String representation methods in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __str__ vs __repr__

What is the output of the following code?

Python

class Fruit:
    def __init__(self, name):
        self.name = name
    def __str__(self):
        return f"Fruit: {self.name}"
    def __repr__(self):
        return f"Fruit('{self.name}')"

f = Fruit('Apple')
print(str(f))
print(repr(f))

A

Fruit: Apple
Apple

B

Fruit('Apple')
Fruit: Apple

C

Apple
Fruit('Apple')

D

Fruit: Apple
Fruit('Apple')

💡 Hint

Remember __str__ is for user-friendly display, __repr__ is for developer-friendly output.

The str() function calls __str__ which returns 'Fruit: Apple'. The repr() function calls __repr__ which returns 'Fruit('Apple')'.

❓ Predict Output

intermediate

Using __repr__ in a list

What is the output of this code?

Python

class Animal:
    def __init__(self, species):
        self.species = species
    def __repr__(self):
        return f"Animal('{self.species}')"

animals = [Animal('Cat'), Animal('Dog')]
print(animals)

A [Animal('Cat'), Animal('Dog')]

B ['Cat', 'Dog']

C [Animal(Cat), Animal(Dog)]

D [<Animal object>, <Animal object>]

💡 Hint

Lists use __repr__ of their elements when printed.

When printing a list, Python calls __repr__ on each element. So it shows the string returned by __repr__ of each Animal.

🔧 Debug

advanced

Fix the __str__ method to avoid error

This code raises an error when printing the object. Which option fixes the error?

Python

class Book:
    def __init__(self, title):
        self.title = title
    def __str__(self):
        return 'Book title is ' + self.title.upper

b = Book('Python 101')
print(b)

A

def __str__(self):
    return 'Book title is ' + self.title.upper()

B

def __str__(self):
    return 'Book title is ' + str(self.title.upper)

C

def __str__(self):
    return 'Book title is ' + self.title.upper

D

def __str__(self):
    return 'Book title is ' + self.title

💡 Hint

Check if you are calling the method or just referencing it.

The error happens because self.title.upper is a method, not a string. You must call it with parentheses: self.title.upper().

🧠 Conceptual

advanced

Difference between __str__ and __repr__

Which statement best describes the difference between __str__ and __repr__ methods in Python?

A <code>__str__</code> is only used when printing lists; <code>__repr__</code> is used everywhere else.

B <code>__str__</code> is for a readable string for users; <code>__repr__</code> is for unambiguous string for developers.

C Both methods return the same string but <code>__repr__</code> is faster.

D <code>__repr__</code> is for readable string for users; <code>__str__</code> is for debugging.

💡 Hint

Think about who the string is for: user or developer.

__str__ returns a friendly string for users, while __repr__ returns a detailed string useful for developers to identify the object.

❓ Predict Output

expert

Output of custom __repr__ with eval

What is the output of this code?

Python

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __repr__(self):
        return f"Point({self.x}, {self.y})"

p = Point(3, 4)
q = eval(repr(p))
print(p == q)
print(q)

A

False
<__main__.Point object at 0x...>

B

True
Point(3, 4)

C

False
Point(3, 4)

D

True
<__main__.Point object at 0x...>

💡 Hint

Does eval(repr(p)) create an equal object? What about equality check?

The repr returns 'Point(3, 4)', but eval tries to call Point(3, 4). Since Point constructor exists, q is a new object with same values but different identity. The default == compares identity, so p == q is False.

Practice

1. Which method in Python is used to define the informal string representation of an object, typically for end users, and is called by the print() function?

easy

A. __init__

B. __repr__

C. __str__

D. __del__

Solution

1. Step 1: Understand the purpose of __str__

   The __str__ method returns a readable string for users, used by print().

2. Step 2: Compare with __repr__

   The __repr__ method returns a detailed string for developers, not usually for printing.

