Method invocation flow in Python - Mini Project: Build & Apply

Method Invocation Flow

📖 Scenario: Imagine you have a simple robot that can greet people and say goodbye. You want to organize its actions using methods inside a class.

🎯 Goal: You will create a class with methods and then call these methods in the right order to see the robot greet and say goodbye.

📋 What You'll Learn

Create a class called Robot

Add a method called greet that prints 'Hello!'

Add a method called say_goodbye that prints 'Goodbye!'

Create an instance of Robot called my_robot

Call the greet method on my_robot

Call the say_goodbye method on my_robot

💡 Why This Matters

🌍 Real World

Classes and methods help organize code for robots, games, apps, and many programs that need actions grouped together.

💼 Career

Understanding how to define and call methods in classes is a key skill for software developers working on object-oriented programming.

1

Create the Robot class

Create a class called Robot with no methods inside.

Python

# Your code here

Hint

A class is created using the class keyword followed by the class name and a colon.

2

Add greet and say_goodbye methods

Inside the Robot class, add a method called greet that prints 'Hello!' and another method called say_goodbye that prints 'Goodbye!'.

Python

class Robot:
    # Your code here

Hint

Methods inside a class need self as the first parameter. Use print() to show messages.

3

Create an instance of Robot

Create an instance of the Robot class called my_robot.

Python

class Robot:
    def greet(self):
        print('Hello!')

    def say_goodbye(self):
        print('Goodbye!')

# Your code here

Hint

To create an object, write the class name followed by parentheses.

4

Call greet and say_goodbye methods

Call the greet method on my_robot and then call the say_goodbye method on my_robot.

Python

class Robot:
    def greet(self):
        print('Hello!')

    def say_goodbye(self):
        print('Goodbye!')

my_robot = Robot()
# Your code here

Hint

Use dot notation to call methods on an object, like my_robot.greet().

Practice

1. What does method invocation flow describe in Python?

easy

A. The order in which methods are called and executed

B. The way variables are declared inside methods

C. How to write comments inside methods

D. The syntax rules for defining methods

Solution

1. Step 1: Understand the term 'method invocation'

   Method invocation means calling a method to run its code.

2. Step 2: Understand 'flow' in this context

   Flow means the order or sequence in which these method calls happen.

3. Final Answer:

   The order in which methods are called and executed -> Option A

4. Quick Check:

   Method invocation flow = method call order [OK]

Hint: Think: When you call methods, what order do they run? [OK]

Common Mistakes:

• Confusing method flow with variable declaration
• Thinking it means method syntax rules
• Mixing it up with comments inside methods

2. Which of the following is the correct way to call a method named greet on an object person?

easy

A. greet.person()

B. person.greet()

C. person->greet()

D. greet(person)

Solution

1. Step 1: Recall Python method call syntax

   In Python, to call a method on an object, use dot notation: object.method()

2. Step 2: Check each option

   person.greet() uses correct dot notation with parentheses. Others use invalid syntax or function call style.

3. Final Answer:

   person.greet() -> Option B

4. Quick Check:

   Object.method() is correct call syntax [OK]

Hint: Remember: object.method() calls a method in Python [OK]

Common Mistakes:

• Using arrow (->) like other languages
• Reversing object and method order
• Calling method without parentheses

3. What is the output of this code?

class A:
    def first(self):
        print('First')
        self.second()
    def second(self):
        print('Second')

obj = A()
obj.first()

medium

A. Second

B. Second\nFirst

C. First\nSecond

D. First

Solution

1. Step 1: Trace method calls

   Calling obj.first() prints 'First' then calls self.second(), which prints 'Second'.

2. Step 2: Determine output order

   Output is 'First' then 'Second' on separate lines.

3. Final Answer:

   First\nSecond -> Option C

4. Quick Check:

   Method calls run in order called [OK]

Hint: Follow method calls step-by-step to find output order [OK]

Common Mistakes:

• Assuming second() runs before first()
• Missing the call to second() inside first()
• Thinking only first print runs

4. Find the error in this code:

class B:
    def start(self):
        self.middle()
    def middle(self):
        self.end()
    def end(self):
        print('Done')

b = B()
b.middle()

medium

A. No output because start() is not called

B. AttributeError because end() is not defined

C. TypeError due to missing arguments

D. No error, prints 'Done'

Solution

1. Step 1: Check method definitions

   All methods are defined correctly with self parameter.

2. Step 2: Check method call

   b.middle() calls self.end(), which prints 'Done'. So output is 'Done' with no error.

3. Final Answer:

   No error, prints 'Done' -> Option D

4. Quick Check:

   Calling middle() runs end() correctly [OK]

Hint: Trace calls from the method you invoke to see output [OK]

Common Mistakes:

• Expecting start() must be called first
• Thinking end() is undefined
• Confusing missing arguments error

5. Given this code, what will be printed?

class C:
    def a(self):
        print('A')
        self.b()
    def b(self):
        print('B')
        self.c()
    def c(self):
        print('C')

c = C()
c.a()

hard

A. A\nB\nC

B. A\nC\nB

C. C\nB\nA

D. B\nC\nA

Solution

1. Step 1: Follow method calls starting from c.a()

   c.a() prints 'A' then calls self.b()

2. Step 2: Trace self.b() and self.c()

   self.b() prints 'B' then calls self.c(), which prints 'C'.

3. Step 3: Combine outputs in order

   Output is 'A' then 'B' then 'C' each on new lines.

4. Final Answer:

   A\nB\nC -> Option A

5. Quick Check:

   Method calls chain in order a->b->c [OK]

Hint: Follow each method call in order to find print sequence [OK]

Common Mistakes:

• Mixing order of method calls
• Skipping intermediate method calls
• Assuming methods run independently