Functions in Python — def, Parameters, return and Scope

A function is a named block of code you can run again and again. It keeps programs short, readable and easy to test.

1. Define and call

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

greet("Asha")
greet("Ravi")

Hello, Asha!
Hello, Ravi!

• def starts a function, followed by its name and parameters in brackets.
• The body is indented.
• Calling it with a value — an argument — runs the body.

2. return: give a value back

print only shows a value; return sends it back so you can use it.

def area(length, width=1):
    return length * width

a = area(5, 3)
print(a, area(4), area(width=2, length=6))

15 4 12

• width=1 is a default value used when no width is given.
• area(width=2, length=6) uses keyword arguments — order doesn't matter.
• A function without return returns None.

Returning several values

def stats(nums):
    return min(nums), max(nums), sum(nums) / len(nums)

low, high, avg = stats([4, 8, 6])
print(low, high, avg)

4 8 6.0

3. *args and **kwargs

• *args collects any number of positional arguments into a tuple.
• **kwargs collects keyword arguments into a dictionary.

def total(*args):
    return sum(args)

def profile(**kwargs):
    for key, value in kwargs.items():
        print(key, "=", value)

print(total(1, 2, 3, 4))
profile(name="Neha", city="Indore")

10
name = Neha
city = Indore

4. Scope: local vs global

Variables created inside a function are local — they exist only while it runs.

count = 10          # global

def show():
    count = 99      # a NEW local variable
    print("inside:", count)

show()
print("outside:", count)

inside: 99
outside: 10

To change a global variable inside a function, declare global count — but usually it's cleaner to pass values in and return results.

5. The mutable default trap

def add_item(item, basket=[]):     # the same list is reused every call!
    basket.append(item)
    return basket

print(add_item("pen"))
print(add_item("book"))

['pen']
['pen', 'book']

Fix it with None:

def add_item(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

print(add_item("pen"))
print(add_item("book"))

['pen']
['book']

This is a favourite interview question.

6. Docstrings

Put a description in triple quotes as the first line of the function. Tools and help() show it.

def is_even(n):
    """Return True if n is even."""
    return n % 2 == 0

print(is_even(10), is_even.__doc__)

True Return True if n is even.

Common mistakes

• Using print inside a function when you need return.
• Forgetting the brackets when calling: greet vs greet("Asha").
• Mutable default arguments (def f(x=[])).

Quick recap

• def name(params): + indented body; call with name(args).
• return gives a value back; no return → None.
• Defaults, keyword arguments, *args (tuple), **kwargs (dict).
• Local variables vanish after the call; prefer return over global.