DICT¶

In Python, the term "hashable" is used to describe an object that has a hash value that remains constant during its lifetime. In the context of dictionaries, hashable objects are critical because they are used as keys.

What Does Hashable Mean?¶

Hash Value:
A hash value is a fixed-size integer that uniquely identifies an object, generated by the hash() function.
Immutability:
To be hashable, an object must be immutable. This ensures that its hash value does not change over time, which is a key requirement for dictionary keys.
Equality and Hashing:
Hashable objects must implement the __hash__() method to compute the hash value and the __eq__() method for equality checks. Python dictionaries use these methods to organize and retrieve keys efficiently.

Why Hashable Is Important for Dictionaries¶

Key Uniqueness:
Dictionary keys must be unique. Python uses the hash value of a key to quickly determine where to store or find a value in the dictionary.
Efficient Lookups:
The hash value allows the dictionary to find a key in O(1) time complexity on average.

Hashable Objects¶

Objects that are immutable (and hence hashable) include:

Strings:

my_dict = {"name": "Alice"}  # 'name' is hashable

Numbers (int, float, etc.):

my_dict = {42: "Answer"}  # 42 is hashable

Tuples (if all elements are hashable):

my_dict = {(1, 2): "coordinates"}  # (1, 2) is hashable

Unhashable Objects¶

Objects that are mutable (and hence not hashable) include:

Lists:
Lists are mutable and cannot be used as dictionary keys.

my_dict = {[1, 2]: "List"}  # Raises TypeError: unhashable type: 'list'

Sets:
Like lists, sets are mutable and unhashable.

my_dict = {set([1, 2]): "Set"}  # Raises TypeError

Custom Hashable Objects¶

You can make a custom object hashable by defining the __hash__() and __eq__() methods.

Example¶

class MyHashableClass:
    def __init__(self, value):
        self.value = value

    def __hash__(self):
        return hash(self.value)  # Use the hash of 'value'

    def __eq__(self, other):
        return isinstance(other, MyHashableClass) and self.value == other.value

# Using custom hashable object as dictionary key
key = MyHashableClass(42)
my_dict = {key: "Custom Object"}
print(my_dict[key])  # Output: Custom Object

Key Points in the Context of Dictionaries¶

A dictionary key must be hashable.
Hashable objects allow Python to store dictionary keys in a hash table for fast lookups.
If you attempt to use an unhashable object as a key, Python will raise a TypeError.

Example¶

# Hashable example
hashable_key = "name"
my_dict = {hashable_key: "Alice"}
print(my_dict[hashable_key])  # Output: Alice

# Unhashable example
unhashable_key = [1, 2, 3]  # A list
my_dict = {unhashable_key: "List"}  # Raises TypeError

By ensuring that objects used as dictionary keys are hashable, Python maintains the efficiency and reliability of its dictionary implementation.

📘 Python Dictionary Data Structure¶

In Python, List, Tuple, and Set are used to represent a group of individual objects as a single entity.

However, if we want to represent a group of objects as key-value pairs, we should use a Dictionary.

Examples of Dictionary Use Cases¶

rollno → name
phone number → address
IP address → domain name

🔑 Key Characteristics of Dictionaries¶

Duplicate keys are not allowed, but values can be duplicated.
Heterogeneous objects are allowed as both keys and values.
Insertion order is preserved (since Python 3.7+).
Dictionaries are mutable (modifiable).
Dictionaries are dynamic (can grow/shrink in size).
Indexing and slicing are not applicable.
In other languages:
Known as Map in C++ and Java.
Known as Hash in Perl and Ruby.

🛠️ Creating a Dictionary¶

Creating an Empty Dictionary¶

d = {}
# or
d = dict()

Adding Elements¶

d[100] = "Durga"
d[200] = "Ravi"
d[300] = "Shiva"
print(d)
# Output: {100: 'Durga', 200: 'Ravi', 300: 'Shiva'}

Creating with Predefined Data¶

d = {100: 'Durga', 200: 'Ravi', 300: 'Shiva'}

🔍 Accessing Dictionary Elements¶

d = {100: 'Durga', 200: 'Ravi', 300: 'Shiva'}
print(d[100])  # Output: Durga
print(d[300])  # Output: Shiva

