Constructor parameters in Python - Step-by-Step Execution

Concept Flow - Constructor parameters

Start object creation

↓

Call __init__ with parameters

↓

Assign parameters to attributes

↓

Object ready with attributes set

↓

Use object

When creating an object, parameters are passed to the constructor (__init__) to set up initial values.

Execution Sample

Python

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

my_dog = Dog("Buddy", 3)
print(my_dog.name, my_dog.age)

This code creates a Dog object with name and age set by constructor parameters, then prints them.

Execution Table

Step	Action	Parameters Passed	Attribute 'name'	Attribute 'age'	Output
1	Call Dog constructor	name='Buddy', age=3	Not set	Not set
2	Inside __init__: assign self.name = name	name='Buddy', age=3	Buddy	Not set
3	Inside __init__: assign self.age = age	name='Buddy', age=3	Buddy	3
4	Constructor returns, object created	name='Buddy', age=3	Buddy	3
5	Print my_dog.name and my_dog.age		Buddy	3	Buddy 3

💡 Constructor finishes, object has attributes set, program prints values.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
self.name	Not set	Buddy	Buddy	Buddy
self.age	Not set	Not set	3	3
name (parameter)	N/A	Buddy	Buddy	N/A
age (parameter)	N/A	3	3	N/A

Key Moments - 2 Insights

Why do we assign parameters to self.name and self.age inside __init__?

Because parameters are temporary values passed in; assigning them to self.name and self.age stores them as object attributes for later use, as shown in steps 2 and 3 of the execution_table.

What happens if we forget to assign a parameter to self.attribute?

The object won't have that attribute set, so trying to access it later will cause an error or show 'Not set', as seen before step 2 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of self.age after step 3?

A 3

B Not set

C Buddy

D None

💡 Hint

Check the 'Attribute age' column in row for step 3 in execution_table.

At which step does the constructor finish creating the object?

A Step 2

B Step 5

C Step 4

D Step 3

💡 Hint

Look for the row mentioning 'Constructor returns' in execution_table.

If we change the parameter name to 'Max', what will be printed at step 5?

A Buddy 3

B Max 3

C None 3

D 3 Max

💡 Hint

Refer to variable_tracker for self.name and the print output in execution_table step 5.

Concept Snapshot

Constructor parameters are inputs given when creating an object.
Inside __init__, these parameters are assigned to self attributes.
This sets up the object's initial state.
Without assignment, attributes won't exist.
Use self.attribute = parameter to store values.

Full Transcript

When you create an object in Python, you often want to give it some starting information. This is done by passing parameters to the constructor method called __init__. Inside __init__, these parameters are assigned to the object's attributes using self.attribute = parameter. This way, the object remembers these values. For example, if you create a Dog with a name and age, you pass those values to __init__, and it saves them as self.name and self.age. Later, you can use these attributes to get information about the object. If you forget to assign a parameter to self.attribute, the object won't have that attribute, and trying to use it will cause an error. The execution table shows each step: calling the constructor, assigning attributes, finishing creation, and printing the values. The variable tracker shows how self.name and self.age change from not set to their final values. This helps you understand how constructor parameters work step-by-step.

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