.. _quickstart: Quickstart ========== Eager to get started? This page gives a good introduction to Flask. It assumes you already have Flask installed. If you do not, head over to the :ref:`installation` section. A Minimal Application --------------------- A minimal Flask application looks something like this:: from flask import Flask app = Flask(__name__) @app.route('/') def hello_world(): return 'Hello World!' if __name__ == '__main__': app.run() Just save it as `hello.py` (or something similar) and run it with your Python interpreter. Make sure to not call your application `flask.py` because this would conflict with Flask itself. :: $ python hello.py * Running on http://127.0.0.1:5000/ Now head over to `http://127.0.0.1:5000/ `_, and you should see your hello world greeting. So what did that code do? 1. First we imported the :class:`~flask.Flask` class. An instance of this class will be our WSGI application. The first argument is the name of the application's module. If you are using a single module (as in this example), you should use `__name__` because depending on if it's started as application or imported as module the name will be different (``'__main__'`` versus the actual import name). For more information, have a look at the :class:`~flask.Flask` documentation. 2. Next we create an instance of this class. We pass it the name of the module or package. This is needed so that Flask knows where to look for templates, static files, and so on. 3. We then use the :meth:`~flask.Flask.route` decorator to tell Flask what URL should trigger our function. 4. The function is given a name which is also used to generate URLs for that particular function, and returns the message we want to display in the user's browser. 5. Finally we use the :meth:`~flask.Flask.run` function to run the local server with our application. The ``if __name__ == '__main__':`` makes sure the server only runs if the script is executed directly from the Python interpreter and not used as imported module. To stop the server, hit control-C. .. _public-server: .. admonition:: Externally Visible Server If you run the server you will notice that the server is only accessible from your own computer, not from any other in the network. This is the default because in debugging mode a user of the application can execute arbitrary Python code on your computer. If you have `debug` disabled or trust the users on your network, you can make the server publicly available simply by changing the call of the :meth:`~flask.Flask.run` method to look like this:: app.run(host='0.0.0.0') This tells your operating system to listen on all public IPs. .. _debug-mode: Debug Mode ---------- The :meth:`~flask.Flask.run` method is nice to start a local development server, but you would have to restart it manually after each change to your code. That is not very nice and Flask can do better. If you enable debug support the server will reload itself on code changes, and it will also provide you with a helpful debugger if things go wrong. There are two ways to enable debugging. Either set that flag on the application object:: app.debug = True app.run() Or pass it as a parameter to run:: app.run(debug=True) Both methods have the exact same effect. .. admonition:: Attention Even though the interactive debugger does not work in forking environments (which makes it nearly impossible to use on production servers), it still allows the execution of arbitrary code. This makes it a major security risk and therefore it **must never be used on production machines**. Screenshot of the debugger in action: .. image:: _static/debugger.png :align: center :class: screenshot :alt: screenshot of debugger in action Have another debugger in mind? See :ref:`working-with-debuggers`. Routing ------- Modern web applications have beautiful URLs. This helps people remember the URLs, which is especially handy for applications that are used from mobile devices with slower network connections. If the user can directly go to the desired page without having to hit the index page it is more likely they will like the page and come back next time. As you have seen above, the :meth:`~flask.Flask.route` decorator is used to bind a function to a URL. Here are some basic examples:: @app.route('/') def index(): return 'Index Page' @app.route('/hello') def hello(): return 'Hello World' But there is more to it! You can make certain parts of the URL dynamic and attach multiple rules to a function. Variable Rules `````````````` To add variable parts to a URL you can mark these special sections as ````. Such a part is then passed as keyword argument to your function. Optionally a converter can be specified by specifying a rule with ````. Here are some nice examples:: @app.route('/user/') def show_user_profile(username): # show the user profile for that user return 'User %s' % username @app.route('/post/') def show_post(post_id): # show the post with the given id, the id is an integer return 'Post %d' % post_id The following converters exist: =========== =========================================== `int` accepts integers `float` like `int` but for floating point values `path` like the default but also accepts slashes =========== =========================================== .. admonition:: Unique URLs / Redirection Behaviour Flask's URL rules are based on Werkzeug's routing module. The idea behind that module is to ensure beautiful and unique URLs based on precedents laid down by Apache and earlier HTTP servers. Take these two rules:: @app.route('/projects/') def projects(): return 'The project page' @app.route('/about') def about(): return 'The about page' Though they look rather similar, they differ in their use of the trailing slash in the URL *definition*. In the first case, the canonical URL for the `projects` endpoint has a trailing slash. In that sense, it is similar to a folder on a file system. Accessing it without a trailing slash will cause Flask to redirect to the canonical URL with the trailing slash. In the second case, however, the URL is defined without a trailing slash, rather like the pathname of a file on UNIX-like systems. Accessing the URL with a trailing slash will produce a 404 "Not Found" error. This behavior allows relative URLs to continue working if users access the page when they forget a trailing slash, consistent with how with how Apache and other servers work. Also, the URLs will stay unique, which helps search engines avoid indexing the same page twice. .. _url-building: URL Building ```````````` If it can match URLs, can Flask also generate them? Of course it can. To build a URL to a specific function you can use the :func:`~flask.url_for` function. It accepts the name of the function as first argument and a number of keyword arguments, each corresponding to the variable part of the URL rule. Unknown variable parts are appended to the URL as query parameters. Here are some examples: >>> from flask import Flask, url_for >>> app = Flask(__name__) >>> @app.route('/') ... def index(): pass ... >>> @app.route('/login') ... def login(): pass ... >>> @app.route('/user/') ... def profile(username): pass ... >>> with app.test_request_context(): ... print url_for('index') ... print url_for('login') ... print url_for('login', next='/') ... print url_for('profile', username='John Doe') ... / /login /login?next=/ /user/John%20Doe (This also uses the :meth:`~flask.Flask.test_request_context` method, explained below. It tells Flask to behave as though it is handling a request, even though we are interacting with it through a Python shell. Have a look at the explanation below. :ref:`context-locals`). Why would you want to build URLs instead of hard-coding them into your templates? There are three good reasons for this: 1. Reversing is often more descriptive than hard-coding the URLs. More importantly, it allows you to change URLs in one go, without having to remember to change URLs all over the place. 2. URL building will handle escaping of special characters and Unicode data transparently for you, so you don't have to deal with them. 3. If your application is placed outside the URL root (say, in ``/myapplication`` instead of ``/``), :func:`~flask.url_for` will handle that properly for you. HTTP Methods ```````````` HTTP (the protocol web applications are speaking) knows different methods for accessing URLs. By default, a route only answers to `GET` requests, but that can be changed by providing the `methods` argument to the :meth:`~flask.Flask.route` decorator. Here are some examples:: @app.route('/login', methods=['GET', 'POST']) def login(): if request.method == 'POST': do_the_login() else: show_the_login_form() If `GET` is present, `HEAD` will be added automatically for you. You don't have to deal with that. It will also make sure that `HEAD` requests are handled as the `HTTP RFC`_ (the document describing the HTTP protocol) demands, so you can completely ignore that part of the HTTP specification. Likewise, as of Flask 0.6, `OPTIONS` is implemented for you automatically as well. You have no idea what an HTTP method is? Worry not, here is a quick introduction to HTTP methods and why they matter: The HTTP method (also often called "the verb") tells the server what the clients wants to *do* with the requested page. The following methods are very common: `GET` The browser tells the server to just *get* the information stored on that page and send it. This is probably the most common method. `HEAD` The browser tells the server to get the information, but it is only interested in the *headers*, not the content of the page. An application is supposed to handle that as if a `GET` request was received but to not deliver the actual content. In Flask you don't have to deal with that at all, the underlying Werkzeug library handles that for you. `POST` The browser tells the server that it wants to *post* some new information to that URL and that the server must ensure the data is stored and only stored once. This is how HTML forms usually transmit data to the server. `PUT` Similar to `POST` but the server might trigger the store procedure multiple times by overwriting the old values more than once. Now you might be asking why this is useful, but there are some good reasons to do it this way. Consider that the connection is lost during transmission: in this situation a system between the browser and the server might receive the request safely a second time without breaking things. With `POST` that would not be possible because it must only be triggered once. `DELETE` Remove the information at the given location. `OPTIONS` Provides a quick way for a client to figure out which methods are supported by this URL. Starting with Flask 0.6, this is implemented for you automatically. Now the interesting part is that in HTML4 and XHTML1, the only methods a form can submit to the server are `GET` and `POST`. But with JavaScript and future HTML standards you can use the other methods as well. Furthermore HTTP has become quite popular lately and browsers are no longer the only clients that are using HTTP. For instance, many revision control system use it. .. _HTTP RFC: http://www.ietf.org/rfc/rfc2068.txt Static Files ------------ Dynamic web applications also need static files. That's usually where the CSS and JavaScript files are coming from. Ideally your web server is configured to serve them for you, but during development Flask can do that as well. Just create a folder called `static` in your package or next to your module and it will be available at `/static` on the application. To generate URLs that part of the URL, use the special ``'static'`` URL name:: url_for('static', filename='style.css') The file has to be stored on the filesystem as ``static/style.css``. Rendering Templates ------------------- Generating HTML from within Python is not fun, and actually pretty cumbersome because you have to do the HTML escaping on your own to keep the application secure. Because of that Flask configures the `Jinja2 `_ template engine for you automatically. To render a template you can use the :func:`~flask.render_template` method. All you have to do is provide the name of the template and the variables you want to pass to the template engine as keyword arguments. Here's a simple example of how to render a template:: from flask import render_template @app.route('/hello/') @app.route('/hello/') def hello(name=None): return render_template('hello.html', name=name) Flask will look for templates in the `templates` folder. So if your application is a module, this folder is next to that module, if it's a package it's actually inside your package: **Case 1**: a module:: /application.py /templates /hello.html **Case 2**: a package:: /application /__init__.py /templates /hello.html For templates you can use the full power of Jinja2 templates. Head over to the the official `Jinja2 Template Documentation `_ for more information. Here is an example template: .. sourcecode:: html+jinja Hello from Flask {% if name %}

Hello {{ name }}!

