Python functions become powerful when they can accept data from outside. This is done using function arguments. In this lecture, we will clearly understand every type of argument with simple explanations and examples.

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1. What Are Function Arguments?

Functions in Python can receive information from the caller. This information is passed as arguments inside the parentheses () when calling a function.

Why arguments are important?

• They make functions reusable
• They allow functions to work with different data
• They make programs dynamic instead of fixed

Example

def my_function(fname):
    print(fname + " Thapa")

my_function("Subin")
my_function("Ayush")
my_function("Amor")
my_function("Yubraj")
my_function("Saubhagya")

Explanation:

• fname → parameter (defined inside the function)
• Values like "Subin", "Ayush" → arguments (passed while calling)

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2. Parameters vs Arguments

Many beginners get confused between parameters and arguments.

Term	Meaning
Parameter	Variable written inside the function definition
Argument	Actual value sent to the function when calling it

Example

def my_function(name):  # parameter
    print("Hello", name)

my_function("Nikilesh")  # argument

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3. Number of Arguments

A function must be called with the exact number of arguments it expects.

Example

def my_function(fname, lname):
    print(fname + " " + lname)

my_function("Subin", "Thapa")  # Correct
my_function("Subin")            # Error: missing argument

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4. Default Parameter Values

Python allows parameters to have default values.

• If no argument is passed → default value is used
• If argument is passed → default value is overridden

Example 1

def my_function(name="friend"):
    print("Hello", name)

my_function("Babina")
my_function()

Example 2

def my_function(district="Dhading"):
    print("I am from", district)

my_function("Kathmandu")
my_function("Pokhara")
my_function()
my_function("Chitwan")

Important Rule:
Default parameters must always come after non-default parameters.

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5. Keyword Arguments

Arguments can be passed using key = value format.

Benefits

• Order does not matter
• Code becomes more readable

Example

def my_function(animal, name):
    print("I have a", animal)
    print("My", animal + "'s name is", name)

my_function(animal="dog", name="Buddy")
my_function(name="Buddy", animal="dog")

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6. Positional Arguments

When arguments are passed without keywords, Python assigns them based on position.

Example

def my_function(animal, name):
    print("I have a", animal)
    print("My", animal + "'s name is", name)

my_function("dog", "Buddy")

⚠️ Order matters

my_function("Buddy", "dog")  # Wrong meaning

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7. Mixing Positional and Keyword Arguments

You can mix both types, but:

Positional arguments must come first

Correct Example

def my_function(animal, name, age):
    print("I have a", age, "year old", animal, "named", name)

my_function("dog", name="Buddy", age=5)

Incorrect Example

my_function(name="Buddy", "dog", age=5)  # Error

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8. Passing Different Data Types as Arguments

Python functions can accept any data type:

• string
• integer
• float
• list
• tuple
• dictionary

Example: Passing a List

def my_function(fruits):
    for fruit in fruits:
        print(fruit)

my_fruits = ["apple", "banana", "cherry"]
my_function(my_fruits)

Example: Passing a Dictionary

def my_function(person):
    print("Name:", person["name"])
    print("Age:", person["age"])

my_person = {"name": "Emil", "age": 25}
my_function(my_person)

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9. Return Values

A function can send data back using the return keyword.

Example

def my_function(x, y):
    return x + y

result = my_function(5, 3)
print(result)

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10. Returning Different Data Types

Functions can return any type of data.

Returning a List

def my_function():
    return ["apple", "banana", "cherry"]

fruits = my_function()
print(fruits)

Returning a Tuple

def my_function():
    return (10, 20)

x, y = my_function()
print(x, y)

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11. Positional-Only Arguments (Python 3.8+)

Use / to force parameters to be positional-only.

Example

def my_function(name, /):
    print("Hello", name)

my_function("Emil")      # Valid
my_function(name="Emil") # Error

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12. Keyword-Only Arguments

Use * to force parameters to be keyword-only.

Example

def my_function(*, name):
    print("Hello", name)

my_function(name="Babina")
my_function("Babina")  # Error

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13. Combining Positional-Only and Keyword-Only Arguments

You can combine both / and * in a single function.

Example

def my_function(a, b, /, *, c, d):
    return a + b + c + d

result = my_function(5, 10, c=15, d=20)
print(result)

Explanation:

• a, b → positional-only
• c, d → keyword-only

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Challenges: Day 2

Challenge 1

Create a function that accepts name and age (default = 18).

def person_info(name, age=18):
    print(f"Name: {name}, Age: {age}")

person_info("Subin")
person_info("Subin", 20)

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Challenge 2

def student_report(name, marks, grade="A"):
    print("Name:", name)
    print("Marks:", marks)
    print("Grade:", grade)

student_report("Subin", 85, "A+")
student_report("Subin", 85)

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Challenge 3

def employee_info(name, /, *, department, salary=30000):
    print("Employee:", name)
    print("Department:", department)
    print("Salary:", salary)

employee_info("Subin", department="IT")
employee_info("Subin", department="Finance", salary=50000)

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