← Python

OOP – String Representation (str)

__str__ defines what is displayed when an object is passed to print() or str()

What it does: Python calls __str__ when you use print(obj) or str(obj). __str__ must return a string — f-strings work well here. __str__ defines the human-readable string for the object — used by print(). print(obj) and str(obj) both trigger __str__ automatically. Without __str__, Python uses a default representation showing the class name and memory address. __str__ only produces a string for display — it doesn't alter the object's data. print() calls __str__ — which returns 'Rex', so that's what prints. __str__ has access to self so it can read any of the object's attributes.

Common mistakes: __str__ must RETURN a string, not print it. __str__ must return a string, not an integer.

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Learning

Duo

What does __str__ control?

What print(object) displaysHow to create an object

Matches

❌ def __str__(self): return 42

↓

Error: __str__ must return a string

Applying

Meanings

Term

__str__(self)

Meaning

Returns a human-readable string used by print() and str().

Practice

class Student:

Testing

Gaps

def (self):

Check Answer

Spots

2 def __str__(self):

3 print('Dog: ' + self.name)

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Quick Questions

1. What does __str__ control?

A. What print(object) displays ✓

B. How to create an object

__str__ defines the human-readable string for the object — used by print()

2. When is __str__ called?

A. When you create the object

B. When you print() or str() the object ✓

print(obj) and str(obj) both trigger __str__ automatically

3. What must __str__ return?

A. A string ✓

B. None

__str__ must return a string — Python will raise TypeError if it doesn't

4. What does print(obj) show without __str__?

A. Nothing

B. Something like <ClassName object at 0x...> ✓

Without __str__, Python uses a default representation showing the class name and memory address

5. Does __str__ change the object?

A. No it just returns a string description ✓

B. Yes

__str__ only produces a string for display — it doesn't alter the object's data

Match the Pairs

def __str__(self): return 42 → Error: __str__ must return a string

def Str(self): → Error: Magic method must be __str__

def __str__(self): → Valid: Defines string representation of an object

return f'Dog: {self.name}' → Ok: Returns a formatted string from __str__

str(3.14) → Output: 3.14

str([1, 2]) → Output: [1, 2]

Key Term

__str__(self)

Returns a human-readable string used by print() and str().~~__repr__(self)

Fill in the Blanks

1. class Dog: def __str__(self): return 'Dog: ' + self.name

2. class Dog: def __str__(self): return self.name

3. class Dog: def __str__(self): return 'Dog: ' + self.name

4. dog = Dog('Rex') print(dog)

5. class Point: def __str__(self): return '(' + str(self.x) + ')'

Spot the Error

Question 1

A class Dog:

B def __str__(self):

C print('Dog: ' + self.name)

Line C — __str__ must RETURN a string, not print it

Question 2

A class Dog:

B def __str__(self):

C return 42

Line C — __str__ must return a string, not an integer

Question 3

A class Dog:

B def __str__():

C return 'Dog'

Line B — Missing self parameter

Question 4

A class Dog:

B def _str_(self):

C return 'Dog'

Line B — Need double underscores: __str__ not _str_

Question 5

A class Dog:

B def str(self):

C return 'Dog'

Line B — Must be __str__ with double underscores not just str

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