Methods with return values in Python - Step-by-Step Execution

Concept Flow - Methods with return values

Define method

↓

Call method

↓

Execute method body

↓

Return value

↓

Receive returned value

↓

Use returned value

This flow shows how a method is defined, called, executes, returns a value, and how that value is used.

Execution Sample

Python

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

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

This code defines a method that adds two numbers and returns the sum, then prints the result.

Execution Table

Step	Action	Evaluation	Result
1	Define method 'add'	Store function with parameters a, b	Method ready to call
2	Call add(3, 4)	Parameters a=3, b=4	Enter method body
3	Calculate a + b	3 + 4	7
4	Return 7	Return value sent back	Method call replaced by 7
5	Assign result = 7	Store 7 in variable 'result'	result = 7
6	Print result	Output value of result	7
7	End	No more code	Program stops

💡 Program ends after printing the returned value 7

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
a	N/A	3	3	3
b	N/A	4	4	4
result	Undefined	Undefined	7	7

Key Moments - 2 Insights

Why do we need the 'return' statement inside the method?

The 'return' sends the result back to where the method was called, as shown in step 4 of the execution_table. Without it, the method would not give any output to use.

What happens to the method call after it returns a value?

After returning, the method call is replaced by the returned value (step 4). This value can then be assigned or used elsewhere, like in step 5 where 'result' gets the value 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'result' after step 5?

A 3

B Undefined

C 7

D 4

💡 Hint

Check the 'result' variable in variable_tracker after step 5

At which step does the method return its value?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Return 7' action in the execution_table

If the method did not have a return statement, what would 'result' be after step 5?

A 7

B None

C Error

D 3

💡 Hint

In Python, a method without return returns None by default

Concept Snapshot

def method_name(params):
    return value

- 'return' sends value back to caller
- Caller can store or use returned value
- Without return, method returns None
- Use returned value to continue program logic

Full Transcript

This lesson shows how methods with return values work in Python. First, a method is defined with parameters. When called, the method runs its code and uses 'return' to send a value back. The returned value replaces the method call and can be stored in a variable or used directly. For example, a method adding two numbers returns their sum, which can then be printed or used elsewhere. The 'return' statement is essential to get output from a method. Without it, the method returns None by default. This flow helps organize code and reuse logic with results.

Practice

1. What does a method with a return statement do in Python?

easy

A. It sends a value back to where the method was called.

B. It prints a value on the screen.

C. It stops the program immediately.

D. It creates a new variable automatically.

Solution

1. Step 1: Understand the purpose of return

   The return statement sends a value back from the method to the caller.

2. Step 2: Differentiate from printing or stopping

   Printing shows output but does not send a value back; stopping ends execution.

3. Final Answer:

   It sends a value back to where the method was called. -> Option A

4. Quick Check:

   Method return = sends value back [OK]

Hint: Return sends value back, print shows it only [OK]

Common Mistakes:

• Confusing return with print
• Thinking return stops the program
• Believing return creates variables

2. Which of the following is the correct syntax for a method that returns the sum of two numbers a and b?

easy

A. def add(a, b): return a - b

B. def add(a, b): print(a + b)

C. def add(a, b): return a + b

D. def add(a, b): a + b

Solution

1. Step 1: Identify correct return usage

   The method must use return to send back the sum a + b.

2. Step 2: Check each option

   The version with return a - b returns the difference. The version with print(a + b) prints but returns None. The version with just a + b lacks return. Only def add(a, b): return a + b correctly returns the sum.

3. Final Answer:

   def add(a, b): return a + b -> Option C

4. Quick Check:

   Return sum = def add(a, b): return a + b [OK]

Hint: Return must be followed by value to send back [OK]

Common Mistakes:

• Using print instead of return
• Omitting return keyword
• Returning wrong expression

3. What is the output of this code?

def multiply(x, y):
    return x * y

result = multiply(3, 4)
print(result)

medium

A. 7

B. 12

C. 34

D. None

Solution

1. Step 1: Understand the method call

   The method multiply returns the product of 3 and 4, which is 12.

2. Step 2: Print the returned value

   The variable result stores 12, so print(result) outputs 12.

3. Final Answer:

   12 -> Option B

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs, return result, print shows it [OK]

Common Mistakes:

• Adding instead of multiplying
• Printing None by missing return
• Confusing string concatenation with multiplication

4. Find the error in this method and choose the correct fix:

def greet(name):
    print("Hello, " + name)

message = greet("Alice")
print(message)

medium

A. Remove the argument name from the method.

B. Add print before calling greet.

C. Change message to greet in the last print.

D. Change print to return inside the method.

Solution

1. Step 1: Identify the problem with return value

   The method prints but does not return a value, so message is None.

2. Step 2: Fix by returning the greeting string

   Replace print with return to send the greeting back.

3. Final Answer:

   Change print to return inside the method. -> Option D

4. Quick Check:

   Return greeting to assign message [OK]

Hint: Use return to get value, not print [OK]

Common Mistakes:

• Expecting print to return a value
• Removing needed parameters
• Changing variable names incorrectly

5. You want to write a method that returns a dictionary with keys as numbers from 1 to n and values as their squares. Which method below correctly does this?

hard

A. def squares(n):
result = {}
for i in range(1, n+1):
result[i] = i * i
return result

B. def squares(n):
result = []
for i in range(n):
result.append(i * i)
return result

C. def squares(n):
return {i: i + i for i in range(1, n)}

D. def squares(n):
for i in range(1, n+1):
print(i * i)

Solution

1. Step 1: Understand the requirement

   The method must return a dictionary with keys 1 to n and values as squares.

2. Step 2: Check each option

   The loop building result = {}, setting result[i] = i * i for i in range(1, n+1), and returning result is correct. Returning a list fails. The comprehension {i: i + i for i in range(1, n)} uses doubles instead of squares and misses key n. Printing without returning fails.

3. Final Answer:

   def squares(n): result = {} for i in range(1, n+1): result[i] = i * i return result -> Option A

4. Quick Check:

   Return dict with squares = def squares(n): result = {} for i in range(1, n+1): result[i] = i * i return result [OK]

Hint: Return dict with keys and squares using loop [OK]

Common Mistakes:

• Returning list instead of dict
• Using wrong range limits
• Printing instead of returning