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python manage.py <command_name>. Django provides a mechanism for defining custom commands by creating a management/commands/ directory inside your app, and then creating a Python file that defines your command. NSNotificationCenter is a powerful mechanism that allows objects to communicate with each other in a loose coupling way. It is used to broadcast messages (notifications) to interested objects without the need for them to be directly linked. This is especially useful for event-driven programming and for objects that need to respond to changes or specific events in other parts of the application. POST, PUT, etc.) and the environment (browser, Node.js, etc.). Below are the most common ways to send JSON data in an HTTP request, particularly using JavaScript. lambda keyword. Unlike a regular function defined with the def keyword, a lambda function can have any number of arguments but can only contain a single expression. The result of the expression is automatically returned by the lambda function. @selector is a compiler directive used to represent a selector, which is a type that refers to the name of a method. A selector is essentially a way to refer to a method by its name so that it can be called dynamically during runtime. It is frequently used in contexts like message sending, target-action patterns, and performing selectors. with statement in Python is used to simplify exception handling and ensure that certain resources are properly managed, such as files, network connections, or database connections. It is often used in conjunction with context managers, which are objects that define the behavior of the with statement. @property is a directive used to declare a property on an object, which is a getter and setter method for an instance variable (ivariable). It allows the automatic creation of accessors (getter and setter methods) and provides encapsulation and memory management for the instance variables, ensuring a clean and simple interface to interact with an object's data. @synthesize is a directive used to automatically generate the getter and setter methods for a property, as well as the instance variable (ivar) associated with that property. However, with the introduction of Automatic Reference Counting (ARC) and modern Objective-C, @synthesize has become less necessary in most cases, as the compiler now automatically synthesizes properties for you by default. But understanding its historical usage and functionality is still important. **IActionResult** is an interface in ASP.NET MVC (and also in ASP.NET Core MVC) that represents the result of an action method. In MVC (Model-View-Controller) applications, an action method in a controller returns an instance of IActionResult, which is used to define the response that will be sent back to the client (typically a web browser). async and await are keywords introduced in Swift 5.5 to simplify writing asynchronous code. These keywords help manage concurrency and make asynchronous programming more readable and less error-prone compared to traditional callback-based methods like closures. AppModule), which is responsible for bootstrapping and launching the application. HttpClient module, which is part of Angular's @angular/common/http package. The HttpClient provides a simplified API for making HTTP requests such as GET, POST, PUT, DELETE, etc., and returns Observables that you can subscribe to for handling the asynchronous data. @IBOutlet and @IBAction are attributes used in iOS development with UIKit (or SwiftUI in some cases, although the syntax differs). These attributes are used to connect UI elements in Interface Builder (the visual tool within Xcode) to your Swift code, enabling interaction between your user interface (UI) and the logic of your application. struct) are used to create custom data types, but they differ in several key aspects, primarily in how they manage memory, behavior, and mutability. Understanding these differences is crucial for choosing the right type for your application. map, filter, and reduce are powerful higher-order functions that operate on collections (such as arrays, sets, etc.) and allow you to transform, filter, or combine values in a concise and functional programming style. These functions are commonly used to manipulate data in a functional programming way. defer is a powerful control flow statement that allows you to execute a block of code just before the current scope exits, no matter how the scope is exited (whether through normal return, early exit due to an error, or other control flow mechanisms). The purpose of defer is to ensure cleanup code is always executed, even in the face of errors or early exits from functions. useEffect hook is one of the most important hooks in React. It allows you to perform side effects in functional components, such as fetching data, updating the DOM, subscribing to events, or cleaning up resources like timers and subscriptions. It is React's answer to lifecycle methods (componentDidMount, componentDidUpdate, and componentWillUnmount) in class components. LIMIT/OFFSET clause (for MySQL) or ROW_NUMBER() function (for SQL Server, PostgreSQL, or Oracle). async/await in Node.js is syntactic sugar built on top of Promises that allows you to write asynchronous code in a more readable, synchronous-like manner. It simplifies the process of working with Promises and makes asynchronous code easier to manage and understand. merge() function, which combines two data frames by common columns or row names, similar to SQL joins. You can control how the data frames are merged (e.g., inner, outer, left, or right join) by specifying different options. ggplot2 is a popular data visualization package in R that provides a powerful and flexible framework for creating a wide range of static graphics. It is based on the Grammar of Graphics (hence the gg), which provides a systematic approach to building visualizations by layering different components. ROW_NUMBER() function in SQL is a window function that assigns a unique sequential integer to rows within a result set. The numbering starts at 1 for the first row and increments by 1 for each subsequent row in the specified order. It is typically used to assign a rank or row number to rows in a result set based on a particular order. ::) to distinguish them from pseudo-classes (which are prefixed with one colon :). SELECT, INSERT, UPDATE, or DELETE statements and can help with complex queries where a direct comparison or