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.

480 comments

  • #126 Eddie Montes said 2018-04-30T07:52:06Z

    Hi Miguel,

    I would like to ask, how to give an alias to git commits. When we do for instance: git checkout 3a and gets to commit related to chapter 3a. How to set this '3a? Thanks in advance.

  • #127 Miguel Grinberg said 2018-05-02T01:35:04Z

    @Eddie: after you do the commit, you need to use the "git tag" command to add a tag to it.

  • #128 Anders Wang said 2018-05-03T04:16:59Z

    I have a question before I purchase your book. The Flask has already been updated to 1.0, so I am wondering that is this new flask book has also updated to 1.0? If not, do you plan to update the ebook for Flask 1.0?

  • #129 Miguel Grinberg said 2018-05-03T06:02:35Z

    @Anders: Is there anything specific that you think I need to update? The tutorial is perfectly adequate for version 1.0.

  • #130 Driss said 2018-05-05T15:52:52Z

    Hey Miguel. Just started out your tutorial as a complete beginner and I have a weird situation going on here.

    The only way I can run the application is by having project structure different than yours, see below /microblog /venv /app init.py routes.py microblog.py

    The difference is in the last file 'microblog.py'. If I place it in the root (microblog) folder when running flask I get an error saying 'Could not import "app.microblog". '

    If I move it to the 'app' folder, then I can normally start the app and continue developing. This is probably a silly question, but please bear with me. I could not find a (clear) answer to why this is happening. Everywhere I've read, makes me believe your folder structure is correct, but it simply doesn't work for me and I have no idea why.

    P.S I'm using python 2.7

    Regards

  • #131 Miguel Grinberg said 2018-05-06T05:27:26Z

    @Driss: The file microblog.py goes in the root directory of the project. Nowhere in this application I import "app.microblog". You need to look at the stack trace for this error to find where in your version of the application you are importing this. My guess is that you need to make a correction in that place.

  • #132 Froot said 2018-05-10T13:12:36Z

    Hi Miguel,

    Thanks for your tutorial on flask. All working well on Linux.:

    But i get this with the end result.:

    ~/project/microblog $ flask run Environment: production WARNING: Do not use the development server in a production environment. Use a production WSGI server instead. * Debug mode: off * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

    The line with Warning thus this mean that a need to install a local WSGI server.

    Best regard,

    Frt

  • #133 Miguel Grinberg said 2018-05-13T02:46:07Z

    @Froot: the warning is letting you know that you should not use this web server in production. I assume you are not, so you are fine. When you get to the deployment chapters you will lean how to use a production WSGI server instead of this one.

  • #134 Peter van Dooren said 2018-05-13T21:37:02Z

    hello Miguel I bought your Flask tutorial (and I'am a beginner in Flask) and I started with the "Hello World" script. When executing the "(venv) $ mkdir app" there are no scripts is this app-directory? The project structure is the same as in the tutorial. What can be the reason? thank you for the help.

  • #135 michele said 2018-05-14T13:55:13Z

    if i put microbolog.py in the same folder with init and routes it works, but if i put out of the "app" folder it doesn't work, he sad:

    Error: Failed to find application in module "microblog". Are you sure it contains a Flask application? Maybe you wrapped it in a WSGI middleware or you are using a factory function.

  • #136 Miguel Grinberg said 2018-05-14T21:13:26Z

    @Peter: the "mkdir app" command creates a sub-directory with the name app.

  • #137 Miguel Grinberg said 2018-05-14T21:33:31Z

    @michele: compare the contents of microblog.py with my version, Flask is telling you that it could not find the application in this module. The location is outside the app directory, so the contents of this module must be wrong in some way.

  • #138 Ankur said 2018-05-16T02:53:11Z

    If I change 'app' to 'application' everywhere, in .py files and the folder name too, then i get following error in the bash -

    Usage: flask run [OPTIONS]

    Error: Could not locate a Flask application. You did not provide the "FLASK_APP" environment variable, and a "wsgi.py" or "app.py" module was not found in the current directory.

    I am changing 'app' to 'application' to make it suitable for aws hosting.

