help", " >>> _ The Python interpreter is now waiting at an interactive prompt, where you can enter Python statements. In future chapters you will learn what kinds of things this interactive prompt is useful for. But for now, you have confirmed that Python is installed on your system. To exit the interactive prompt, you can type exit() and press Enter. On the Linux and Mac OS X versions of Python you can also exit the interpreter by pressing CtrlD. On Windows, the exit shortcut is Ctrl-Z followed by Enter. Installing Flask The next step is to install Flask, but before I go into that I want to tell you about the best practices associated with installing Python packages. In Python, packages such as Flask are available in a public repository, from where anybody can download them and install them. The official Python package repository is called PyPI, which stands for Python Package Index (some people also refer to this repository as the "cheese shop"). Installing a package from PyPI is very simple, because Python comes with a tool called pip that does this work (in Python 2.7 pip does not come bundled with Python and needs to be installed separately). To install a package on your machine, you use pip as follows: $ pip install Interestingly, this method of installing packages will not work in most cases. If your Python interpreter was installed globally for all the users of your computer, chances are your regular user account is not going to have permission to make modifications to it, so the only way to make the command above work is to run it from an administrator account. But even without that complication, consider what happens when you install a package as above. The pip tool is going to download the package from PyPI, and then add it to your Python installation. From that point on, every Python script that you have on your system will have access to this package. Imagine a situation where you have completed a web application using version 0.11 of Flask, which was the most current version of Flask when you started, but now has been superseeded by version 0.12. You now want to start a second application, for which you'd like to use the 0.12 version, but if you replace the 0.11 version that you have installed you risk breaking your older application. Do you see the problem? It would be ideal if it was possible to install Flask 0.11 to be used by your old application, and also install Flask 0.12 for your new one. To address the issue of maintaining different versions of packages for different applications, Python uses the concept of virtual environments. A virtual environment is a complete copy of the Python interpreter. When you install packages in a virtual environment, the system-wide Python interpreter is not affected, only the copy is. So the solution to have complete freedom to install any versions of your packages for each application is to use a different virtual environment for each application. Virtual environments have the added benefit that they are owned by the user who creates them, so they do not require an administrator account. Let's start by creating a directory where the project will live. I'm going to call this directory microblog, since that is the name of the application: $ mkdir microblog $ cd microblog If you are using a Python 3 version, virtual environment support is included in it, so all you need to do to create one is this: $ python3 -m venv venv With this command, I'm asking Python to run the venv package, which creates a virtual environment named venv. The first venv in the command is the name of the Python virtual environment package, and the second is the virtual environment name that I'm going to use for this particular environment. If you find this confusing, you can replace the second venv with a different name that you want to assign to your virtual environment. In general I create my virtual environments with the name venv in the project directory, so whenever I cd into a project I find its corresponding virtual environment. Note that in some operating systems you may need to use python instead of python3 in the command above. Some installations use python for Python 2.x releases and python3 for the 3.x releases, while others map python to the 3.x releases. After the command completes, you are going to have a directory named venv where the virtual environment files are stored. If you are using any version of Python older than 3.4 (and that includes the 2.7 release), virtual environments are not supported natively. For those versions of Python, you need to download and install a third-party tool called virtualenv before you can create virtual environments. Once virtualenv is installed, you can create a virtual environment with the following command: $ virtualenv venv Regardless of the method you used to create it, you should have your virtual environment created. Now you have to tell the system that you want to use it, and you do that by activating it. To activate your brand new virtual environment you use the following command: $ source venv/bin/activate (venv) $ _ If you are using a Microsoft Windows command prompt window, the activation command is slightly different: $ venv\Scripts\activate (venv) $ _ When you activate a virtual environment, the configuration of your terminal session is modified so that the Python interpreter stored inside it is the one that is invoked when you type python. Also, the terminal prompt is modified to include the name of the activated virtual environment. The changes made to your terminal session are all temporary and private to that session, so they will not persist when you close the terminal window. If you work with multiple terminal windows open at the same time, it is perfectly fine to