condition is needed. !important rule in CSS is used to give a CSS property higher priority or specificity when applying styles to an element. It is used to override any other conflicting styles, even if the other styles are more specific or come later in the CSS declaration order. GROUP BY clause in MySQL is used to group rows that have the same values in specified columns into summary rows, often used with aggregate functions like COUNT(), SUM(), AVG(), MIN(), and MAX() to perform calculations on each group. Essentially, it allows you to aggregate data across multiple rows and summarize it into a smaller set of results. @objc attribute in Swift is used to expose Swift code (such as methods, properties, or classes) to the Objective-C runtime. This allows Swift code to be used with Objective-C features, such as dynamic dispatch, selectors, KVO (Key-Value Observing), and targets for actions. The @objc attribute is essential for interoperability between Swift and Objective-C, especially when working with APIs that require Objective-C runtime features. UIViewController lifecycle refers to the sequence of methods that are called as a UIViewController object goes through different stages in its existence. These stages include creation, presentation, interaction, and destruction. Understanding the lifecycle helps developers manage view-related tasks, such as data loading, view configuration, and cleanup. $.ajax() method in jQuery is used to perform asynchronous HTTP (AJAX) requests to load data from a web server without refreshing the page. It is a powerful and flexible function that allows you to interact with servers, send data, and receive responses in various formats like JSON, HTML, XML, or plain text. inline keyword in C++ is used to suggest to the compiler that it should attempt to generate inline code for a function call, instead of the normal function call mechanism (i.e., jumping to the function's code location). This can improve performance by avoiding the overhead associated with function calls, especially for small functions that are called frequently. $_FILES superglobal array, which allows you to access file data sent via an HTML form. The process involves setting up an HTML form for file uploads, configuring your PHP script to handle the uploaded file, and ensuring proper validation and security. foreach loop in PHP is a control structure used to iterate over arrays or objects. It is specifically designed for iterating through the elements of an array without needing to use an index or counter. The foreach loop is generally preferred when you want to loop through all elements of an array or object in a simple and clean way. __get() and __set() are magic methods that allow you to intercept access to properties of an object. These methods are part of PHP's object-oriented programming and are used to handle cases where you want to manage how properties are read and written dynamically, especially when those properties are not directly accessible or do not exist. .animate() function in jQuery is used to create custom animations on CSS properties of elements. It allows you to animate various properties, such as width, height, opacity, margin, padding, and more. The .animate() method enables the creation of smooth transitions, providing a way to modify an element's CSS properties over a specified duration. $.ajaxSetup() method in jQuery is used to set default values for all AJAX requests that are made on a page. It allows you to define global settings for AJAX requests, such as the default HTTP method, data type, timeout, or headers, so you don't need to specify them each time you make an AJAX request using methods like $.ajax(), $.get(), or $.post(). .toggleClass() method. This method checks if the class is already applied to the element and then adds it if it's not present, or removes it if it is. It's useful for creating interactive elements, like buttons or links, that change their style when clicked. .fadeIn() and .fadeOut() methods in jQuery are used to gradually change the opacity of an element, creating a smooth fading effect. These methods are commonly used for animating the visibility of elements on the web page, making the transition between visible and invisible states smooth and visually appealing. $(document).ready() function in jQuery is used to run code as soon as the DOM (Document Object Model) is fully loaded, but before all the external resources (such as images, stylesheets, etc.) are completely loaded. This is particularly useful to ensure that your JavaScript and jQuery code is executed as soon as the HTML structure is available, which helps prevent errors caused by trying to manipulate elements that haven't been loaded yet. this keyword is a reference to the current instance of the class or struct in which it is used. It is used to access the members (fields, properties, methods) of the current object from within its instance methods or constructors. It provides a way to refer to the current object explicitly, which can be useful to disambiguate between class members and method parameters, or to pass the current object as an argument. Application class in Android plays a crucial role in managing the global state of the application. It is a base class for maintaining global application state, and it is instantiated before any activity, service, or receiver in the application. The Application class is designed to provide a central location to initialize resources that need to persist throughout the app's lifecycle, such as singletons, global variables, or shared resources. open() function. final keyword in Java provides a way to define constants, enforce method behavior (prevent overriding), and prevent inheritance. It is a powerful tool for ensuring that certain aspects of your program remain immutable and fixed, which can help improve code reliability, security, and maintainability. try, catch, finally, throw, and throws. switch-case statement to handle different arithmetic operations. The basic operations we’ll include are addition, subtraction, multiplication, and division. We'll take input from the user for two numbers and the operation they want to perform. this keyword in JavaScript refers to the context in which a function is called. Its value depends on how a function is invoked and can change dynamically. It is commonly used to access the properties and methods of the current object, but its behavior varies across different execution contexts.Here’s a breakdown of how this works in different situations: