Python Functions, Arguments, *args / **kwargs, and Scope

Before we begin, here are the concepts you must already understand. If any of these feel unfamiliar, take a few minutes to review them.

• Variables · storing a value in a name (e.g., x = 5)
• print() · displaying output in the terminal
• Basic data types · strings ("hello"), integers (5), floats (3.14), lists ([1, 2, 3])
• if / else · making decisions in code
• for loops · repeating an action multiple times

No prior experience with functions or scope is needed.

1.1 What Is a Function?

A function is a named, reusable block of code. You define it once, and then you can call (run) it as many times as you like from anywhere in your program.

Real-world analogy: Think of a function like a vending machine button. The machine (your function) is built once. Every time someone pushes the button (calls the function), the machine does the same job and delivers a result.

Why do functions exist?

• DRY principle · "Don't Repeat Yourself." Write the logic once, use it everywhere.
• Readability · Code broken into named functions is far easier to read and understand.
• Maintenance · If a rule changes, you fix it in one place instead of hunting through hundreds of lines.
• Testing · Small, focused functions are much easier to test and debug.

1.2 Defining a Function

In Python, you create a function using the def keyword.

Syntax:

def function_name():
    # code that runs when the function is called

Breaking down the syntax line by line:

1.3 Your First Function

def say_hello():
    print("Hello from inside a function!")

Important: Defining a function does not run it. It just stores the recipe. Nothing appears on screen yet.

1.4 Calling a Function

To actually run the function, you call it by writing its name followed by parentheses:

def say_hello():
    print("Hello from inside a function!")

say_hello()

Expected output:

Hello from inside a function!

You can call the same function multiple times:

say_hello()
say_hello()
say_hello()

Expected output:

Hello from inside a function!
Hello from inside a function!
Hello from inside a function!

💡 Think about it: What happens if you never call the function? Try removing say_hello() from the code. What do you see?

1.5 Why Indentation Matters

Python uses indentation (spaces at the start of a line) to know which lines belong inside a function. This is not optional · it is the rule.

def greet():
    print("Line 1 – inside the function")
    print("Line 2 – also inside the function")

print("Line 3 – OUTSIDE the function, runs immediately")
greet()

Expected output:

Line 3 – OUTSIDE the function, runs immediately
Line 1 – inside the function
Line 2 – also inside the function

🧠 Notice: Line 3 runs first because it is outside the function and sits at the top level of the program. Line 1 and Line 2 only run when greet() is called.

1.6 Common Beginner Mistake · Forgetting the Call

# ❌ WRONG – defines the function but never calls it
def show_score():
    print("Your score is 100")

# Nothing is printed because we forgot to call it!

Corrected version:

# ✅ CORRECT
def show_score():
    print("Your score is 100")

show_score()  # This line actually runs the function

Expected output:

Your score is 100

1.7 Common Beginner Mistake · Indentation Error

# ❌ WRONG – missing indentation inside function body
def greet():
print("Hello")  # IndentationError!

Corrected version:

# ✅ CORRECT
def greet():
    print("Hello")  # 4 spaces of indentation

1.8 The pass Statement

Sometimes you want to create a function skeleton now and fill in the logic later. An empty function body causes a syntax error · use pass as a placeholder:

def my_future_function():
    pass  # Does nothing, but Python is happy

my_future_function()  # No output, no error

This is common in professional development when planning out a project structure before writing all the logic.

2.1 What Are Arguments?

So far our functions do the exact same thing every time. But real functions need to work with different data each time · like a calculator that can add any two numbers, not just 2 + 2.

Parameters and arguments let you pass information into a function.

The difference:

Think of a parameter as an empty slot, and an argument as the value you drop into that slot.

2.2 A Function with One Parameter

def greet_person(name):       # 'name' is the parameter
    print("Hello, " + name + "!")

greet_person("Alice")         # "Alice" is the argument
greet_person("Bob")           # "Bob" is the argument
greet_person("Dr. Johnson")   # "Dr. Johnson" is the argument

Expected output:

Hello, Alice!
Hello, Bob!
Hello, Dr. Johnson!

🧠 What changed? The same function ran three times. Each time, the name parameter held a different value. The function code never changed · only the input did.

2.3 A Function with Multiple Parameters

You can define as many parameters as you need, separated by commas:

def describe_student(first_name, subject, grade):
    print(first_name + " studies " + subject + " and scored " + str(grade) + "%.")

describe_student("Maria", "Maths", 94)
describe_student("James", "Biology", 78)

Expected output:

Maria studies Maths and scored 94%.
James studies Biology and scored 78%.

