Constructor method
A constructor is a type-associated operation or callable that creates or initializes an instance and establishes its initial state.
Python How-To
__init__ is a Python instance-initialization method that runs when an object is created and establishes its initial state.
- How it is used
- Establishes the attributes a custom Python class’s instances need when constructed.
- How deep it goes
- Works through parameters, attribute placement, and the __new__-then-__init__ sequence without requiring a custom __new__ implementation.
- Examples
- Uses task objects, attribute dictionaries, an AttributeError, memory-address traces, and a generated bill constructor.
- What you can do after
- Build it from scratch by defining __init__ with required arguments and initializing every instance attribute before other methods use it.
The Quick Python Book, Fourth Edition
__init__ is Python's conventional instance-initialization method invoked when an object is initialized.
- How it is used
- Uses `__init__` to set instance fields when objects are created and to validate initial contents in a dictionary subclass.
- How deep it goes
- Explains automatic invocation, `self`, defaults, subclass calls, and the distinction from `__new__`; leaves unusual object-creation overrides aside.
- Examples
- Shows circles initialized with default or supplied values, inherited position fields, and a string-only dictionary requiring initial validation.
- What you can do after
- Build it from scratch: write an initializer that sets instance fields, calls a base initializer when needed, and checks initial dictionary contents.
Read how The Quick Python Book, Fourth Edition explains constructor method →
Python Workout: 50 ten-minute exercises
__init__ is a Python instance method that initializes an object during instance creation.
- How it is used
- Initializes instance attributes during class calls and supports reusable state setup across inheritance hierarchies.
- How deep it goes
- Explains execution order, self, attributes, defaults, inheritance, and super, while stopping short of implementing __new__.
- Examples
- Uses Scoop, Person, Employee, Animal, Beverage, and LogFile examples to demonstrate attributes, inheritance, defaults, and resources.
- What you can do after
- Build it from scratch by defining classes that validate arguments informally, assign instance attributes, and delegate shared setup with super().
Read how Python Workout: 50 ten-minute exercises explains constructor method →
Python Workout, Second Edition
The __init__ method is Python's special instance-initialization method, called after an object is created to initialize its attributes.
- How it is used
- Initializes instance attributes when classes are called and coordinates initialization across inherited classes.
- How deep it goes
- Explains attribute assignment, self, inheritance, super, and the __new__ distinction, while stopping short of implementing __new__.
- Examples
- Uses Scoop, Person, Employee, Animal subclasses, beverages, files, and a Cage to demonstrate initialization choices.
- What you can do after
- Build instance initialization, delegate parent setup with super, and distinguish initialization from object creation.
Read how Python Workout, Second Edition explains constructor method →
The Well-Grounded Rubyist, Fourth Edition
A Ruby constructor is the class method conventionally named new that creates and returns an instance of the class.
- How it is used
- Creates class instances for subsequent method calls, array initialization, and file reading.
- How deep it goes
- Defines `new` as a class-directed constructor and works through array arguments and blocks, without detailing constructor implementation.
- Examples
- Uses `Object.new`, `Ticket.new`, several `Array.new` forms, and `File.new`.
- What you can do after
- Apply `new` to create instances, choose safe array initialization, and close file handles opened with `File.new`.
Read how The Well-Grounded Rubyist, Fourth Edition explains constructor method →
Essential TypeScript 5, Third Edition
A constructor is a class member that initializes a newly created instance and its properties.
- How it is used
- Initializes class instances and their properties when they are created with `new`.
- How deep it goes
- Works through property assignment, TypeScript parameter properties, and subclass initialization; contrasts classes with constructor functions and their typing limitations.
- Examples
- Uses product creation, an employee with parameter properties, and a subclass that initializes a tax rate.
- What you can do after
- Build it from scratch: initialize class instances, shorten repetitive TypeScript constructors, and call a superclass constructor when extending a class.
Read how Essential TypeScript 5, Third Edition explains constructor method →
Pro Angular 16
A constructor is a class member that initializes an instance when the class is instantiated.
- How it is used
- Initializes class instances and declares the services Angular components need when they are created.
- How deep it goes
- Works through property assignment, TypeScript parameter properties, inherited initialization, and Angular's inspection of constructor argument types.
