Counter

🎤 Dictionaries — Interview Questions

💡 Dictionaries — Extra Tips

✍️ Dictionaries — Extra Practice Questions

Dictionaries Complete

`items()`

`values()`

3. What does `data.get("x", 0)` return when `"x"` is absent?

5. What does `"name" in student` check?