What Are the Basics of Python Programming You Must Know

# Variable assignment
name = "Alice"       # str
age = 25             # int
salary = 45000.50    # float
is_employed = True   # bool
middle_name = None   # NoneType

# Check types
print(type(name))    # <class 'str'>
print(type(age))     # <class 'int'>

# f-strings (modern formatting)
print(f"{name} is {age} years old")

# Type conversion
age_str = "25"
age_int = int(age_str)      # str → int
price = float("99.99")      # str → float
count = str(100)            # int → str

# Important: input() always returns a string
user_age = input("Your age: ")
user_age = int(user_age)    # must convert!

# Common error
# "25" + 5 → TypeError
# "25" is str, 5 is int

# Arithmetic
a, b = 10, 3
print(a + b)    # 13
print(a - b)    # 7
print(a * b)    # 30
print(a / b)    # 3.333...
print(a // b)   # 3 (floor division)
print(a % b)    # 1 (remainder)
print(a ** b)   # 1000 (power)

# Comparison
print(10 == 10)   # True
print(10 != 5)    # True
print(10 > 5)     # True

# Logical operators
is_adult = True
has_id = False

print(is_adult and has_id)   # False
print(is_adult or has_id)    # True
print(not has_id)            # True

# Membership
fruits = ["apple", "banana", "mango"]
print("apple" in fruits)     # True
print("orange" in fruits)    # False

# Identity
x = None
print(x is None)             # True
print(x is not None)         # False

# if / elif / else
score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Grade: {grade}")  # Grade: B

# Ternary (one-line if)
status = "Pass" if score >= 40 else "Fail"

# for loop — iterate over a sequence
fruits = ["apple", "banana", "mango"]
for fruit in fruits:
    print(fruit)

# range() — iterate over numbers
for i in range(1, 6):
    print(i)  # 1 2 3 4 5

# while loop — repeat until condition is False
count = 0
while count < 3:
    print(f"Count: {count}")
    count += 1

# break and continue
for i in range(10):
    if i == 5:
        break       # exit loop
    if i % 2 == 0:
        continue    # skip this iteration
    print(i)        # 1 3

# Define a function
def greet(name):
    return f"Hello, {name}!"

# Call it
message = greet("Alice")
print(message)  # Hello, Alice!

# Function with no return
def say_hi():
    print("Hi!")

say_hi()  # Hi!

# Default parameters
def greet_default(name="Guest"):
    return f"Hello, {name}!"

print(greet_default())        # Hello, Guest!
print(greet_default("Bob"))   # Hello, Bob!

# Multiple parameters
def add(a, b):
    return a + b

print(add(3, 5))        # 8
print(add(a=3, b=5))    # 8 (keyword args)

# Returning multiple values
def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 4, 1, 5])
print(low, high)  # 1 5

# *args — variable positional arguments
def total(*args):
    return sum(args)

print(total(1, 2, 3, 4))  # 10

# **kwargs — variable keyword arguments
def show(**kwargs):
    for k, v in kwargs.items():
        print(k, v)

show(name="Alice", age=25)

# Lists — ordered, mutable
fruits = ["apple", "banana", "mango"]
fruits.append("orange")
fruits.remove("banana")
print(fruits[0])         # apple
print(fruits[-1])        # orange
print(len(fruits))       # 3

# List slicing
numbers = [10, 20, 30, 40, 50]
print(numbers[1:3])      # [20, 30]
print(numbers[:2])       # [10, 20]
print(numbers[::2])      # [10, 30, 50]

# Tuples — ordered, immutable
point = (10, 20)
x, y = point             # unpacking
# point[0] = 5           # TypeError — can't modify

# Dicts — key-value pairs
user = {
    "name": "Alice",
    "age": 25,
    "city": "Delhi"
}

print(user["name"])           # Alice
print(user.get("email"))      # None (safe access)

user["email"] = "alice@x.com" # add
user["age"] = 26              # update
del user["city"]              # delete

# Iterate
for key, value in user.items():
    print(f"{key}: {value}")

# Sets — unique items, no order
unique = {1, 2, 3, 3, 2, 1}
print(unique)                 # {1, 2, 3}

a = {1, 2, 3}
b = {3, 4, 5}
print(a & b)                  # {3} — intersection
print(a | b)                  # {1,2,3,4,5} — union

# Writing to a file
with open("notes.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")

# Reading a file
with open("notes.txt", "r") as f:
    content = f.read()
    print(content)

# Reading line by line
with open("notes.txt", "r") as f:
    for line in f:
        print(line.strip())

# Appending
with open("notes.txt", "a") as f:
    f.write("Third line\n")

# Always use "with" — it closes the file automatically

# try / except — handle errors gracefully
try:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number")
except ZeroDivisionError:
    print("Can't divide by zero")
else:
    print("No exceptions occurred")
finally:
    print("This always runs")

# Raising your own errors
def validate_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

# Custom exceptions
class InvalidInputError(Exception):
    pass