Function definition and syntax in Python - Step-by-Step Execution

Concept Flow - Function definition and syntax

Start

↓

Define function with def

↓

Function name and parameters

↓

Function body indented

↓

Function ends

↓

Call function to run it

↓

Function runs and returns

↓

End

This flow shows how a function is defined with def, given a name and parameters, has an indented body, and then is called to run.

Execution Sample

Python

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

greet("Alice")

Defines a function greet that prints a hello message, then calls it with 'Alice'.

Execution Table

Step	Action	Evaluation	Result
1	Define function greet with parameter name	No output	Function greet created
2	Call greet with argument 'Alice'	name = 'Alice'	Function greet starts
3	Execute print inside greet	Prints 'Hello, Alice!'	Output: Hello, Alice!
4	Function greet ends	No return value	Back to main program

💡 Function greet has finished running after printing the message.

Variable Tracker

Variable	Start	After call greet('Alice')	Final
name	undefined	'Alice'	undefined after function ends

Key Moments - 3 Insights

Why do we indent the function body?

Indentation shows which lines belong inside the function, as seen in step 3 where print is indented under greet.

What happens if we call the function without parentheses?

Without parentheses, the function does not run; step 2 shows calling greet() with parentheses runs the function.

Is the variable 'name' available outside the function?

No, 'name' exists only inside the function during step 2 and 3, then disappears after step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'name' during step 3?

A 'Alice'

B undefined

C None

D greet

💡 Hint

Check the 'Evaluation' column at step 3 in the execution table.

At which step does the function greet actually run its code?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for the step where the print statement executes in the execution table.

If we remove the parentheses in greet("Alice"), what changes in the execution table?

A Error occurs at step 2

B Function greet runs twice

C Function greet is defined but never called

D Output prints twice

💡 Hint

Refer to step 2 where the function call happens with parentheses.

Concept Snapshot

def function_name(parameters):
    indented function body

- Use def to define a function.
- Function body must be indented.
- Call function with parentheses to run it.
- Parameters receive input values.

Full Transcript

This example shows how to define a function in Python using def, give it a name and parameters, write an indented body, and then call it with parentheses to run. The variable inside the function exists only during the function call. Indentation is important to mark the function body. Calling the function without parentheses does not run it.

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