Note: str(grade) converts the number 94 into the string "94" so it can be joined with the other strings using +. This is called type conversion.

2.4 Positional Arguments

When you pass arguments in order, Python matches them to parameters by position:

def show_order(item, quantity, price):
    print(item + " × " + str(quantity) + " = £" + str(price))

show_order("Apples", 6, 3.00)
# item = "Apples", quantity = 6, price = 3.00

Expected output:

Apples × 6 = £3.0

⚠️ Order matters! If you mix up the order, you get incorrect results · or an error:

show_order(6, "Apples", 3.00)   # quantity and item are swapped!

Incorrect output:

6 × Apples = £3.0

2.5 Keyword Arguments

You can also pass arguments using the parameter name so order does not matter:

def show_order(item, quantity, price):
    print(item + " × " + str(quantity) + " = £" + str(price))

show_order(quantity=6, price=3.00, item="Apples")

Expected output:

Apples × 6 = £3.0

Using keyword arguments makes your code self-documenting · anyone reading it knows exactly what each value means.

2.6 Default Parameter Values

You can give a parameter a default value. If the caller does not provide that argument, the default is used automatically:

def greet(name, language="English"):
    if language == "English":
        print("Hello, " + name + "!")
    elif language == "Spanish":
        print("Hola, " + name + "!")
    elif language == "French":
        print("Bonjour, " + name + "!")

greet("Alice")               # uses default: English
greet("Carlos", "Spanish")   # overrides default
greet("Marie", "French")     # overrides default

Expected output:

Hello, Alice!
Hola, Carlos!
Bonjour, Marie!

💡 Rule: Parameters with defaults must come after parameters without defaults in the function definition.

# ❌ WRONG
def greet(language="English", name):   # SyntaxError
    pass

# ✅ CORRECT
def greet(name, language="English"):
    pass

2.7 Returning Values with return

So far our functions have printed things. But often you want a function to calculate a result and send it back so your program can use it elsewhere.

The return keyword sends a value back to wherever the function was called:

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

total = add(10, 25)
print("The total is:", total)

Expected output:

The total is: 35

What happens line by line:

1. add(10, 25) is called · a = 10, b = 25
2. Inside: result = 10 + 25 → result = 35
3. return result sends 35 back to the caller
4. total receives the returned value 35
5. print(...) displays it

🛑 After return, the function stops. Any code after return in the same function body will never run.

def demo():
    return "I am returned"
    print("This line never runs!")  # unreachable code

value = demo()
print(value)

Expected output:

I am returned

2.8 Practical Example · Temperature Converter

def celsius_to_fahrenheit(celsius):
    fahrenheit = (celsius * 9 / 5) + 32
    return fahrenheit

temp_c = 100
temp_f = celsius_to_fahrenheit(temp_c)
print(str(temp_c) + "°C = " + str(temp_f) + "°F")

print(str(0) + "°C = " + str(celsius_to_fahrenheit(0)) + "°F")
print(str(37) + "°C = " + str(celsius_to_fahrenheit(37)) + "°F")

Expected output:

100°C = 212.0°F
0°C = 32.0°F
37°C = 98.6°F

💡 Real-world use: Weather apps, scientific instruments, and cooking sites use this exact type of function. Write it once, use it everywhere.

2.9 Passing a List as an Argument

Any data type can be passed as an argument, including lists:

def show_fruits(fruits):
    for fruit in fruits:
        print("🍎 " + fruit)

my_fruits = ["Apple", "Banana", "Mango", "Grape"]
show_fruits(my_fruits)

Expected output:

🍎 Apple
🍎 Banana
🍎 Mango
🍎 Grape

3.1 The Problem: What If You Don't Know How Many Arguments There Will Be?

Imagine writing a function that adds numbers. How many numbers should it accept? Two? Five? Twenty?

If you hard-code three parameters, a caller who wants to sum six numbers is stuck. Python solves this with arbitrary arguments.

3.2 \*args · Arbitrary Positional Arguments

Adding a single * before a parameter name tells Python: "collect all the extra positional arguments into a tuple."

