Function definition and syntax in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax to define a function in Python?

Use the def keyword, followed by the function name, parentheses (), and a colon :. Then indent the function body below. Example:

def my_function():
    print("Hello")

beginner

What does the return statement do inside a function?

It sends a value back to where the function was called. This lets the function give back a result. Without return, the function returns None by default.

beginner

Can a Python function have parameters? How are they used?

Yes, parameters are names inside the parentheses in the function definition. They act like placeholders for values you give when calling the function. Example:

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

intermediate

What happens if you call a function without parentheses?

You get a reference to the function itself, not the result of running it. To run the function and get its output, you must use parentheses, even if empty.

beginner

Why is indentation important in Python function definitions?

Indentation shows which lines belong inside the function. Python uses indentation instead of braces. Without correct indentation, Python will give an error or run code incorrectly.

Which keyword is used to define a function in Python?

A define

B function

C func

D def

The correct keyword to define a function in Python is def.

What will this function return if called without a return statement?

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

A The sum of a and b

B None

C An error

D a + b as a string

Without a return statement, Python functions return None by default.

How do you call a function named say_hello with no parameters?

A say_hello()

B say_hello

C call say_hello

D def say_hello()

You call a function by writing its name followed by parentheses, even if empty.

What is the purpose of parameters in a function?

A To return values from the function

B To store the function's name

C To hold values passed into the function

D To indent the function body

Parameters are placeholders for values you pass into the function when calling it.

Why must the function body be indented in Python?

A Because Python uses indentation to group code blocks

B To separate the function from other code

C To make the code look pretty

D To add comments inside the function

Python uses indentation to know which lines belong inside the function or other blocks.

Explain how to define a simple function in Python that takes one parameter and returns a value.

Think about the parts you need to write a function that gives back a result.

Describe why indentation is important in Python function definitions and what happens if it is missing.

Consider how Python knows which lines belong inside the function.

Practice

1. What keyword do you use to start defining a function in Python?

easy

A. function

B. define

C. func

D. def

Solution

1. Step 1: Recall Python function syntax

   In Python, functions are defined using the keyword def.

2. Step 2: Compare options

   Only def uses the correct keyword def. Others are not valid Python keywords.

3. Final Answer:

   def -> Option D

4. Quick Check:

   Function definition starts with def [OK]

Hint: Remember: def starts every function in Python [OK]

Common Mistakes:

• Using 'function' instead of 'def'
• Using 'func' or 'define' which are not Python keywords

2. Which of the following is the correct syntax to define a function named greet that takes no parameters?

easy

A. def greet():

B. function greet():

C. def greet[]:

D. def greet:

Solution

1. Step 1: Check function header syntax

   The correct syntax uses def, function name, parentheses for parameters, and a colon.

2. Step 2: Validate each option

   def greet(): matches this exactly: def greet():. Others have wrong keywords, brackets, or missing parentheses.

3. Final Answer:

   def greet(): -> Option A

4. Quick Check:

   def + name + () + : is correct syntax [OK]

Hint: Function name + () + : after def is always needed [OK]

Common Mistakes:

• Omitting parentheses after function name
• Using square brackets instead of parentheses
• Using 'function' keyword instead of 'def'

3. What will be the output of this code?

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

print(add(3, 4))

medium

A. 7

B. 34

C. TypeError

D. None

Solution

1. Step 1: Understand function behavior

   The function add takes two numbers and returns their sum.

2. Step 2: Calculate the return value

   Calling add(3, 4) returns 3 + 4 = 7, which is printed.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Return sends back the sum; print shows it [OK]

Common Mistakes:

• Concatenating numbers as strings (getting '34')
• Forgetting to return value (getting None)
• Passing wrong number of arguments causing TypeError

4. Find the error in this function definition:

def multiply(a, b)
    return a * b

medium

A. Indentation error in return statement

B. Missing colon at the end of function header

C. Wrong function name

D. Parameters should be in square brackets

Solution

1. Step 1: Check function header syntax

   The function header must end with a colon (:).

2. Step 2: Identify missing colon

   The code misses the colon after def multiply(a, b), causing a syntax error.

3. Final Answer:

   Missing colon at the end of function header -> Option B

4. Quick Check:

   Function header must end with : [OK]

Hint: Always put : after function header line [OK]

Common Mistakes:

• Forgetting colon after function header
• Incorrect indentation of return line
• Using wrong brackets for parameters

5. You want to write a function is_even that returns True if a number is even, otherwise False. Which is the correct function?

hard

A. def is_even(n):
return n % 2

B. def is_even(n):
return n / 2 == 0

C. def is_even(n):
if n % 2 == 0:
return True
else:
return False

D. def is_even(n):
if n % 2:
return True
else:
return False

Solution

1. Step 1: Understand even number check

   A number is even if remainder when divided by 2 is zero (n % 2 == 0).

2. Step 2: Evaluate each option

   def is_even(n): if n % 2 == 0: return True else: return False correctly returns True if remainder is zero, else False. def is_even(n): return n / 2 == 0 uses division instead of modulo. def is_even(n): if n % 2: return True else: return False returns True when remainder is non-zero (odd). def is_even(n): return n % 2 returns remainder directly (not boolean).

3. Final Answer:

   def is_even(n): if n % 2 == 0: return True else: return False -> Option C

4. Quick Check:

   Use modulo == 0 to check even [OK]

Hint: Use n % 2 == 0 to check even numbers [OK]

Common Mistakes:

• Using division instead of modulo
• Returning remainder instead of boolean
• Confusing condition for even check