Python Functions

A function is a named block of code that runs only when you call it.

Creating and Calling a Function

Use the def keyword, a name, brackets and a colon. The body is indented.

def greet():
    print("Assalam o Alaikum!")
    print("Welcome to Workownix")

greet()
greet()

Assalam o Alaikum!
Welcome to Workownix
Assalam o Alaikum!
Welcome to Workownix

Write the code once, use it many times.

Parameters and Arguments

A function can receive values. The names in the brackets are parameters. The values you pass when calling are arguments.

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

greet("Ayesha", "Karachi")
greet("Usman", "Multan")

Hello Ayesha from Karachi
Hello Usman from Multan

Returning a Value

return sends a result back to the caller. The function stops at return.

def total_bill(price, qty):
    return price * qty

bill = total_bill(250, 4)
print(bill)
print(total_bill(99, 2) + 50)

1000
248

A function without return gives back None.

Default Values and Keyword Arguments

def make_tea(sugar=2, milk="full"):
    return f"Tea with {sugar} sugar and {milk} milk"

print(make_tea())
print(make_tea(1))
print(make_tea(milk="less", sugar=0))

Tea with 2 sugar and full milk
Tea with 1 sugar and full milk
Tea with 0 sugar and less milk

With keyword arguments (milk="less"), the order does not matter.

Any Number of Arguments

*args collects extra positional arguments into a tuple. **kwargs collects extra keyword arguments into a dictionary.

def add_all(*numbers):
    return sum(numbers)

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

print(add_all(5, 10, 15, 20))
profile(name="Sara", age=21)

50
name = Sara
age = 21

Docstrings

A string on the first line of a function explains what it does. Tools and editors show it as help.

def area(length, width):
    """Return the area of a rectangle."""
    return length * width

print(area(12, 8))

96

Scope

Variables created inside a function are local. They exist only inside that function.

def show():
    message = "inside"
    print(message)

show()
print(message)

inside
NameError: name 'message' is not defined

Lambda Functions

A lambda is a tiny function written in one line, often used with sorted(), map() or filter():

square = lambda x: x * x
print(square(6))

students = [("Ali", 72), ("Sana", 91), ("Omar", 85)]
print(sorted(students, key=lambda s: s[1], reverse=True))

36
[('Sana', 91), ('Omar', 85), ('Ali', 72)]

Exercise