Python Variables and Data Types Complete Guide

# Variable declaration and assignment
name = "John Doe"        # String variable
age = 25                 # Integer variable
price = 19.99            # Float variable
is_student = True        # Boolean variable

# Multiple assignment
x, y, z = 10, 20, 30

# Same value to multiple variables
a = b = c = 100

# Dynamic typing - change variable type
variable = 10           # Initially integer
print(type(variable))   # 

variable = "Hello"      # Now string
print(type(variable))   # 

# VALID variable names
name = "John"
first_name = "Jane"
age_2024 = 25
_student = True
MAX_VALUE = 100  # Constants (convention)
user2 = "active"

# INVALID variable names (will cause errors)
# 2name = "John"      # Starts with digit
# first-name = "Jane" # Contains hyphen
# class = "Python"    # Python keyword
# $price = 19.99      # Special character
# user name = "John"  # Contains space

# Different variable types
name = "Python"           # str
age = 25                  # int
price = 19.99             # float
is_valid = True           # bool
numbers = [1, 2, 3]       # list
person = {"name": "John"} # dict

# Using type() function
print(type(name))        # 
print(type(age))         # 
print(type(price))       # 
print(type(is_valid))    # 
print(type(numbers))     # 
print(type(person))      # 

# Compare types directly
print(type(name) == str)   # True
print(type(age) == int)    # True
print(type(price) == float)# True

# Using isinstance() - better for inheritance
print(isinstance(name, str))     # True
print(isinstance(age, int))      # True
print(isinstance(price, float))  # True
print(isinstance(price, (int, float)))  # True (multiple types)

# Check if variable is mutable/immutable
print(isinstance(numbers, list))  # True
print(hasattr(numbers, 'append')) # True (list has append method)

def check_variable_type(var, var_name):
    """Check and display variable type information"""
    var_type = type(var)
    
    print(f"Variable: {var_name}")
    print(f"Value: {var}")
    print(f"Type: {var_type}")
    print(f"Type name: {var_type.__name__}")
    
    # Check specific types
    if isinstance(var, int):
        print("✓ This is an integer")
    elif isinstance(var, float):
        print("✓ This is a float")
    elif isinstance(var, str):
        print(f"✓ This is a string with {len(var)} characters")
    elif isinstance(var, list):
        print(f"✓ This is a list with {len(var)} items")
    elif isinstance(var, dict):
        print(f"✓ This is a dictionary with {len(var)} keys")
    
    print("-" * 40)

# Test the function
check_variable_type(42, "answer")
check_variable_type(3.14, "pi")
check_variable_type("Hello Python", "greeting")
check_variable_type([1, 2, 3], "numbers")
check_variable_type({"name": "John", "age": 30}, "person")

# Type conversion examples
# int() - converts to integer
x = int(3.14)      # 3 (truncates decimal)
y = int("100")     # 100
# z = int("hello") # ValueError

# float() - converts to float
a = float(10)      # 10.0
b = float("3.14")  # 3.14
c = float("100")   # 100.0

# str() - converts to string
s1 = str(100)      # "100"
s2 = str(3.14)     # "3.14"
s3 = str([1,2,3])  # "[1, 2, 3]"

# list() - converts to list
lst1 = list("hello")        # ['h', 'e', 'l', 'l', 'o']
lst2 = list((1, 2, 3))      # [1, 2, 3]
lst3 = list({1, 2, 3})      # [1, 2, 3]

# tuple() - converts to tuple
t1 = tuple([1, 2, 3])       # (1, 2, 3)
t2 = tuple("hello")         # ('h', 'e', 'l', 'l', 'o')

# set() - converts to set (removes duplicates)
s1 = set([1, 2, 2, 3, 3])   # {1, 2, 3}
s2 = set("hello")           # {'h', 'e', 'l', 'o'}

# dict() - converts to dictionary
# dict requires sequence of (key, value) pairs
d = dict([("a", 1), ("b", 2)])  # {'a': 1, 'b': 2}

# bool() - converts to boolean
b1 = bool(10)      # True (non-zero)
b2 = bool(0)       # False
b3 = bool("")      # False (empty string)
b4 = bool("Hello") # True (non-empty string)
b5 = bool([])      # False (empty list)
b6 = bool([1,2])   # True (non-empty list)

# Variables and Data Types Practice
# Run this script and observe the output

# 1. Variable declaration and type checking
print("=== Example 1: Basic Variables ===")
name = "Alice"
age = 30
height = 5.6
is_programmer = True

print(f"Name: {name}, Type: {type(name)}")
print(f"Age: {age}, Type: {type(age)}")
print(f"Height: {height}, Type: {type(height)}")
print(f"Is Programmer: {is_programmer}, Type: {type(is_programmer)}")

# 2. Type conversion
print("\n=== Example 2: Type Conversion ===")
number_str = "100"
number_int = int(number_str)
print(f"String '{number_str}' → Integer: {number_int}")

pi_str = "3.14159"
pi_float = float(pi_str)
print(f"String '{pi_str}' → Float: {pi_float}")

# 3. List operations
print("\n=== Example 3: Lists ===")
fruits = ["apple", "banana", "cherry"]
print(f"Fruits list: {fruits}")
print(f"Type: {type(fruits)}")
print(f"First fruit: {fruits[0]}")

# Add new fruit
fruits.append("orange")
print(f"After adding orange: {fruits}")

# 4. Dictionary example
print("\n=== Example 4: Dictionary ===")
student = {
    "name": "Bob",
    "age": 22,
    "courses": ["Math", "Science", "English"]
}
print(f"Student dictionary: {student}")
print(f"Type: {type(student)}")
print(f"Student name: {student['name']}")
print(f"Student courses: {student['courses']}")

# 5. Set operations (unique values)
print("\n=== Example 5: Sets ===")
numbers = {1, 2, 2, 3, 3, 4, 5}
print(f"Original numbers (with duplicates): [1, 2, 2, 3, 3, 4, 5]")
print(f"Set (duplicates removed): {numbers}")
print(f"Type: {type(numbers)}")

print("\n=== Try It Yourself ===")
print("Modify the values and add your own variables!")
print("Use type() to check different variable types")