2017-12-05T17:15:48Z

The Flask Mega-Tutorial Part I: Hello, World!

Welcome! You are about to start on a journey to learn how to create web applications with Python and the Flask framework. The video above will give you an overview of the contents of this tutorial. In this first chapter, you are going to learn how to set up a Flask project. By the end of this chapter you are going to have a simple Flask web application running on your computer!

For your reference, below is a list of the articles in this series.

Note 1: If you are looking for the legacy version of this tutorial, it's here.

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 courses.miguelgrinberg.com.

All the code examples presented in this tutorial are hosted on a GitHub repository. Downloading the code from GitHub can save you a lot of typing, but I strongly recommend that you type the code yourself, at least for the first few chapters. Once you become more familiar with Flask and the example application you can access the code directly from GitHub if the typing becomes too tedious.

At the beginning of each chapter, I'm going to give you three GitHub links that can be useful while you work through the chapter. The Browse link will open the GitHub repository for Microblog at the place where the changes for the chapter you are reading were added, without including any changes introduced in future chapters. The Zip link is a download link for a zip file including the entire application up to and including the changes in the chapter. The Diff link will open a graphical view of all the changes that were made in the chapter you are about to read.

The GitHub links for this chapter are: Browse, Zip, Diff.

Installing Python

If you don't have Python installed on your computer, go ahead and install it now. If your operating system does not provide you with a Python package, you can download an installer from the Python official website. If you are using Microsoft Windows along with WSL or Cygwin, note that you will not be using the Windows native version of Python, but a Unix-friendly version that you need to obtain from Ubuntu (if you are using WSL) or from Cygwin.

To make sure your Python installation is functional, you can open a terminal window and type python3, or if that does not work, just python. Here is what you should expect to see:

$ python3
Python 3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> _

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 Ctrl-D. 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 <package-name>

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 Chapter 2. 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 http://127.0.0.1:5000/ (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:

    http://localhost:5000/

Alternatively you can use this other URL:

    http://localhost:5000/index

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.

Hello, World!

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!

Before I end this chapter, I want to mention one more thing. Since environment variables aren't remembered across terminal sessions, you may find tedious to always have to set the FLASK_APP environment variable when you open a new terminal window. Starting with version 1.0, Flask allows you to register environment variables that you want to be automatically imported when you run the flask command. To use this option you have to install the python-dotenv package:

(venv) $ pip install python-dotenv

Then you can just write the environment variable name and value in a .flaskenv file in the top-level directory of the project:

.flaskenv: Environment variables for flask command

FLASK_APP=microblog.py

Doing this is optional. If you prefer to set the environment variable manually, that is perfectly fine, as long as you always remember to do it.

472 comments

  • #401 Miguel Grinberg said 2020-04-23T09:52:16Z

    @Jack: have you compared your code against mine? There is a download link at the top of the article.

  • #402 Kenneth said 2020-04-25T01:02:41Z

    Hello MIguel,

    Is pip install flask the same the same as pip install Flask? Does lowercase or uppercase matter when using pip install?

  • #403 Miguel Grinberg said 2020-04-25T21:56:00Z

    @Kenneth: Doesn't matter, pip installs are not case sensitive.

  • #404 Mark Tigson said 2020-04-26T18:29:26Z

    I was getting this error with the same exact setup as described in the tutorial:

    Traceback (most recent call last): File "d:\microblog\venv\lib\site-packages\flask\cli.py", line 240, in locate_app import(module_name) File "D:\microblog\microblog.py", line 1, in from app import app ImportError: cannot import name 'app' from 'app' (D:\microblog\app__init__.py)

    Changed the 'app' package name to 'myapp' (the directory name itself and in all the files created to 'from myapp import app') and it fixed it.

    Also, another slightly confusing piece is which directory to run 'set FLASK_APP=microblog.py' and 'flask run' in cmd. I backed out of the microblog directory at first since the example looked like this:

    (venv)$ set FLASK_APP=microblog.py

    But it's really from within the microblog directory like this:

    (venv)microblog$ set FLASK_APP=microblog.py

    This is probably obvious to someone experienced with cmd, but might not be to beginners.

  • #405 Miguel Grinberg said 2020-04-26T22:52:27Z

    @Mark: My guess is that when you went to rename all the "from app" to "from myapp" you may have inadvertently fixed the problem. The import should work either way.

    Regarding the current directory, the tutorial asks you to cd into the microblog directory. Unless specifically mentioned you should not change directories. The prompt that I use to show CLI commands is generic. You probably have a prompt configured to show your current directory and for that reason expected I would do something like that, but no, I do not show current directory because it is always microblog throughout the whole tutorial, as far as I remember.

  • #406 Per-Olof Hermansson said 2020-04-30T18:50:06Z

    Hi Just started the tutorial,

    I named my folder web_project. And then it did not work. After renaming the microblog.py file to web_project.py and setting the FLASK_APP = web_project.py it works.

    So must the folder and the .py file have the same name?

    Per-Olof

  • #407 Miguel Grinberg said 2020-05-01T10:50:34Z

    @Per-Olof: that is very strange, the name of the top-level directory does not really matter, it can be anything you want. Maybe you want to give me more details on how it did not work? It must have been something else that cause your problem.

  • #408 Bidon Saha said 2020-05-03T17:51:00Z

    I am getting the error: Error: Failed to find Flask application or factory in module "microblog".Use "FLASK_APP=microblog:name to specify one.

    I have exactly followed the tutorial and double checked in many times. But I am unable to find the solution

  • #409 Miguel Grinberg said 2020-05-03T21:38:15Z

    @Bidon: the error suggests you do not have a "app" variable in your microblog.py file. Maybe you used a different name for your application instance? In that case, do what the error message suggests, set your FLASK_APP variable to microblog:name, where name is the name of your Flask object.