- Examples
- Creates products with optional categories and discounted subclasses, then injects a discount service into display and editor components.
- What you can do after
- Build it from scratch to initialize TypeScript objects, or apply it in Angular to request a service through a component constructor.
Learn Go with Pocket-Sized Projects
A constructor is a mechanism or operation that creates and initializes an instance of a type.
- How it is used
- Provides a package-level creation API that enforces required initialization and hides implementation details.
- How deep it goes
- Explains zero values, required parameters, map setup, naming, pointer returns, and functional options, but not language-level constructor syntax.
- Examples
- Demonstrates New for a generic cache with an initialized map and for a logger requiring a logging threshold.
- What you can do after
- Build it from scratch using a package-level New function that validates or applies required initialization before returning an instance.
Read how Learn Go with Pocket-Sized Projects explains constructor method →
Unit Testing Principles, Practices, and Patterns
A constructor is a class member invoked when an object is created to initialize its state.
- How it is used
- Uses constructors to initialize objects and evaluates their role in test-fixture setup and ORM-compatible object restoration.
- How deep it goes
- Compares constructor-based setup with factory methods and base classes, then examines private-constructor contracts; stops short of language semantics.
- Examples
- Uses CustomerTests fixture setup, factory-method refactoring, integration-test database setup, and an Inquiry private constructor.
- What you can do after
- Diagnose constructor-based test coupling and choose explicit factories or shared infrastructure without exposing unnecessary implementation details.
Read how Unit Testing Principles, Practices, and Patterns explains constructor method →
Julia as a Second Language
A constructor is a function or language mechanism that creates an instance of a type from supplied component values.
- How it is used
- Creates composite-type instances while offering convenient inputs or protecting valid initialization.
- How deep it goes
- Works through default, outer, and inner methods, including new and the removal of defaults; does not detail constructor dispatch internals.
- Examples
- Uses rocket fuel tanks, angle units stored as seconds, and numeric conversions.
- What you can do after
- Build it from scratch: add convenient outer methods or an inner method that controls object initialization.
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Practices of the Python Pro
__init__ is the Python instance initializer method that sets up an object's initial state after the instance has been constructed.
- How it is used
- Sets an instance’s initial state while organizing related data and behavior inside a class.
- How deep it goes
- Defines initialization, self, arguments, and instance creation, then demonstrates attribute updates; it stops short of advanced initialization patterns.
- Examples
- Uses a RockPaperScissorsSimulator whose initializer sets human_choice and computer_choice to None.
- What you can do after
- Build instances with predictable starting attributes and let methods access or change that state through self.
Read how Practices of the Python Pro explains constructor method →
F# in Action: A Revised Edition of Get Programming with F#
A constructor is a member or function-like operation that creates and initializes an instance of a type.
- How it is used
- Creates initialized class instances in F# applications, especially when consuming .NET and C# object-oriented code.
- How deep it goes
- Explains declaration, implicit member access, and concise versus explicit invocation, but does not detail compiler internals.
- Examples
- Uses FileInfo, a C# Person class, and an ATM class receiving a Customer through its constructor.
- What you can do after
- Apply it via F# syntax to construct .NET objects and instances of classes defined in C# or F#.
Read how F# in Action: A Revised Edition of Get Programming with F# explains constructor method →
Get Programming: Learn to code with Python
A Python special method that is automatically invoked to initialize a newly created instance.
- How it is used
- Initializes newly created objects with the data attributes needed by their class.
- How deep it goes
- Explains class placement, self, attributes, parameters, and missing arguments through small Circle and Rectangle examples.
- Examples
- Uses Circle with a zero radius and Rectangle with length and width, including a missing-width TypeError.
- What you can do after
- Builds classes whose instances begin with chosen attributes and accepts required starting values.
Read how Get Programming: Learn to code with Python explains constructor method →
Related concepts
- objectproduces · A computational object is a runtime entity or structured value representing data, state, or a resource, with properties or operations defined by its programming model.
- new keywordused by · In JavaScript, new is an operator that creates an object instance by invoking a constructable function or class as its constructor.
- functionkind of · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.
- pattern matchinguses · Pattern matching is the process of comparing data or values with specified or learned patterns to identify correspondence, conformity, or a matching case.
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