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Python for Beginners

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Functions

Basics10 min readLesson 8 of 8

A function is a named block of code that you can call whenever you need it. You give input through parameters, and the function can send a result back with return. A short function that does one clear job is easier to test and reuse than the same lines copied through a program. A function with no parameters is still called with parentheses.

python
def greet(name):
    message = 'Hello, ' + name
    print(message)

greet('Asha')

Output

Hello, Asha

8.1 Defining and calling

def starts the definition, then the function name, parentheses, and a colon. The indented body runs when you call the function by name. The names in the definition are parameters; the values you pass in the call are arguments. A return statement ends the function immediately and sends a value back to the caller. The caller can store that value, print it, or use it in another expression.

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

result = add(3, 4)
print(result * 2)

Output

14

8.2 Common mistake

A function that uses print() has shown the value and returned nothing. print() displays something; return sends a value back to the caller. A function with no return statement gives back None, even when it printed helpful text. If the caller stores that value and adds a number, Python raises TypeError. Return the value when you want to use it later, and print when seeing the text is enough.

python
def greet(name, greeting='Hello'):
    return f'{greeting}, {name}!'

print(greet('Ravi'))
print(greet('Mira', 'Good morning'))

Output

Hello, Ravi!
Good morning, Mira!

8.3 Defaults and local names

  • Default values are assigned when the function is defined, so they suit settings that usually stay the same.
  • Parameters without defaults must come before parameters with defaults.
  • Callers can override a default by passing a value in the matching position, or by naming the parameter, as in greet('Mira', greeting='Hi').

A name assigned inside a function is local and exists only while the function runs, so the same name outside the function keeps its own value.

python
score = 10
def show():
    score = 3
    print(score)
show()
print(score)

Output

3
10

8.4 Practice

  1. Write area(length, width) that returns the area of a rectangle, then print the result outside the function.
  2. Give width a default value and call the function with one argument and with two.
  3. Add a local variable named label inside the function and a different label outside it, then print both.

Interview questions

3 questions from this note. Open one to see the answer; no account needed.

  • 01What is the difference between `print()` and `return` inside a function?

    `print()` writes text to the screen and returns `None`; `return` sends a value back to the caller. Use `return` when the result needs to be stored or reused.

  • 02Why must parameters with default values come after parameters without them?

    Python raises a syntax error when a parameter without a default follows one with a default. Put required parameters first.

  • 03What happens to a variable assigned inside a function when the function ends?

    It is local to that function and normally disappears when the call ends. A same-named variable outside the function keeps its own value.

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