🧠 Mastering Python Functions

Everything You Need to Know (Beginner to Pro)

---

📌 So, What Exactly is a Function?

In Python, a function is just a reusable block of code that takes some input, does some work, and gives you an output.

Here’s a simple one:

def is_even(num):
    """
    This function returns whether a number is odd or even.
    input - any valid integer
    output - odd/even
    created on - 16th Nov 2022
    """
    if type(num) == int:
        if num % 2 == 0:
            return 'even'
        else:
            return 'odd'
    else:
        return 'pagal hai kya?'

Calling the Function:

for i in range(1, 11):
    print(is_even(i))

Want to Know What the Function Does?

Use the docstring!

print(is_even.__doc__)

---

⚙️ Parameters vs Arguments

A parameter is what you write in the function definition.
An argument is what you pass in the function call.

def greet(name):  # 'name' is the parameter
    print(f"Hello, {name}!")

greet("Zainab")   # "Zainab" is the argument

---

🧪 Types of Arguments

Let’s break down how you can pass values to a function:

✅ Default Arguments

def power(a=1, b=1):
    return a ** b

print(power())  # 1

✅ Positional Arguments

Position matters…

print(power(2, 3))  # 8

✅ Keyword Arguments

Called by key.

print(power(b=3, a=2))  # 8 (same result, but more readable!)

---

✨ Meet *args and **kwargs

They make your functions super flexible!

*args = Multiple positional arguments

when you don’t want to bound your function with limited number of inputs.

Now you can use function for many inputs

def multiply(*args):
    product = 1
    for i in args:
        product *= i
    return product

multiply(2, 3, 4)  # 24

**kwargs = Multiple keyword arguments

def display(**kwargs):
    for key, value in kwargs.items():
        print(f"{key} -> {value}")

display(india='delhi', srilanka='colombo')

---

🧠 Behind the Scenes: How Functions Live in Memory

• When your script starts, Python creates a global frame.

• Every time you call a function, it gets its own memory frame.

• Once the function finishes, that frame is destroyed — neat and clean.

---

❌ What If a Function Doesn’t Return Anything?

It still works... but it returns None.

def is_even(num):
    if num % 2 == 0:
        print("even")
    else:
        print("odd")

x = is_even(3)
print(x)  # Output: None

Use return if you want to do something with the result.

---

🌍 Local vs Global Scope

x = 5

def f():
    x = 1
    x += 1
    print(x)  # 2

f()
print(x)      # 5

Want to change the outer variable from inside the function? Use global.

x = 5

def f():
    global x
    x += 1

f()
print(x)  # 6

---

🔁 Functions Inside Functions (a.k.a. Nested)

Yes, you can define a function inside another:

def outer():
    def inner():
        print("Inside inner")
    inner()
    print("Inside outer")

---

🎯 Functions Are First-Class Citizens

In Python, functions are treated just like variables.

1. You can assign them:

f = lambda x: x * x

2. You can pass them around:

def func_b(z):
    return z()

func_b(lambda: print("Called"))

3. You can return them:

def f():
    def x(a, b):
        return a + b
    return x

print(f()(3, 4))  # 7

---

✅ Wrapping Up

Functions are the heart of Python programming. They help you:

• Avoid repetition

• Keep your code clean

• Organize logic like a pro

Once you master functions, you're no longer a beginner — you're leveling up. 🚀

---