Handling Missing Keys¶

# Raises KeyError
print(d[400])

# Use `in` operator (Python 3)
if 400 in d:
    print(d[400])

🧪 Program: Store and Display Student Records¶

rec = {}
n = int(input("Enter number of students: "))
for i in range(n):
    name = input("Enter Student Name: ")
    marks = input("Enter % of Marks of Student: ")
    rec[name] = marks

print("Name of Student", "\t", "% of Marks")
for name in rec:
    print("\t", name, "\t\t", rec[name])

✏️ Updating Dictionary Elements¶

d = {100: "Durga", 200: "Ravi", 300: "Shiva"}
d[400] = "Pavan"     # Adds new key
d[100] = "Sunny"     # Updates value for key 100
print(d)

❌ Deleting Elements from a Dictionary¶

Delete by Key¶

del d[100]

Clear All Entries¶

d.clear()

Delete Entire Dictionary¶

del d

⚙️ Dictionary Built-in Functions¶

1. `dict()`¶

d = dict()
d = dict({100: "Durga", 200: "Ravi"})
d = dict([(100, "Durga"), (200, "Shiva")])

2. `len()`¶

len(d)

3. `clear()`¶

d.clear()

4. `get()`¶

d.get(key)                 # Returns value or None
d.get(key, default_value)  # Returns value or default

5. `pop()`¶

d.pop(key)  # Removes and returns the value

6. `popitem()`¶

d.popitem()  # Removes and returns an arbitrary item

7. `keys()`¶

d.keys()

8. `values()`¶

d.values()

9. `items()`¶

d.items()  # Returns key-value pairs

10. `copy()`¶

d2 = d.copy()

11. `setdefault()`¶

d.setdefault(key, value)

12. `update()`¶

d.update(other_dict)

💡 Program: Sum of Dictionary Values¶

d = eval(input("Enter dictionary: "))
s = sum(d.values())
print("Sum = ", s)

Example¶

Input: {'A': 100, 'B': 200, 'C': 300}
Output: Sum = 600

🔁 Program: Count Character Occurrences¶

word = input("Enter any word: ")
d = {}
for char in word:
    d[char] = d.get(char, 0) + 1

for k, v in d.items():
    print(k, "occurred", v, "times")

🔠 Program: Count Vowel Occurrences¶

word = input("Enter any word: ")
vowels = {'a', 'e', 'i', 'o', 'u'}
d = {}

for char in word:
    if char in vowels:
        d[char] = d.get(char, 0) + 1

for k, v in sorted(d.items()):
    print(k, "occurred", v, "times")

🧑‍🎓 Program: Student Marks Query System¶

n = int(input("Enter the number of students: "))
d = {}

for _ in range(n):
    name = input("Enter Student Name: ")
    marks = input("Enter Student Marks: ")
    d[name] = marks

while True:
    name = input("Enter Student Name to get Marks: ")
    marks = d.get(name, -1)
    if marks == -1:
        print("Student Not Found")
    else:
        print("The Marks of", name, "are", marks)

    option = input("Do you want to find another student marks? [Yes/No]: ")
    if option.lower() == "no":
        break

print("Thanks for using our application!")

43. Python Program to Count the Number of Each Vowel¶

# Program to count the number of each vowels

# string of vowels
vowels = 'aeiou'

ip_str = 'Hello, have you tried our tutorial section yet?'

# make it suitable for caseless comparisions
ip_str = ip_str.casefold()

# make a dictionary with each vowel a key and value 0
count = {}.fromkeys(vowels,0)

# count the vowels
for char in ip_str:
   if char in count:
       count[char] += 1

print(count)

{'a': 2, 'e': 5, 'i': 3, 'o': 5, 'u': 3}

# Using a list and a dictionary comprehension
# Using dictionary and list comprehension

ip_str = 'Hello, have you tried our tutorial section yet?'

# make it suitable for caseless comparisions
ip_str = ip_str.casefold()

# count the vowels
count = {x:sum([1 for char in ip_str if char == x]) for x in 'aeiou'}

print(count)

vowels = { "a":0, "e":0, "i":0, "o":0, "u":0 }

s = "ashish Bindru"

for word in s:
    if word in vowels:
        vowels[word]+=1

vowels

vowels = { "a":0, "e":0, "i":0, "o":0, "u":0 }

s = "ashish Bindru"

vowels =  {word:s.count(word) for word in s if word in vowels}

vowels

vowels = { "a":0, "e":0, "i":0, "o":0, "u":0 }

s = "ashish Bindru"

vowels = {word: s.count(word) for word in 'aeiou'}
vowels

w = "ashish Bindru"

s=set(w) 

v={'a','e','i','o','u'} 
d=s.intersection(v) 

print("The different vowel present in",w,"are",d)

s = "ashish Bindru"

v="aeiou" 

for w in v:
    print(s.count(w))
# print("The different vowel present in",w,"are",d)