3. Final Answer:

   __str__ -> Option C

4. Quick Check:

   Informal string for print() = __str__ [OK]

Hint: Use __str__ for user-friendly print output [OK]

Common Mistakes:

• Confusing __repr__ with __str__
• Thinking __init__ controls string output
• Assuming __del__ affects printing

2. Which of the following is the correct syntax to define the __repr__ method inside a Python class?

easy

A. def __repr__(self): self.return 'object info'

B. def repr(self): return 'object info'

C. def __repr__(self): print('object info')

D. def __repr__(self): return 'object info'

Solution

1. Step 1: Check method name and parameters

   The method must be named __repr__ and take self as parameter.

2. Step 2: Verify return statement

   __repr__ must return a string, not print or use invalid syntax.

3. Final Answer:

   def __repr__(self): return 'object info' -> Option D

4. Quick Check:

   Correct __repr__ syntax returns string [OK]

Hint: __repr__ must return a string, not print it [OK]

Common Mistakes:

• Omitting underscores in __repr__
• Using print instead of return
• Wrong method name without underscores

3. What is the output of this code?

class Cat:
    def __repr__(self):
        return 'Cat()'
    def __str__(self):
        return 'A cute cat'

c = Cat()
print(c)
print(repr(c))

medium

A. A cute cat
Cat()

B. Cat()
A cute cat

C. A cute cat
A cute cat

D. Cat()
Cat()

Solution

1. Step 1: Understand print(c) calls __str__

   The print(c) calls __str__, which returns 'A cute cat'.

2. Step 2: Understand print(repr(c)) calls __repr__

   The repr(c) calls __repr__, which returns 'Cat()'.

3. Final Answer:

   A cute cat Cat() -> Option A

4. Quick Check:

   print() = __str__, repr() = __repr__ [OK]

Hint: print() uses __str__, repr() uses __repr__ [OK]

Common Mistakes:

• Mixing __str__ and __repr__ outputs
• Assuming print calls __repr__
• Confusing repr() with str()

4. Identify the error in this class definition related to string representation methods:

class Dog:
    def __str__(self):
        return 'Dog'
    def __repr__(self):
        print('Dog object')

print(Dog())

medium

A. __repr__ should return a string, not print it

B. __str__ method is missing self parameter

C. print(Dog()) should be print(Dog)

D. __repr__ method name is incorrect

Solution

1. Step 1: Check __repr__ method body

   The __repr__ method uses print() instead of returning a string, which is incorrect.

2. Step 2: Understand consequences

   Because __repr__ returns None, printing the object calls __str__ but repr() would fail to give a string.

3. Final Answer:

   __repr__ should return a string, not print it -> Option A

4. Quick Check:

   __repr__ must return string, not print [OK]

Hint: Always return string in __repr__, never print [OK]

Common Mistakes:

• Using print instead of return in __repr__
• Forgetting self parameter in methods
• Confusing print(Dog()) with print(Dog)

5. You want to create a class Book where print(book) shows the title nicely, but repr(book) shows a string that can recreate the object. Which implementation correctly achieves this?

class Book:
    def __init__(self, title):
        self.title = title

    def __str__(self):
        return f"Book titled '{self.title}'"

    def __repr__(self):
        # Which line below is correct?
        pass

hard

A. return f"Book('{self.title}')"

B. return f"Book(title='{self.title}')"

C. return f"Book(title={self.title})"

D. return f"Book({self.title})"

Solution

1. Step 1: Understand __str__ output

   The __str__ method returns a user-friendly string with the title.

2. Step 2: Create __repr__ that recreates object

   The __repr__ should return a string that looks like the constructor call with a keyword argument and quotes around the title.

3. Step 3: Check options for correct syntax

   return f"Book(title='{self.title}')" returns Book(title='title') which can be used to recreate the object. Others miss quotes or keyword.

4. Final Answer:

   return f"Book(title='{self.title}')" -> Option B

5. Quick Check:

   __repr__ returns constructor call string [OK]

Hint: __repr__ should return code to recreate object [OK]

Common Mistakes:

• Missing quotes around string in __repr__
• Not using keyword argument in __repr__
• Returning informal string in __repr__