Constructor parameters in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constructor in Python?

A constructor is a special method named __init__ that runs automatically when you create a new object from a class. It sets up the object’s initial state.

beginner

How do constructor parameters help when creating an object?

Constructor parameters let you give values to the object right when you create it. This way, each object can start with its own unique data.

beginner

What does this code do?<br><pre>class Car:
    def __init__(self, color):
        self.color = color

my_car = Car('red')</pre>

This code defines a class Car with a constructor that takes a color parameter. When my_car is created, it sets its color to 'red'.

intermediate

Can constructor parameters have default values? How?

Yes! You can give constructor parameters default values by assigning them in the __init__ method, like def __init__(self, color='blue'):. If no value is given, the default is used.

beginner

Why do we use self in constructor parameters?

self represents the object being created. It lets us store the parameter values inside the object so they can be used later.

What is the name of the constructor method in Python?

A __start__

B __init__

C constructor

D init

The constructor method in Python is always named __init__.

What does the self parameter represent in a constructor?

A The object being created

B The class itself

C A global variable

D A function parameter unrelated to the object

self refers to the specific object being created or used.

How can you give a constructor parameter a default value?

A By calling a separate method

B By setting it outside the class

C By using a global variable

D By assigning a value in the parameter list, e.g., <code>def __init__(self, x=5):</code>

Default values are set by assigning them in the parameter list of the constructor.

What happens if you create an object without passing a required constructor parameter?

A Python raises an error

B The object is created with default values

C The parameter is set to None automatically

D The program ignores the missing parameter

If a required parameter is missing, Python raises a TypeError.

Why do we assign constructor parameters to self attributes?

A To make the parameters global

B To delete the parameters after use

C To save the values inside the object for later use

D To print the values immediately

Assigning to self stores the values inside the object so they can be accessed anytime.

Explain what constructor parameters are and why they are useful.

Think about how you give information to a new object when you create it.

Describe the role of self in a constructor and how it relates to parameters.

Consider how the object keeps its own information.

Practice

1. What is the purpose of constructor parameters in a Python class?

easy

A. To provide initial values to an object when it is created

B. To define methods that the class can use

C. To create global variables outside the class

D. To delete an object from memory

Solution

1. Step 1: Understand what constructor parameters do

   Constructor parameters allow you to pass values to an object when you create it, so it starts with specific data.

2. Step 2: Compare options with this understanding

   Only To provide initial values to an object when it is created describes this purpose correctly. Other options describe unrelated concepts.

3. Final Answer:

   To provide initial values to an object when it is created -> Option A

4. Quick Check:

   Constructor parameters = initial values [OK]

Hint: Constructor parameters set starting values for new objects [OK]

Common Mistakes:

• Confusing constructor parameters with methods
• Thinking constructor parameters delete objects
• Believing constructor parameters create global variables

2. Which of the following is the correct way to define a constructor with parameters in Python?

easy

A. def __start__(self, name):

B. def constructor(name):

C. def init(self):

D. def __init__(self, name):

Solution

1. Step 1: Recall Python constructor syntax

   Python uses the special method named __init__ with self as the first parameter to define constructors.

2. Step 2: Check each option

   Only def __init__(self, name): uses the correct method name and includes self and a parameter.

3. Final Answer:

   def __init__(self, name): -> Option D

4. Quick Check:

   Constructor method = __init__ with self [OK]

Hint: Constructor method is always named __init__ with self first [OK]

Common Mistakes:

• Using wrong method names like constructor or init
• Omitting self parameter
• Using incorrect special method names

3. What will be the output of this code?

class Dog:
    def __init__(self, name):
        self.name = name
    def speak(self):
        return f"{self.name} says Woof!"

my_dog = Dog("Buddy")
print(my_dog.speak())

medium

A. Buddy says Woof!

B. Woof!

C. Dog says Woof!

D. Error: missing parameter

Solution

1. Step 1: Understand constructor parameter usage

   The constructor takes a name and assigns it to self.name. The object my_dog is created with name "Buddy".

2. Step 2: Analyze the speak method output

   The speak method returns a string using self.name, so it returns "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option A

4. Quick Check:

   Object name used in speak = Buddy says Woof! [OK]

Hint: Constructor sets name; speak uses it to print message [OK]

Common Mistakes:

• Ignoring the name parameter and expecting just 'Woof!'
• Confusing class name with instance name
• Assuming missing parameters cause error here

4. Identify the error in this class definition:

class Car:
    def __init__(color):
        self.color = color

my_car = Car("red")

medium

A. Missing quotes around "red"

B. Missing self parameter in __init__ method

C. Cannot assign to self.color

D. Wrong method name, should be __start__

Solution

1. Step 1: Check __init__ method parameters

   The first parameter of __init__ must be self to refer to the instance. Here, self is missing.

2. Step 2: Confirm other parts are correct

   Method name __init__ is correct, assignment to self.color is valid, and "red" is properly quoted.

3. Final Answer:

   Missing self parameter in __init__ method -> Option B

4. Quick Check:

   __init__ first parameter = self [OK]

Hint: Always include self as first parameter in __init__ [OK]

Common Mistakes:

• Forgetting self parameter
• Changing __init__ method name
• Misunderstanding self usage

5. You want to create a class Book that stores title and author when you create a new book object. Which constructor correctly sets these attributes?

hard

A. def __init__(title, author):
self.title = title
self.author = author

B. def __init__(self):
title = title
author = author

C. def __init__(self, title, author):
self.title = title
self.author = author

D. def __init__(self, title, author):
title = self.title
author = self.author

Solution

1. Step 1: Check parameter list and self usage

   The constructor must have self as first parameter, then title and author. def __init__(self, title, author): self.title = title self.author = author has this correct.

2. Step 2: Verify attribute assignment

   Attributes must be assigned as self.title = title and self.author = author to store values in the object. def __init__(self, title, author): self.title = title self.author = author does this correctly.

3. Final Answer:

   def __init__(self, title, author): self.title = title self.author = author -> Option C

4. Quick Check:

   Use self to assign attributes in __init__ [OK]

Hint: Use self.attribute = parameter inside __init__ [OK]

Common Mistakes:

• Omitting self parameter
• Assigning parameters to themselves instead of self
• Reversing assignment order