  • #410 FFFutureflo said 2020-05-04T21:54:51Z

    Thank you for this tutorial! I have a small hint if someone is having problems with autosaving in VSCode, like I had: The "from app import routes" line was set to the top of the file. To prevent this, just use the command: Ctrl + K & Ctrl + Shift + S

    The solution can be read here: https://stackoverflow.com/questions/44831313/how-to-exclude-files-from-format-on-save-in-vscode

  • #411 Miguel Grinberg said 2020-05-04T22:28:09Z

    @FFFutureflo: this happened because at some point you must have enabled the option that reformats your code when you save it.

  • #412 Gabriel Bożek said 2020-05-05T19:05:51Z

    Hi Miguel

    I have problem with logs in my console when I run flask. Output is: 127.0.0.1 - - [05/May/2020 20:48:15] "?[32mGET / HTTP/1.1?[0m" 302 - 127.0.0.1 - - [05/May/2020 20:48:15] "?[37mGET /login?next=%2F HTTP/1.1?[0m" 200 - and I don't know how can I remove ?[37m from logs. I found in google that ?[37m is terminal control characters, but I don't know what can I do with it. Thanks in advance.

    By the way, great tutorial.

  • #413 Miguel Grinberg said 2020-05-06T13:29:04Z

    @Gabriel: these control characters provide colors for the log output. I'm not sure what the exact rules are to determine if colored logs should be used or not, but I know there is an open bug for Flask regarding these characters appearing on Windows, which does not support colors. See https://github.com/pallets/flask/issues/3595.

  • #414 Hassan said 2020-05-17T23:49:55Z

    I have used your way in setting flask via using the init.py to initiate the Flask app. Things work just fine. I have just 2 questions. The First: Why should I put the "routes" import within the init.py? If I don't put it, how would that affect my app running? If I put the import of routes.py within a run.py file within the root of the project and initialize the Environment with FLASK_APP and FLASK_ENV (or FLASK_DEBUG), how would this affect my application? I have personally tried this as follows: I have created the app folder being named "blog" and kept the init.py empty, I created an app.py file where I initiated Flask app after importing it and didn't include, I created the config.py too at the root of the project. Flask running went just fine till I tried to use flask-sqlalchemy. Creating users with my configuration was not error-free. I just didn't figure out why I went into trouble with my configuration. PS. I have deleted the whole thing and restarted it your way. I just wanted to know why my way was wrong?

    If my way is possible what should have I done in order to make db records to be created successfully? Thanks for your kind patience.

  • #415 Miguel Grinberg said 2020-05-18T10:24:55Z

    If you don't import routes.py in __init__.py, where are you importing them? If you don't import them at all then your routes will not be registered with Flask. These are functions that you are never going to call directly, but they need to be imported somewhere just so that the app.route decorators register them with Flask. I can't really comment on the errors that you had since you did not describe them in detail, sorry.

  • #416 Hassan said 2020-05-18T10:54:25Z

    Hello, again

    I was just figuring out why importing the routes within the init.py. So if instead I used a module called application.py and initiated the flask app within it I should import routes(views), models and forms at the bottom just as you did in order to get registered with the Flask app?

    Anyway, thanks again for your prompt reply and being so helpful.

  • #417 Miguel Grinberg said 2020-05-18T16:55:20Z

    @Hassan: yes, that should work. You can import your models and your routes anywhere, it does not really matter where you do it. Typically those imports go at the bottom to avoid circular dependencies but that really depends on how you structure your application and your imports.

  • #418 Nick said 2020-05-27T09:50:45Z

    Hi! I have a little nit-pick.

    You said pip is not bundled with py2.7, but it actually is!

    Since Python 2.7.9, it is included by default with the Python binary installers, proof: https://docs.python.org/2/installing/index.html#key-terms

    Anyway, thanks for this amazing tutorial :)

  • #419 Miguel Grinberg said 2020-05-27T14:50:33Z

    @Nick: Good to know. I haven't used Python 2.7 in several years. Pip was not included in 2.7 back when this tutorial was written in 2012/2013.

  • #420 Rohan said 2020-05-28T12:03:49Z

    Hi Miguel - this is fantastic set of tutorials especially for beginners like me.

    Any suggestions on best practice to save and render images using external storage like S3 with flask?

  • #421 Miguel Grinberg said 2020-05-28T12:43:18Z

    @Rohan: The Flask-S3 extension overrides the url_for() function so that you can return URLs to static assets on an S3 bucket without having to modify your templates.

  • #422 Carlos Piña said 2020-05-28T16:58:01Z

    Hi to everyone! I just shared on Medium an article to understand better all these things related to the beginning with Flask and Python. Feel free to share and comment ;) https://medium.com/@carlospinamartinezdm/what-they-should-had-told-me-about-flask-and-python-since-the-beginning-fcfca24d7d3b

  • #423 Miguel Grinberg said 2020-05-28T23:06:23Z

    @Carlos: an article on Flask's use of the __name__ argument is going to come on the blog soon!

  • #424 Michael Gardner said 2020-06-05T04:16:53Z

    Looking forward to the name argument blog.

    For the routes import at the bottom, why do we even have to import them in the first place? If it somehow registers the routes with the Flask Class shouldn't there be a method to register them within the Flask Class that could be used in our routes.py to avoid the import? Sorry if you have already answered this elsewhere.

  • #425 Michael Gardner said 2020-06-05T04:37:41Z

    You can ignore my previous comment. It just occurred to me that routes.py is never used elsewhere so you have to import it into your init.py for the Flask Class to discover it.

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