By convention, the name used is args, but any name after works.

def add_all(*numbers):
    total = 0
    for n in numbers:
        total = total + n
    return total

print(add_all(1, 2))
print(add_all(1, 2, 3, 4, 5))
print(add_all(10, 20, 30, 40, 50, 60))

Expected output:

3
15
210

What is numbers here? It is a tuple · an immutable, ordered collection of all the values passed in:

def inspect_args(*items):
    print(type(items))   # <class 'tuple'>
    print(items)

inspect_args("cat", "dog", "fish")

Expected output:

<class 'tuple'>
('cat', 'dog', 'fish')

3.3 \*args With Other Parameters

args can be combined with regular parameters. Regular parameters must come before args:

def show_menu(restaurant_name, *dishes):
    print("Restaurant: " + restaurant_name)
    print("Today's menu:")
    for dish in dishes:
        print("  - " + dish)

show_menu("La Piazza", "Pasta", "Pizza", "Tiramisu", "Risotto")

Expected output:

Restaurant: La Piazza
Today's menu:
  - Pasta
  - Pizza
  - Tiramisu
  - Risotto

🧠 How it works: "La Piazza" fills the restaurant_name parameter. Everything else · "Pasta", "Pizza", "Tiramisu", "Risotto" · is collected into the dishes tuple.

3.4 \\kwargs · Arbitrary Keyword Arguments

kwargs does the same thing as *args, but for keyword arguments (name=value pairs). The double collects them into a dictionary.

By convention the name is kwargs (keyword arguments), but you can use any name after .

def describe_person(**details):
    print("Person Profile:")
    for key, value in details.items():
        print("  " + key + ": " + str(value))

describe_person(name="Sarah", age=28, city="London", job="Engineer")

Expected output:

Person Profile:
  name: Sarah
  age: 28
  city: London
  job: Engineer

What is details here? It is a dictionary:

def inspect_kwargs(**info):
    print(type(info))   # <class 'dict'>
    print(info)

inspect_kwargs(colour="blue", size=12)

Expected output:

<class 'dict'>
{'colour': 'blue', 'size': 12}

3.5 \\kwargs · Practical Example: Product Builder

def create_product(**specs):
    print("=== New Product ===")
    for spec, value in specs.items():
        print(spec.capitalize() + ": " + str(value))
    print()

create_product(name="Laptop", brand="TechPro", ram="16GB", price=999.99)
create_product(name="Headphones", brand="SoundWave", colour="Black", wireless=True)

Expected output:

=== New Product ===
Name: Laptop
Brand: TechPro
Ram: 16GB
Price: 999.99

=== New Product ===
Name: Headphones
Brand: SoundWave
Colour: Black
Wireless: True

💡 Real-world use: Web frameworks like Flask and Django use **kwargs everywhere. When you create a database model or an HTML element with attributes, the framework accepts them as keyword arguments.

3.6 Combining Regular, \args, and \\*kwargs

You can use all three together in one function. The order must be:

def function(regular_params, *args, **kwargs):

def full_order(table_number, *items, **extras):
    print("Table " + str(table_number) + " ordered:")
    for item in items:
        print("  - " + item)
    if extras:
        print("  Special requests:")
        for key, val in extras.items():
            print("    " + key + ": " + str(val))

full_order(7, "Pizza", "Salad", "Water", sauce="extra", gluten_free=True)

Expected output:

Table 7 ordered:
  - Pizza
  - Salad
  - Water
  Special requests:
    sauce: extra
    gluten_free: True

3.7 The \ and \\* Unpacking Operators in Function Calls

The and can also be used when calling a function, to unpack* a list or dictionary into separate arguments:

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

numbers = [1, 2, 3]
print(add(*numbers))   # unpacks list into: add(1, 2, 3)

Expected output:

6

def greet(name, language):
    print("Hello " + name + " in " + language)

settings = {"name": "Aiko", "language": "Japanese"}
greet(**settings)   # unpacks dict into: greet(name="Aiko", language="Japanese")

Expected output:

Hello Aiko in Japanese

3.8 Common Beginner Mistakes with \args and \\*kwargs

Mistake 1 · Wrong order:

# ❌ WRONG – **kwargs before *args
def bad_func(**kwargs, *args):   # SyntaxError
    pass

# ✅ CORRECT – *args always before **kwargs
def good_func(*args, **kwargs):
    pass

Mistake 2 · Treating \*args as a list (it's a tuple):

def my_func(*numbers):
    numbers.append(99)   # AttributeError! Tuples don't have append

# ✅ CORRECT – convert to list if you need to modify it
def my_func(*numbers):
    nums_list = list(numbers)
    nums_list.append(99)
    print(nums_list)

my_func(1, 2, 3)

Expected output:

[1, 2, 3, 99]

4.1 What Is Scope?

Scope describes where in your code a variable is visible and accessible. Not every variable can be seen from every part of a program · and this is intentional.

Why does scope exist? Imagine a huge program with hundreds of functions. If every variable was visible everywhere, any function could accidentally overwrite another function's total or count. Scope creates walls between different parts of code, preventing accidental interference.

