Method invocation flow in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of chained method calls

What is the output of this Python code involving chained method calls?

Python

class TextProcessor:
    def __init__(self, text):
        self.text = text
    def to_upper(self):
        self.text = self.text.upper()
        return self
    def add_exclamation(self):
        self.text += '!'
        return self
    def get_text(self):
        return self.text

result = TextProcessor('hello').to_upper().add_exclamation().get_text()
print(result)

A "hello"

B "hello!"

C "HELLO"

D "HELLO!"

💡 Hint

Look at how each method modifies the text and returns self for chaining.

The methods to_upper and add_exclamation both modify the text and return the object itself, allowing chaining. Finally, get_text returns the modified string 'HELLO!'.

❓ Predict Output

intermediate

Method call order and output

What will be printed when this code runs?

Python

class Counter:
    def __init__(self):
        self.count = 0
    def increment(self):
        self.count += 1
        return self
    def double(self):
        self.count *= 2
        return self
    def get(self):
        return self.count

c = Counter()
print(c.increment().double().increment().get())

A 4

B 3

C 2

D 5

💡 Hint

Trace the count value step by step through each method call.

Starting at 0: increment() makes 1, double() makes 2, increment() makes 3, get() returns 3.

🔧 Debug

advanced

Identify the error in method chaining

This code tries to chain methods but raises an error. What is the error type?

Python

class Builder:
    def __init__(self):
        self.value = ''
    def add_a(self):
        self.value += 'a'
    def add_b(self):
        self.value += 'b'
    def get_value(self):
        return self.value

b = Builder()
print(b.add_a().add_b().get_value())

A NoneType has no attribute 'add_b'

B TypeError

C SyntaxError

D AttributeError

💡 Hint

Check what each method returns and how chaining works.

Methods add_a and add_b do not return self, so add_a() returns None. Then None.add_b() causes an AttributeError: 'NoneType' object has no attribute 'add_b'.

🧠 Conceptual

advanced

Understanding method invocation flow with inheritance

Given these classes, what will be the output of the code below?

Python

class Base:
    def action(self):
        return 'Base'
class Child(Base):
    def action(self):
        return 'Child' + super().action()

obj = Child()
print(obj.action())

A "ChildBase"

B "BaseChild"

C "Child"

D "Base"

💡 Hint

Look at how super() calls the parent method inside the child method.

Child's action method returns 'Child' plus the result of Base's action method, which is 'Base', so the output is 'ChildBase'.

❓ Predict Output

expert

Complex method invocation with side effects

What is the final output of this code?

Python

class Tracker:
    def __init__(self):
        self.log = []
    def step1(self):
        self.log.append('1')
        return self
    def step2(self):
        self.log.append('2')
        return self
    def step3(self):
        self.log.append('3')
        return self
    def get_log(self):
        return ''.join(self.log)

tracker = Tracker()
result = tracker.step1().step2().step1().step3().get_log()
print(result)

A "123"

B "1123"

C "1213"

D "213"

💡 Hint

Follow the order of method calls and what each appends to the log.

The calls append '1', then '2', then '1' again, then '3', so the log is '1213'.

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