Topic: Python Functions – Part 1 of 3: Basics, Syntax, and Parameters (Long Lesson)

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### 1. What is a Function in Python?

A function is a reusable block of code that performs a specific task. Functions help:

• Avoid code duplication
• Improve code readability
• Enable modular programming

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### 2. Why Use Functions?

• Reusability – Write once, use many times
• Modularity – Split large tasks into smaller blocks
• Debuggability – Easier to test/debug small units
• Abstraction – Hide complex logic behind a name

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### 3. Function Syntax

def function_name(parameters):
    # block of code
    return result

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### 4. Creating a Simple Function

def greet():
    print("Hello, welcome to Python functions!")
    
greet()  # Calling the function

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### 5. Function with Parameters

def greet_user(name):
    print(f"Hello, {name}!")

greet_user("Hussein")

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### 6. Function with Return Value

def add(a, b):
    return a + b

result = add(10, 5)
print(result)  # Output: 15

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### 7. Positional vs Keyword Arguments

def student_info(name, age):
    print(f"Name: {name}, Age: {age}")

student_info("Ali", 22)  # Positional
student_info(age=22, name="Ali")  # Keyword

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### 8. Default Parameter Values

def greet(name="Guest"):
    print(f"Hello, {name}!")

greet()            # Output: Hello, Guest!
greet("Hussein")   # Output: Hello, Hussein!

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### 9. Variable Number of Arguments

#### \*args – Multiple positional arguments:

def sum_all(*numbers):
    total = 0
    for num in numbers:
        total += num
    return total

print(sum_all(1, 2, 3, 4))  # Output: 10

#### \*\*kwargs – Multiple keyword arguments:

def print_details(**info):
    for key, value in info.items():
        print(f"{key}: {value}")

print_details(name="Ali", age=24, country="Egypt")

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### **10. Scope of Variables**

#### Local vs Global Variables

x = "global"

def func():
    x = "local"
    print(x)

func()     # Output: local
print(x)   # Output: global

Use global keyword if you want to modify a global variable inside a function.

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### 11. Docstrings (Function Documentation)

def square(n):
    """Returns the square of a number."""
    return n * n

print(square.__doc__)  # Output: Returns the square of a number.

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### 12. Best Practices

• Use descriptive names for functions
• Keep functions short and focused
• Avoid side effects unless needed
• Add docstrings for documentation

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### Exercise

• Create a function that takes a list and returns the average
• Create a function that takes any number of scores and returns the highest
• Create a function with default arguments for greeting a user by name and language

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#Python #Functions #CodingBasics #ModularProgramming #CodeReuse #PythonBeginners

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