What is a Dictionary?

A dictionary in Python is a collection of unordered, changeable, and key-value paired items.
It is represented by curly braces {} with keys and values separated by a colon :.

If you want to store data in key-value pairs, you can use a Dictionary. For example, to store a collection of related data like a person’s name and age, you can use a Dictionary. The Dictionary is represented by curly braces {} with keys and values separated by a colon :

person = {"name": "Alice", "age": 25, "city": "New York"}

Here, we have created a dictionary named person. It contains three key-value pairs: “name”: “Alice”, “age”: 25, and “city”: “New York”.

Dictionary Features

Unordered: The items in a dictionary are stored without any specific order. The order of items may vary.
Changeable: You can change, add, or remove key-value pairs in a dictionary after it is created.
Key-Value Pairs: Each item in a dictionary consists of a key and a corresponding value.

Important Methods Used in Dictionary:

Method / Operation	Description	Example
`dict.clear()`	Removes all items from the dictionary.	`d.clear()`
`dict.copy()`	Returns a shallow copy of the dictionary.	`new = d.copy()`
`dict.fromkeys(seq, value)`	Creates a new dictionary with keys from a sequence, all set to the same value.	`dict.fromkeys(['a','b'], 0)` → `{'a':0,'b':0}`
`dict.get(key, default)`	Returns value of the key; if not found, returns `default` (or `None`).	`d.get("x", "Not Found")`
`dict.items()`	Returns a view object of key-value pairs (`dict_items`).	`d.items()`
`dict.keys()`	Returns a view object of keys (`dict_keys`).	`d.keys()`
`dict.pop(key, default)`	Removes key and returns its value. If key not found, returns `default` (or error).	`d.pop("x", "NA")`
`dict.popitem()`	Removes and returns the last inserted key-value pair.	`d.popitem()`
`dict.setdefault(key, default)`	Returns value of key. If not present, inserts with `default`.	`d.setdefault("x", 0)`
`dict.update(other)`	Updates dictionary with items from another dict/iterable.	`d.update({"x": 10})`
`dict.values()`	Returns a view object of values (`dict_values`).	`d.values()`
`len(d)`	Returns number of items in the dictionary.	`len(d)`
`key in d`	Checks if a key exists in the dictionary.	`"name" in d`
`del d[key]`	Deletes a key-value pair by key.	`del d["age"]`
`for k in d:`	Iterates over all keys in dictionary.	`for k in d: print(k)`
`for v in d.values():`	Iterates over all values.	`for v in d.values(): print(v)`
`for k, v in d.items():`	Iterates over key-value pairs.	`for k, v in d.items(): print(k, v)`

Methods that don’t work with Dictionary:

append()
extend()
insert()
sort()
reverse()
count()
index()

Since dictionaries are not sequences but mappings (key → value), these sequence-based methods cannot be used directly on dictionaries. If you want to use such methods, you have to convert the dictionary into a list or tuple first (e.g., `list(d.items())`) and then apply them.

# Original dictionary
d = {"a": 1, "b": 2, "c": 3}
print("Original Dictionary:", d)

# Convert dictionary to list of key-value tuples
lst = list(d.items())   # [('a',1), ('b',2), ('c',3)]
print("List from dictionary:", lst)

# Using list methods
lst.append(("d", 4))             # append a new item
lst.insert(1, ("z", 100))        # insert at index 1
lst.remove(("b", 2))             # remove specific item
lst.reverse()                     # reverse the list
print("List after list operations:", lst)

# Convert back to dictionary
d_new = dict(lst)
print("Dictionary after converting back:", d_new)

Using Dictionaries in Everyday Life

Create a Dictionary:

car = {"brand": "Toyota", "model": "Corolla", "year": 2025}

Access a Value by Key:

car_model = car["model"] # "Corolla"

Change a Value:

car["year"] = 2026

Add a New Key-Value Pair:

car["color"] = "blue"

Remove a Key-Value Pair:

car.pop("model")

Example 1: Storing Contact Information

contact = {"name": "Subin Thapa", "phone": "9761875043", "email": "subinthapa2092.com"}
phone_number = contact["phone"]
print("Phone Number:", phone_number)
contact["email"] = "subinthapa2022@gmail.com"
print("Updated Contact:", contact)
contact["address"] = "Dhunibeshi"
print("Contact with Address:", contact)
contact.pop("phone")
print("Contact after Removing Phone:", contact)

Example 2: Language Translation

translation = {"hello": "hola", "goodbye": "adiós", "please": "por favor", "thank you": "gracias"}
spanish_hello = translation["hello"]
print("Hello in Spanish:", spanish_hello)

