This chapter and the next take a quick look at program execution—how you launch code, and how Python runs it. In this chapter, we’ll study how the Python interpreter executes programs in general. Chapter 3 will then show you how to get your own programs up and running. Startup details are inherently platform-specific, and some of the material in these two chapters may not apply to the platform you work on, so more advanced readers should feel free to skip parts not relevant to their intended use. Likewise, readers who have used similar tools in the past and prefer to get to the meat of the language quickly may want to file some of these chapters away as “for future reference.” For the rest of us, let’s take a brief look at the way that Python will run our code, before we learn how to write it.
时间: 2024-02-14 20:03:53 浏览: 35
本章和下一章将快速浏览程序执行 - 如何启动代码以及Python如何运行它。在本章中,我们将研究Python解释器如何一般执行程序。第三章将向您展示如何运行您自己的程序。启动详细信息固有地依赖于特定平台,这两章中的一些材料可能不适用于您所使用的平台,因此更高级的读者应随意跳过与其预期使用无关的部分。同样,过去使用过类似工具并希望快速了解语言核心的读者可能希望将一些章节存为“供将来参考”。对于我们其他人来说,在学习如何编写代码之前,让我们简要了解Python将如何运行我们的代码。
相关问题
Explain how Looper and Handler work together for threads execution in Android.
In Android, Looper and Handler work together to execute tasks on a separate thread, also known as a background thread.
A Looper is an object that allows a thread to handle incoming messages or tasks. It runs in an infinite loop, waiting for new tasks to arrive. When a new task arrives, the Looper dispatches it to the appropriate Handler for processing.
A Handler is an object that receives tasks from the Looper and processes them. It is associated with a specific thread, and when a task is sent to a Handler, it is added to a message queue. The Handler processes the tasks one by one, in the order they were added to the queue.
Together, the Looper and Handler allow us to perform time-consuming tasks on a separate thread, without blocking the main thread. The main thread is responsible for handling UI events, and if it is blocked by a long-running task, the user interface will become unresponsive. By using Looper and Handler, we can keep the main thread free to handle UI events while executing time-consuming tasks on a separate thread.
Here is an example of how Looper and Handler work together:
```
public class MyThread extends Thread {
public Handler handler;
@Override
public void run() {
Looper.prepare();
handler = new Handler() {
@Override
public void handleMessage(Message msg) {
// Process the task here
}
};
Looper.loop();
}
}
```
In this example, we create a new thread called `MyThread`. In the `run()` method, we create a new `Looper` and call `prepare()` to initialize it. We then create a new `Handler` and associate it with the thread. Finally, we call `Looper.loop()` to start the infinite loop and wait for tasks to arrive.
To send a task to `MyThread`, we can use the `handler` object:
```
MyThread myThread = new MyThread();
myThread.start();
Message msg = myThread.handler.obtainMessage();
msg.obj = "Hello";
myThread.handler.sendMessage(msg);
```
In this example, we create a new message and set its `obj` property to `"Hello"`. We then send the message to `MyThread` using its `handler` object. When the message arrives, the `handleMessage()` method of the `Handler` will be called, and we can process the task there.
What is the purpose of declaring exceptions? How do you declare an exception, and where? Can you declare multiple exceptions in a method header?
The purpose of declaring exceptions is to handle error conditions that may occur during the execution of a program. Exceptions can be used to signal that something unexpected has happened, such as an I/O error or an arithmetic overflow.
In Java, you declare an exception using the `throws` keyword in the method signature. For example:
```java
public void readFile() throws IOException {
// code that may throw an IOException
}
```
In this example, the `readFile()` method may throw an `IOException`, so it is declared in the method signature using the `throws` keyword.
You can declare multiple exceptions in a method header by separating them with commas. For example:
```java
public void doSomething() throws IOException, SQLException {
// code that may throw an IOException or a SQLException
}
```
By declaring the exceptions in the method signature, you are indicating to other developers that these exceptions may be thrown by the method and need to be handled appropriately.
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