48. Python Program to Merge Two Dictionaries¶

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = dict1 | dict2
print(merged_dict)

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = {**dict1, **dict2}
print(merged_dict)

dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}
merged_dict = dict1.copy()
merged_dict.update(dict2)
print(merged_dict)

dict1.update(dict2)
dict1

53. Python Program to Iterate Over Dictionaries Using for Loop¶

# Example 1: Access both key and value using items()
dt = {'a': 'juice', 'b': 'grill', 'c': 'corn'}

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

a juice b grill c corn

# Example 2: Access both key and value without using items()
dt = {'a': 'juice', 'b': 'grill', 'c': 'corn'}

for key in dt:
    print(key, dt[key])

54. Python Program to Sort a Dictionary by Value¶

# Example 1: Sort the dictionary based on values
dt = {5:4, 1:6, 6:3}

sorted_dt = {key: value for key, value in sorted(dt.items(), key=lambda item: item[1])}

print(sorted_dt)

{6: 3, 5: 4, 1: 6}

# Example 2: Sort only the values
dt = {5:4, 1:6, 6:3}

sorted_dt_value = sorted(dt.values())
print(sorted_dt_value)

[3, 4, 6]

59. Python Program to Check if a Key is Already Present in a Dictionary¶

# Using in keyword
my_dict = {1: 'a', 2: 'b', 3: 'c'}

if 2 in my_dict:
    print("present")

75. Python Program to Convert Two Lists Into a Dictionary¶

The zip() function takes iterables (can be zero or more), aggregates them in a tuple, and returns it.

keys = ["k","l","m"]
values = ["a", "s", "h","i"]

dict(zip(keys, values))

output

{'k': 'a', 'l': 's', 'm': 'h'}

# Example 2: Using list comprehension
index = [1, 2, 3]
languages = ['python', 'c', 'c++']

dictionary = {k: v for k, v in zip(index, languages)}
print(dictionary)

output:

{1: 'python', 2: 'c', 3: 'c++'}

DICT¶

What Does Hashable Mean?¶

Why Hashable Is Important for Dictionaries¶

Hashable Objects¶

Unhashable Objects¶

Custom Hashable Objects¶

Example¶

Key Points in the Context of Dictionaries¶

Example¶

📘 Python Dictionary Data Structure¶

Examples of Dictionary Use Cases¶

🔑 Key Characteristics of Dictionaries¶

🛠️ Creating a Dictionary¶

Creating an Empty Dictionary¶

Adding Elements¶

Creating with Predefined Data¶

🔍 Accessing Dictionary Elements¶

Handling Missing Keys¶

🧪 Program: Store and Display Student Records¶

✏️ Updating Dictionary Elements¶

❌ Deleting Elements from a Dictionary¶

Delete by Key¶

Clear All Entries¶

Delete Entire Dictionary¶

⚙️ Dictionary Built-in Functions¶

1. dict()¶

2. len()¶

3. clear()¶

4. get()¶

5. pop()¶

6. popitem()¶

7. keys()¶

8. values()¶

9. items()¶

10. copy()¶

11. setdefault()¶

12. update()¶

💡 Program: Sum of Dictionary Values¶

Example¶

🔁 Program: Count Character Occurrences¶

🔠 Program: Count Vowel Occurrences¶

🧑‍🎓 Program: Student Marks Query System¶

43. Python Program to Count the Number of Each Vowel¶

48. Python Program to Merge Two Dictionaries¶

53. Python Program to Iterate Over Dictionaries Using for Loop¶

54. Python Program to Sort a Dictionary by Value¶

59. Python Program to Check if a Key is Already Present in a Dictionary¶

75. Python Program to Convert Two Lists Into a Dictionary¶

1. `dict()`¶

2. `len()`¶

3. `clear()`¶

4. `get()`¶

5. `pop()`¶

6. `popitem()`¶

7. `keys()`¶

8. `values()`¶

9. `items()`¶

10. `copy()`¶

11. `setdefault()`¶

12. `update()`¶