Example 3: Check if Key Available

squares = {2: 4, 4: 16, 6: 36, 8: 64, 10: 100}
is_two_present = 2 in squares
print("Is 2 in the dictionary?", is_two_present)
is_four_present = 4 in squares
print("Is 4 in the dictionary?", is_four_present)
is_sixty_four_key = 64 in squares
print("Is 64 a key in the dictionary?", is_sixty_four_key)

Example 4: Nested Dictionary

students = {
    "101": {"name": "Alice", "age": 20},
    "102": {"name": "Bob", "age": 22},
    "103": {"name": "Charlie", "age": 21}
}
print("Students Dictionary:", students)
print("Name of Roll 101:", students["101"]["name"])
students["102"]["age"] = 23
print("After Updating Roll 102:", students)
students["104"] = {"name": "David", "age": 19}
print("After Adding Roll 104:", students)
students.pop("103")
print("After Removing Roll 103:", students)

Dictionary Comprehension in Python

# Syntax:
# {key_expression: value_expression for item in iterable if condition}

Example 5: Dictionary Comprehension – Squares of Numbers

squares = {x: x*x for x in range(1, 6)}
print("Squares Dictionary:", squares)
print("Square of 3:", squares[3])

Example 6: Dictionary Comprehension – Filter by Value

student_scores = {"Alice": 85, "Bob": 76, "Charlie": 90, "David": 65,"Eve": 88}
high_scorers = {name: score for name, score in student_scores.items() if score > 80}
print("High Scorers:", high_scorers)

Challenge 26:

# Create a dictionary of your favorite stocks and their buy dates.
# Add a new buy stock, update the buy date, and remove a stock.
# Hint: Use a dictionary with stock symbols as keys and buy dates as values.

stocks = {"AAPL": "2023-05-01", "TSLA": "2023-06-10", "AMZN": "2023-07-15"}
stocks["MSFT"] = "2023-08-01"       # Add a new stock
stocks["TSLA"] = "2023-06-20"       # Update buy date
stocks.pop("AMZN")                  # Remove a stock
print("Updated Stocks:", stocks)

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Day-26: Mastering Dictionaries in Python – Key-Value Collections

What is a Dictionary?

If you want to store data in key-value pairs, you can use a Dictionary. For example, to store a collection of related data like a person’s name and age, you can use a Dictionary. The Dictionary is represented by curly braces {} with keys and values separated by a colon :

Here, we have created a dictionary named person. It contains three key-value pairs: “name”: “Alice”, “age”: 25, and “city”: “New York”.

Dictionary Features

Important Methods Used in Dictionary:

Methods that don’t work with Dictionary:

Since dictionaries are not sequences but mappings (key → value), these sequence-based methods cannot be used directly on dictionaries. If you want to use such methods, you have to convert the dictionary into a list or tuple first (e.g., `list(d.items())`) and then apply them.

Using Dictionaries in Everyday Life

Example 1: Storing Contact Information

Example 2: Language Translation

Example 3: Check if Key Available

Example 4: Nested Dictionary

Dictionary Comprehension in Python

Example 5: Dictionary Comprehension – Squares of Numbers

Example 6: Dictionary Comprehension – Filter by Value

Challenge 26:

Leave a Reply Cancel reply

What is a Dictionary?

If you want to store data in key-value pairs, you can use a Dictionary. For example, to store a collection of related data like a person’s name and age, you can use a Dictionary. The Dictionary is represented by curly braces {} with keys and values separated by a colon :

Here, we have created a dictionary named person. It contains three key-value pairs: “name”: “Alice”, “age”: 25, and “city”: “New York”.

Dictionary Features

Important Methods Used in Dictionary:

Methods that don’t work with Dictionary:

Since dictionaries are not sequences but mappings (key → value), these sequence-based methods cannot be used directly on dictionaries. If you want to use such methods, you have to convert the dictionary into a list or tuple first (e.g., list(d.items())) and then apply them.

Using Dictionaries in Everyday Life

Example 1: Storing Contact Information

Example 2: Language Translation

Example 3: Check if Key Available

Example 4: Nested Dictionary

Dictionary Comprehension in Python

Example 5: Dictionary Comprehension – Squares of Numbers

Example 6: Dictionary Comprehension – Filter by Value

Challenge 26:

Leave a Reply Cancel reply

Since dictionaries are not sequences but mappings (key → value), these sequence-based methods cannot be used directly on dictionaries. If you want to use such methods, you have to convert the dictionary into a list or tuple first (e.g., `list(d.items())`) and then apply them.