have different virtual environments activated on each one. Now that you have a virtual environment created and activated, you can finally install Flask in it: (venv) $ pip install flask If you want to confirm that your virtual environment now has Flask installed, you can start the Python interpreter and import Flask into it: >>> import flask >>> _ If this statement does not give you any errors you can congratulate yourself, as Flask is installed and ready to be used. A "Hello, World" Flask Application If you go to the Flask website, you are welcomed with a very simple example application that has just five lines of code. Instead of repeating that trivial example, I'm going to show you a slightly more elaborate one that will give you a good base structure for writing larger applications. The application will exist in a package. In Python, a sub-directory that includes a __init__.py file is considered a package, and can be imported. When you import a package, the __init__.py executes and defines what symbols the package exposes to the outside world. Let's create a package called app, that will host the application. Make sure you are in the microblog directory and then run the following command: (venv) $ mkdir app The __init__.py for the app package is going to contain the following code: app/__init__.py: Flask application instance from flask import Flask app = Flask(__name__) from app import routes The script above simply creates the application object as an instance of class Flask imported from the flask package. The __name__ variable passed to the Flask class is a Python predefined variable, which is set to the name of the module in which it is used. Flask uses the location of the module passed here as a starting point when it needs to load associated resources such as template files, which I will cover in . For all practical purposes, passing __name__ is almost always going to configure Flask in the correct way. The application then imports the routes module, which doesn't exist yet. One aspect that may seem confusing at first is that there are two entities named app. The app package is defined by the app directory and the __init__.py script, and is referenced in the from app import routes statement. The app variable is defined as an instance of class Flask in the __init__.py script, which makes it a member of the app package. Another peculiarity is that the routes module is imported at the bottom and not at the top of the script as it is always done. The bottom import is a workaround to circular imports, a common problem with Flask applications. You are going to see that the routes module needs to import the app variable defined in this script, so putting one of the reciprocal imports at the bottom avoids the error that results from the mutual references between these two files. So what goes in the routes module? The routes are the different URLs that the application implements. In Flask, handlers for the application routes are written as Python functions, called view functions. View functions are mapped to one or more route URLs so that Flask knows what logic to execute when a client requests a given URL. Here is your first view function, which you need to write in the new module named app/routes.py: app/routes.py: Home page route from app import app @app.route('/') @app.route('/index') def index(): return "Hello, World!" This view function is actually pretty simple, it just returns a greeting as a string. The two strange @app.route lines above the function are decorators, a unique feature of the Python language. A decorator modifies the function that follows it. A common pattern with decorators is to use them to register functions as callbacks for certain events. In this case, the @app.route decorator creates an association between the URL given as an argument and the function. In this example there are two decorators, which associate the URLs / and /index to this function. This means that when a web browser requests either of these two URLs, Flask is going to invoke this function and pass the return value of it back to the browser as a response. If this does not make complete sense yet, it will in a little bit when you run this application. To complete the application, you need to have a Python script at the top-level that defines the Flask application instance. Let's call this script microblog.py, and define it as a single line that imports the application instance: microblog.py: Main application module from app import app Remember the two app entities? Here you can see both together in the same sentence. The Flask application instance is called app and is a member of the app package. The from app import app statement imports the app variable that is a member of the app package. If you find this confusing, you can rename either the package or the variable to something else. Just to make sure that you are doing everything correctly, below you can see a diagram of the project structure so far: microblog/ venv/ app/ __init__.py routes.py microblog.py Believe it or not, this first version of the application is now complete! Before running it, though, Flask needs to be told how to import it, by setting the FLASK_APP environment variable: (venv) $ export FLASK_APP=microblog.py If you are using Microsoft Windows, use set instead of export in the command above. Are you ready to be blown away? You can run your first web application, with the following command: (venv) $ flask run * Serving Flask app "microblog" * Running on (Press CTRL+C to quit) After the server initializes it will wait for client connections. The output from flask run indicates that the server is running on IP address 127.0.0.1, which is always the address of your own computer. This address is