  • #139 Miguel Grinberg said 2018-05-16T04:44:47Z

    @Ankur: not sure what is the aws requirement you talk about with regards to app vs. application, but in any case, you can set FLASK_APP=microblog.py:application.

  • #140 J. said 2018-05-16T14:11:44Z

    Could you make an article about unit and integration testing of an flask app.

    I'm having some trouble trying to test the app.

    When I execute the tests I got module imports errors. (can't find flask or something like that)

  • #141 Miguel Grinberg said 2018-05-16T19:11:28Z

    @J: there are a couple of chapters in this tutorial that cover unit testing, see chapters 7 and 15.

  • #142 David Wilson said 2018-05-17T10:40:26Z

    Progress! I've found doc online that tells me I can do: flask run --host=0.0.0.0 and thus I can ssh into my raspberry pi and access the web pages from my Mac. In other words the flask environment is not restricted to the raspberry pi only. What I'd like to do is incorporate that configuration into the appropriate startup file. I'm guessing app/init.py - and this I have not figured out how to do. What do you recommend?

    David
  • #143 Miguel Grinberg said 2018-05-17T18:59:16Z

    @David: the "flask run" command does not work with configuration. There is the app.run() method of running the application, which does accept host and port arguments to set the configuration for the web server.

  • #144 Steve said 2018-05-24T22:05:21Z

    Hi Miguel,

    Thanks for this great tutorial. Just what I was looking for! I have a question about circular import workaround -- putting "from app import routes" at the bottom of init.py. I accept this as Flask convention, but wonder if it could be done by putting "from flask.Flask import routes" at the top of the script. Just curious, since I find curiosity a good attribute in my learning process. Thanks.

  • #145 Miguel Grinberg said 2018-05-24T23:25:25Z

    @Steve: moving the import to the bottom is not really a Flask convention, it is a way to prevent circular dependency errors that would occur if you have that import at the top. I suggest you try it to see how the application fails.

  • #146 Steve said 2018-05-25T16:12:22Z

    Thanks, Miguel. I will play around with that. Also, I realized my error. "from app import routes" refers to the "app" directory I created, NOT the "app" variable declared in "app = Flask(name)". I renamed the directory "my_app" to avoid confusion.

    One more comment. Perhaps I missed your saying this, but it seems that once the server is started it keeps whatever code was loaded. If corrections to my code are made and saved, the server needs to be stopped and restarted for those corrections to take effect. Perhaps you said this and I missed it, or perhaps it is obvious, but for other newbie coders there could be some frustration when they fix their code and don't understand why the server is still showing errors.

  • #147 Miguel Grinberg said 2018-05-26T21:04:31Z

    @Steve: Later in the tutorial you are going to learn about debug mode and in particular about the Flask reloader, which automatically restarts the server when the code changes. For the time being, you can stop the server after each test, and restart it after you made changes.

  • #148 Julio Ribeiro said 2018-06-03T16:09:40Z

    Hi Miguel! I may have lost previous explanations about this, but, anyway here it go: Why not create a directory named something like "webapp" instead of "app" in order to avoid confusion with the "app" instance of Flask class? Seems to be more easy to follow. Just saying.

  • #149 Miguel Grinberg said 2018-06-03T18:06:34Z

    @Julio: I honestly don't see a confusion, there are many instances when writing Python applications where a package and a symbol inside that package are named the same. But if you prefer to use a different name for the package, that's totally fine, no problem at all.

  • #150 Akshay Khurana said 2018-06-04T08:38:20Z

    Hi Miguel, Your blog seems great but unfortunately, I struck on running part. I'm getting below error:

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 325, in call self._flush_bg_loading_exception()

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 313, in _flush_bg_loading_exception reraise(*exc_info)

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask_compat.py", line 35, in reraise raise value

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 302, in _load_app self._load_unlocked()

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 317, in _load_unlocked self._app = rv = self.loader()

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 372, in load_app app = locate_app(self, import_name, name)

    File "D:\Flask_Tutorial\microblog\venv\lib\site-packages\flask\cli.py", line 246, in locate_app 'Could not import "{name}".'.format(name=module_name)

    flask.cli.NoAppException: Could not import ""microblog.py"".

    I'm working on Windows, I have tried all the relevant solutions discussed in the comment section Looking forward to your suggestions.

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