Analogy: Think of scope like rooms in a house. A lamp in your bedroom (local scope) only lights your bedroom. It does not light the kitchen. The main electricity supply (global scope) powers the entire house.

Python has four levels of scope, remembered with the acronym LEGB:

We will focus on Local and Global · the two you will encounter most.

4.2 Local Scope

A variable created inside a function is local · it only exists while the function is running, and it is invisible outside.

def my_function():
    local_variable = "I only exist inside this function"
    print(local_variable)

my_function()
print(local_variable)   # NameError! local_variable doesn't exist here

Expected output:

I only exist inside this function
Traceback (most recent call last):
  ...
NameError: name 'local_variable' is not defined

🧠 Why? When my_function() finishes, Python destroys all local variables. They are temporary · like writing on a whiteboard that gets erased when the meeting ends.

4.3 Local Variables in Different Functions Don't Interfere

Each function has its own private workspace. Two functions can both use a variable called x without conflict:

def function_a():
    x = 100
    print("Inside function_a, x =", x)

def function_b():
    x = 999
    print("Inside function_b, x =", x)

function_a()
function_b()

Expected output:

Inside function_a, x = 100
Inside function_b, x = 999

Both functions have their own x. They never interfere with each other. This is the power of local scope.

4.4 Global Scope

A variable created outside all functions is global · it is visible to all code in the file:

greeting = "Good morning"   # global variable

def say_it():
    print(greeting)         # can READ the global variable

def shout_it():
    print(greeting.upper()) # can also READ it

say_it()
shout_it()

Expected output:

Good morning
GOOD MORNING

4.5 Can a Function Modify a Global Variable?

By default, reading a global variable inside a function is fine. But modifying it is blocked · Python creates a new local variable instead:

counter = 0   # global

def increment():
    counter = counter + 1   # ❌ UnboundLocalError!

increment()

Why the error? Python sees counter = on the left side of = inside the function, assumes you mean a new local variable, then tries to read the (not-yet-created local) counter on the right side · and fails.

4.6 The global Keyword

To tell Python "I want to modify the actual global variable, not create a local one," use the global keyword:

counter = 0   # global

def increment():
    global counter             # declare intent to use the global
    counter = counter + 1
    print("Counter:", counter)

increment()
increment()
increment()
print("Final counter:", counter)

Expected output:

Counter: 1
Counter: 2
Counter: 3
Final counter: 3

⚠️ Use global sparingly. Overusing global is considered bad practice because it makes code hard to trace and debug. The professional approach is to pass values as arguments and return results instead.

Professional alternative to global:

# ❌ Using global (not ideal)
total = 0
def add_to_total(n):
    global total
    total += n

# ✅ Better approach – pass and return
def add_to_total(current_total, n):
    return current_total + n

total = 0
total = add_to_total(total, 5)
total = add_to_total(total, 10)
print(total)  # 15

4.7 Local Takes Priority Over Global

If a local variable and a global variable share the same name, the local one wins inside the function:

name = "Global Name"

def show_name():
    name = "Local Name"   # this is a NEW local variable
    print(name)

show_name()
print(name)   # global variable unchanged

Expected output:

Local Name
Global Name

🧠 Notice: Assigning inside the function did NOT change the global name. The global is completely untouched.

4.8 The nonlocal Keyword (Bonus: Enclosing Scope)

When you have a function inside another function (a nested function), the inner function can access the outer function's variables as enclosing scope. To modify them, use nonlocal:

def outer():
    message = "Hello from outer"

    def inner():
        nonlocal message
        message = "Modified by inner"
        print("Inner says:", message)

    inner()
    print("Outer says:", message)

outer()

Expected output:

Inner says: Modified by inner
Outer says: Modified by inner

This is an advanced pattern. You will encounter it when learning about closures and decorators · but nonlocal follows exactly the same logic as global, just one level in.

4.9 Built-in Scope

Python has many built-in names that are always available · like print, len, range, type, int, str, list. These live in the built-in scope, the outermost layer:

print(len("Hello"))   # 'len' and 'print' are built-in
print(type(42))       # 'type' is built-in

Expected output:

5
<class 'int'>

⚠️ Never shadow built-in names! Don't create variables called list, print, input, id, etc. · this overwrites the built-in and causes confusing errors.

# ❌ WRONG – never do this
list = [1, 2, 3]       # You've now overwritten the built-in 'list'
print(list([1,2,3]))   # TypeError – list is now just a list object, not a function

# ✅ CORRECT – use a descriptive name
my_list = [1, 2, 3]