Use cases for each method type in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instance method in Python?

An instance method is a function defined inside a class that operates on an instance of the class. It can access and modify the object's attributes using 'self'.

beginner

When should you use a class method?

Use a class method when you need to access or modify the class itself, not an individual object. It receives the class as the first argument, usually named 'cls'.

beginner

What is a static method and when is it useful?

A static method is a function inside a class that does not access the instance or class. Use it for utility functions related to the class but that don't need class or instance data.

intermediate

Why use instance methods instead of class or static methods?

Instance methods are used when behavior depends on the specific data of an object. They can read or change the object's attributes.

intermediate

Give an example use case for a class method.

Class methods are useful for alternative constructors, like creating an object from different data formats or counting how many objects have been created.

Which method type can modify the class state but not instance state?

A Class method

B Instance method

C Static method

D None of the above

Class methods receive the class as the first argument and can modify class-level data.

Which method type does NOT receive 'self' or 'cls' as the first argument?

A Instance method

B Static method

C All receive one of them

D Class method

Static methods do not receive 'self' or 'cls' because they don't depend on instance or class data.

When should you use an instance method?

A For utility functions unrelated to class or instance

B To create alternative constructors

C To access or modify instance attributes

D To modify class variables

Instance methods operate on individual objects and their data.

What is a common use case for class methods?

A Utility functions

B Modifying instance attributes

C Performing calculations unrelated to class

D Alternative constructors

Class methods are often used to create objects in different ways than the default constructor.

Which method type is best for a function that does not need access to class or instance data?

A Static method

B Class method

C Instance method

D Property method

Static methods are used for functions related to the class but independent of class or instance data.

Explain the differences and use cases for instance, class, and static methods in Python.

Think about what each method type can access and when you might want to use it.

Describe a scenario where a class method is more appropriate than an instance method.

Consider when you want to work with the class as a whole, not just one object.

Practice

1. Which method type in Python is used to access or modify data unique to each object instance?

easy

A. Global function

B. Instance method

C. Static method

D. Class method

Solution

1. Step 1: Understand instance methods

   Instance methods receive the object itself as the first argument and can access or modify instance-specific data.

2. Step 2: Compare with other methods

   Class methods work with class-level data, static methods don't access instance or class data, and global functions are outside the class.

3. Final Answer:

   Instance method -> Option B

4. Quick Check:

   Instance method = unique object data [OK]

Hint: Instance methods use 'self' to access object data [OK]

Common Mistakes:

• Confusing class methods with instance methods
• Thinking static methods access instance data
• Assuming global functions are methods

2. Which of the following is the correct way to define a class method in Python?

easy

A. def method():

B. def method(self):

C. @staticmethod\ndef method():

D. @classmethod\ndef method(cls):

Solution

1. Step 1: Recall class method syntax

   Class methods use the @classmethod decorator and receive the class as the first argument, usually named 'cls'.

2. Step 2: Check options

   @classmethod\ndef method(cls): matches this syntax exactly; others define instance or static methods or lack decorators.

3. Final Answer:

   @classmethod\ndef method(cls): -> Option D

4. Quick Check:

   Class method = @classmethod + cls parameter [OK]

Hint: Class methods use @classmethod and 'cls' parameter [OK]

Common Mistakes:

• Using 'self' instead of 'cls' for class methods
• Missing the @classmethod decorator
• Confusing static method syntax with class methods

3. What will be the output of this code?

class Example:
    count = 0

    def __init__(self):
        Example.count += 1

    @classmethod
    def get_count(cls):
        return cls.count

obj1 = Example()
obj2 = Example()
print(Example.get_count())

medium

A. 2

B. 0

C. 1

D. Error

Solution

1. Step 1: Understand the constructor behavior

   Each time an Example object is created, the class variable 'count' increases by 1. Two objects are created, so count becomes 2.

2. Step 2: Understand the class method output

   The class method 'get_count' returns the current value of 'count', which is 2 after creating two objects.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Class variable incremented twice = 2 [OK]

Hint: Class methods access shared data like 'count' [OK]

Common Mistakes:

• Thinking count resets per instance
• Confusing instance and class variables
• Expecting an error due to method call

4. Identify the error in this code snippet:

class Calculator:
    @staticmethod
    def add(x, y):
        return x + y

    @classmethod
    def multiply(cls, x, y):
        return x * y

print(Calculator.add(2, 3))
print(Calculator.multiply(2, 3))

medium

A. No error, code runs correctly

B. Class method 'multiply' missing 'self' parameter

C. Static method 'add' missing 'self' parameter

D. Class method 'multiply' missing 'cls' parameter

Solution

1. Step 1: Check static method definition

   Static methods do not require 'self' or 'cls'; 'add' is correctly defined with two parameters.

2. Step 2: Check class method definition

   Class method 'multiply' correctly has 'cls' as the first parameter and two others; usage is correct.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Static and class methods correctly defined [OK]

Hint: Static methods no 'self'; class methods need 'cls' first [OK]

Common Mistakes:

• Expecting 'self' in static methods
• Confusing 'self' and 'cls' in class methods
• Assuming method calls require instance

5. You want to create a method in a class that logs a message but does not need access to instance or class data. Which method type should you use and why?

hard

A. Instance method, because it can access instance data

B. Class method, because it can access class data

C. Static method, because it does not require instance or class data

D. Global function, because it is outside the class

Solution

1. Step 1: Identify method requirements

   The method only logs a message and does not need to access instance or class data.

2. Step 2: Choose appropriate method type

   Static methods are designed for tasks related to the class but do not use instance or class data, making them ideal here.

3. Final Answer:

   Static method, because it does not require instance or class data -> Option C

4. Quick Check:

   Logging without data access = static method [OK]

Hint: Use static methods for utility tasks without data access [OK]

Common Mistakes:

• Using instance or class methods unnecessarily
• Confusing static methods with global functions
• Thinking logging requires instance data