so common that is also has a simpler name that you may have seen before: localhost. Network servers listen for connections on a specific port number. Applications deployed on production web servers typically listen on port 443, or sometimes 80 if they do not implement encryption, but access to these ports require administration rights. Since this application is running in a development environment, Flask uses the freely available port 5000. Now open up your web browser and enter the following URL in the address field: Alternatively you can use this other URL: Do you see the application route mappings in action? The first URL maps to /, while the second maps to /index. Both routes are associated with the only view function in the application, so they produce the same output, which is the string that the function returns. If you enter any other URL you will get an error, since only these two URLs are recognized by the application. When you are done playing with the server you can just press Ctrl-C to stop it. Congratulations, you have completed the first big step to become a web developer! December 12 2017 The Flask Mega-Tutorial Part II: Templates In this second installment of the Flask Mega-Tutorial series, I'm going to discuss how to work with templates. For your reference, below is a list of the articles in this series. • : Hello, World! • : Templates (this article) • : Web Forms • : Database • : User Logins • : Profile Page and Avatars • : Error Handling • : Followers • : Pagination • : Email Support • : Facelift • : Dates and Times • : I18n and L10n • : Ajax • : A Better Application Structure • : Full-Text Search • : Deployment on Linux • : Deployment on Heroku • : Deployment on Docker Containers • : Some JavaScript Magic • : User Notifications • : Background Jobs • : Application Programming Interfaces (APIs) (available on May 8th, 2018) Note 2: If you would like to support my work on this blog, or just don't have patience to wait for weekly articles, I am offering the complete version of this tutorial packaged as an ebook or a set of videos. For more information, visit learn.miguelgrinberg.com. After you complete , you should have a fully working, yet simple web application that has the following file structure: microblog\ venv\ app\ __init__.py routes.py microblog.py To run the application you set the FLASK_APP=microblog.py in your terminal session, and then execute flask run. This starts a web server with the application, which you can open by typing the URL in your web browser's address bar. In this chapter you will continue working on the same application, and in particular, you are going to learn how to generate more elaborate web pages that have a complex structure and many dynamic components. If anything about the application or the development workflow so far isn't clear, please review again before continuing. The GitHub links for this chapter are: Browse, Zip, Diff. What Are Templates? I want the home page of my microblogging application to have a heading that welcomes the user. For the moment, I'm going to ignore the fact that the application does not have the concept of users yet, as this is going to come later. Instead, I'm going to use a mock user, which I'm going to implement as a Python dictionary, as follows: user = {'username': 'Miguel'} Creating mock objects is a useful technique that allows you to concentrate on one part of the application without having to worry about other parts of the system that don't exist yet. I want to design the home page of my application, and I don't want the fact that I don't have a user system in place to distract me, so I just make up a user object so that I can keep going. The view function in the application returns a simple string. What I want to do now is expand that returned string into a complete HTML page, maybe something like this: app/routes.py: Return complete HTML page from view function from app import app @app.route('/') @app.route('/index') def index(): user = {'username': 'Miguel'} return ''' Home Page - Microblog

Hello, ''' + user['username'] + '''!

''' If you are not familiar with HTML, I recommend that you read HTML Markup on Wikipedia for a brief introduction. Update the view function as shown above and give the application a try to see how it looks in your browser. I hope you agree with me that the solution used above to deliver HTML to the browser is not good. Consider how complex the code in this view function will become when I have the blog posts from users, which are going to constantly change. The application is also going to have more view functions that are going to be associated with other URLs, so imagine if one day I decide to change the layout of this application, and have to update the HTML in every view function. This is clearly not an option that will scale as the application grows. If you could keep the logic of your application separate from the layout or presentation of your web pages, then things would be much better organized, don't you think? You could even hire a web designer to create a killer web site while you code the application logic in Python. Templates help achieve this separation between presentation and business logic. In Flask, templates are written as separate files, stored in a templates folder that is inside the application package. So after making sure that you are in the microblog directory, create the directory where templates will be stored: (venv) $ mkdir app/templates Below you can see your first template, which is similar in functionality to the HTML page returned by the index() view function above. Write this file in app/templates/index.html: app/templates/index.html: Main page template {{ title }} - Microblog

Hello, {{ user.username }}!