Python Keywords

🔀 Control Flow Keywords

Keywords that control how your program executes

🔹 Conditional Keywords

# if, elif, else - Make decisions
age = 18
if age >= 18:
    print("Adult")
elif age >= 13:
    print("Teenager")
else:
    print("Child")

# Nested conditions
weather = "sunny"
temperature = 75
if weather == "sunny":
    if temperature > 70:
        print("Perfect day!")
    else:
        print("Sunny but cold")
else:
    print("Not sunny")

🔹 Loop Keywords

# for - Loop through sequences
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

# while - Loop with condition
count = 0
while count < 3:
    print(f"Count: {count}")
    count += 1

# break - Exit loop early
for i in range(10):
    if i == 5:
        break
    print(i)  # Prints 0, 1, 2, 3, 4

# continue - Skip to next iteration
for i in range(5):
    if i == 2:
        continue
    print(i)  # Prints 0, 1, 3, 4

🔹 Flow Control Keywords

# pass - Do nothing (placeholder)
def future_function():
    pass  # Will implement later

if True:
    pass  # Empty block

# return - Exit function with value
def add_numbers(a, b):
    return a + b

result = add_numbers(5, 3)
print(result)  # 8

# Multiple returns
def check_number(n):
    if n > 0:
        return "positive"
    elif n < 0:
        return "negative"
    else:
        return "zero"

🏗️ Definition Keywords

Keywords for creating functions, classes, and scopes

🔹 Function Definition

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

message = greet("Alice")
print(message)

# lambda - Anonymous function
square = lambda x: x ** 2
print(square(5))  # 25

# Lambda with multiple arguments
add = lambda a, b: a + b
print(add(3, 4))  # 7

🔹 Class Definition

# class - Define class
class Dog:
    def __init__(self, name):
        self.name = name
    
    def bark(self):
        return f"{self.name} says woof!"

my_dog = Dog("Buddy")
print(my_dog.bark())  # Buddy says woof!

🔹 Scope Keywords

# global - Access global variable
counter = 0

def increment():
    global counter
    counter += 1

increment()
print(counter)  # 1

# nonlocal - Access enclosing scope
def outer():
    x = 10
    
    def inner():
        nonlocal x
        x += 1
        return x
    
    return inner()

print(outer())  # 11

⚖️ Logical Keywords

Keywords for logical operations and comparisons

🔹 Boolean Operators

# and - Both conditions must be True
age = 25
has_license = True
can_drive = age >= 18 and has_license
print(can_drive)  # True

# or - At least one condition must be True
is_weekend = False
is_holiday = True
can_sleep_in = is_weekend or is_holiday
print(can_sleep_in)  # True

# not - Reverse boolean value
is_raining = False
is_sunny = not is_raining
print(is_sunny)  # True

🔹 Identity and Membership

# is - Check if same object
a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(a is b)  # False (different objects)
print(a is c)  # True (same object)
print(a == b)  # True (same content)

# in - Check membership
fruits = ["apple", "banana", "cherry"]
print("apple" in fruits)     # True
print("orange" in fruits)    # False

text = "Hello World"
print("World" in text)       # True

🔹 Special Values

# True, False - Boolean values
is_active = True
is_deleted = False

if is_active and not is_deleted:
    print("User is active")

# None - Represents absence of value
result = None
if result is None:
    print("No result yet")

def maybe_return_value(condition):
    if condition:
        return "value"
    return None  # Explicit None return

🛡️ Exception Keywords

Keywords for handling errors and exceptions

🔹 Exception Handling

# try, except - Handle errors
try:
    number = int(input("Enter a number: "))
    result = 10 / number
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")

# Multiple exceptions
try:
    data = [1, 2, 3]
    print(data[10])
except (IndexError, KeyError) as e:
    print(f"Error accessing data: {e}")

# finally - Always executes
try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found")
finally:
    print("Cleanup complete")

🔹 Raising Exceptions

# raise - Throw exception
def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

try:
    result = divide(10, 0)
except ValueError as e:
    print(f"Error: {e}")

# assert - Debug check
def calculate_age(birth_year):
    current_year = 2024
    age = current_year - birth_year
    assert age >= 0, "Age cannot be negative"
    return age

try:
    age = calculate_age(2030)  # Will raise AssertionError
except AssertionError as e:
    print(f"Assertion failed: {e}")

🔹 Context Management

# with, as - Context manager
with open("data.txt", "w") as file:
    file.write("Hello, World!")
# File automatically closed

# Multiple context managers
with open("input.txt", "r") as infile, \
     open("output.txt", "w") as outfile:
    content = infile.read()
    outfile.write(content.upper())

# Custom context manager
class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    
    def __exit__(self, *args):
        import time
        print(f"Time taken: {time.time() - self.start:.2f}s")

with Timer():
    # Some time-consuming operation
    sum(range(1000000))

📋 Complete Keywords List

All 35 Python keywords organized by category

# Check all keywords
import keyword
print("All Python keywords:")
print(keyword.kwlist)
print(f"Total keywords: {len(keyword.kwlist)}")

# Check if word is keyword
print(f"Is 'if' a keyword? {keyword.iskeyword('if')}")
print(f"Is 'print' a keyword? {keyword.iskeyword('print')}")

# Keywords cannot be used as variable names
# This would cause SyntaxError:
# if = 5  # Error!
# def = "hello"  # Error!

# But these are fine:
if_statement = 5
def_keyword = "hello"
print(f"Valid variable: {if_statement}")