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Noah Brown
Noah Brown

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In this Flask tutorial, you create a simple Flask app with three pages that use a common base template. Along the way, you experience a number of features of Visual Studio Code including using the terminal, the editor, the debugger, code snippets, and more.




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You now have a self-contained environment ready for writing Flask code. VS Code activates the environment automatically when you use Terminal: Create New Terminal. If you open a separate command prompt or terminal, activate the environment by running source .venv/bin/activate (Linux/macOS) or .venv\Scripts\Activate.ps1 (Windows). You know the environment is activated when the command prompt shows (.venv) at the beginning.


Debugging gives you the opportunity to pause a running program on a particular line of code. When a program is paused, you can examine variables, run code in the Debug Console panel, and otherwise take advantage of the features described on Debugging. Running the debugger also automatically saves any modified files before the debugging session begins.


The decorator used for the new URL route, /hello/, defines an endpoint /hello/ that can accept any additional value. The identifier inside in the route defines a variable that is passed to the function and can be used in your code.


As described in the code comments, always filter arbitrary user-provided information to avoid various attacks on your app. In this case, the code filters the name argument to contain only letters, which avoids injection of control characters, HTML, and so forth. (When you use templates in the next section, Flask does automatic filtering and you won't need this code.)


Output appears in a "Python Debug Console" terminal. Ctrl+click the :5000/ link in that terminal to open a browser to that URL. In the browser's address bar, navigate to :5000/hello/VSCode. Before the page renders, VS Code pauses the program at the breakpoint you set. The small yellow arrow on the breakpoint indicates that it's the next line of code to run.


On the left side of the VS Code window, you see a Variables pane that shows local variables, such as now, as well as arguments, such as name. Below that are panes for Watch, Call Stack, and Breakpoints (see VS Code debugging for details). In the Locals section, try expanding different values. You can also double-click values (or use Enter (Windows, Linux F2)) to modify them. Changing variables such as now, however, can break the program. Developers typically make changes only to correct values when the code didn't produce the right value to begin with.


When a program is paused, the Debug Console panel (which is different from the "Python Debug Console" in the Terminal panel) lets you experiment with expressions and try out bits of code using the current state of the program. For example, once you've stepped over the line now = datetime.now(), you might experiment with different date/time formats. In the editor, select the code that reads now.strftime("%A, %d %B, %Y at %X"), then right-click and select Evaluate in Debug Console to send that code to the debug console, where it runs:


Change the line in the code to use different datetime format, for example now.strftime("%a, %d %b, %y at %X"), and then save the file. The Flask server will automatically reload, which means the changes will be applied without the need to restart the debugger. Refresh the page on the browser to see the update.


During your work with Flask or any other library, you may want to examine the code in those libraries themselves. VS Code provides two convenient commands that navigate directly to the definitions of classes and other objects in any code:


Go to Definition jumps from your code into the code that defines an object. For example, in app.py, right-click on the Flask class (in the line app = Flask(__name__)) and select Go to Definition (or use F12), which navigates to the class definition in the Flask library.


The app you've created so far in this tutorial generates only plain text web pages from Python code. Although it's possible to generate HTML directly in code, developers avoid such a practice because it opens the app to cross-site scripting (XSS) attacks. In the hello_there function of this tutorial, for example, one might think to format the output in code with something like content = "Hello there, " + clean_name + "!", where the result in content is given directly to a browser. This opening allows an attacker to place malicious HTML, including JavaScript code, in the URL that ends up in clean_name and thus ends up being run in the browser.


In the templates folder, create a file named hello_there.html with the contents below. This template contains two placeholders named "name" and "date", which are delineated by pairs of curly braces, and . As you can see, you can also include formatting code in the template directly:


Also in app.py, modify the hello_there function to use render_template to load a template and apply the named values (and add a route to recognize the case without a name). render_template assumes that the first argument is relative to the templates folder. Typically, developers name the templates the same as the functions that use them, but matching names are not required because you always refer to the exact filename in your code.


The second type are those that you want to address in code, such as when you want to implement an API endpoint that returns a static file. For this purpose, the Flask object contains a built-in method, send_static_file, which generates a response with a static file contained within the app's static folder.


Within the static folder, create a file named site.css with the following contents. After entering this code, also observe the syntax highlighting that VS Code provides for CSS files, including a color preview:


Also, because you'll likely create many pages that extend the same template, it's helpful to create a code snippet in VS Code with which you can quickly initialize new page templates. A snippet helps you avoid tedious and error-prone copy-paste operations.


You can run the app at this point, but because you haven't made use of the base template anywhere and haven't changed any code files, the result is the same as the previous step. Complete the remaining sections to see the final effect.


Because the three pages you create in the next section extend layout.html, it saves time to create a code snippet to initialize a new template file with the appropriate reference to the base template. A code snippet provides a consistent piece of code from a single source, which avoids errors that can creep in when using copy-paste from existing code.


When you share your app code through source control or some other means, it doesn't make sense to copy all the files in a virtual environment because recipients can always recreate the environment themselves.


Throughout this Flask tutorial, all the app code is contained in a single app.py file. To allow for further development and to separate concerns, it's helpful to refactor the pieces of app.py into separate files.


In your project folder, create a folder for the app, such as hello_app, to separate its files from other project-level files like requirements.txt and the .vscode folder where VS Code stores settings and debug configuration files.


Get started with the Azure Blob Storage client library for Python to manage blobs and containers. Follow these steps to install the package and try out example code for basic tasks in an interactive console app.


Application requests to Azure Blob Storage must be authorized. Using the DefaultAzureCredential class provided by the Azure Identity client library is the recommended approach for implementing passwordless connections to Azure services in your code, including Blob Storage.


DefaultAzureCredential supports multiple authentication methods and determines which method should be used at runtime. This approach enables your app to use different authentication methods in different environments (local vs. production) without implementing environment-specific code.


For example, your app can authenticate using your Azure CLI sign-in credentials with when developing locally. Your app can then use a managed identity once it has been deployed to Azure. No code changes are required for this transition.


In most cases it will take a minute or two for the role assignment to propagate in Azure, but in rare cases it may take up to eight minutes. If you receive authentication errors when you first run your code, wait a few moments and try again.


Add this code inside the try block. When the code runs on your local workstation, DefaultAzureCredential uses the developer credentials of the prioritized tool you're logged into to authenticate to Azure. Examples of these tools include Azure CLI or Visual Studio Code.


When deployed to Azure, this same code can be used to authorize requests to Azure Storage from an application running in Azure. However, you'll need to enable managed identity on your app in Azure. Then configure your storage account to allow that managed identity to connect. For detailed instructions on configuring this connection between Azure services, see the Auth from Azure-hosted apps tutorial.


This app creates a test file in your local folder and uploads it to Azure Blob Storage. The example then lists the blobs in the container, and downloads the file with a new name. You can compare the old and new files.


You can either download binaries or source code archives for the latest stable or previous release or access the current development (aka nightly) distribution through Git. This software may not be exported in violation of any U.S. export laws or regulations. For more information regarding Export Control matters please go to


Do not worry, selecting this option this will not immediately send the project files to arXiv; instead, it displays another window which lets you download your article, complete with .bbl file, for onward submission to arXiv: 041b061a72


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