{% else %}

Hello World!

{% endif %} Inside templates you also have access to the :class:`~flask.request`, :class:`~flask.session` and :class:`~flask.g` [#]_ objects as well as the :func:`~flask.get_flashed_messages` function. Templates are especially useful if inheritance is used. If you want to know how that works, head over to the :ref:`template-inheritance` pattern documentation. Basically template inheritance makes it possible to keep certain elements on each page (like header, navigation and footer). Automatic escaping is enabled, so if `name` contains HTML it will be escaped automatically. If you can trust a variable and you know that it will be safe HTML (for example because it came from a module that converts wiki markup to HTML) you can mark it as safe by using the :class:`~jinja2.Markup` class or by using the ``|safe`` filter in the template. Head over to the Jinja 2 documentation for more examples. Here is a basic introduction to how the :class:`~jinja2.Markup` class works: >>> from flask import Markup >>> Markup('Hello %s!') % 'hacker' Markup(u'Hello <blink>hacker</blink>!') >>> Markup.escape('hacker') Markup(u'<blink>hacker</blink>') >>> Markup('Marked up » HTML').striptags() u'Marked up \xbb HTML' .. versionchanged:: 0.5 Autoescaping is no longer enabled for all templates. The following extensions for templates trigger autoescaping: ``.html``, ``.htm``, ``.xml``, ``.xhtml``. Templates loaded from a string will have autoescaping disabled. .. [#] Unsure what that :class:`~flask.g` object is? It's something in which you can store information for your own needs, check the documentation of that object (:class:`~flask.g`) and the :ref:`sqlite3` for more information. Accessing Request Data ---------------------- For web applications it's crucial to react to the data a client sent to the server. In Flask this information is provided by the global :class:`~flask.request` object. If you have some experience with Python you might be wondering how that object can be global and how Flask manages to still be threadsafe. The answer is context locals: .. _context-locals: Context Locals `````````````` .. admonition:: Insider Information If you want to understand how that works and how you can implement tests with context locals, read this section, otherwise just skip it. Certain objects in Flask are global objects, but not of the usual kind. These objects are actually proxies to objects that are local to a specific context. What a mouthful. But that is actually quite easy to understand. Imagine the context being the handling thread. A request comes in and the web server decides to spawn a new thread (or something else, the underlying object is capable of dealing with concurrency systems other than threads). When Flask starts its internal request handling it figures out that the current thread is the active context and binds the current application and the WSGI environments to that context (thread). It does that in an intelligent way so that one application can invoke another application without breaking. So what does this mean to you? Basically you can completely ignore that this is the case unless you are doing something like unit testing. You will notice that code which depends on a request object will suddenly break because there is no request object. The solution is creating a request object yourself and binding it to the context. The easiest solution for unit testing is to use the :meth:`~flask.Flask.test_request_context` context manager. In combination with the `with` statement it will bind a test request so that you can interact with it. Here is an example:: from flask import request with app.test_request_context('/hello', method='POST'): # now you can do something with the request until the # end of the with block, such as basic assertions: assert request.path == '/hello' assert request.method == 'POST' The other possibility is passing a whole WSGI environment to the :meth:`~flask.Flask.request_context` method:: from flask import request with app.request_context(environ): assert request.method == 'POST' The Request Object `````````````````` The request object is documented in the API section and we will not cover it here in detail (see :class:`~flask.request`). Here is a broad overview of some of the most common operations. First of all you have to import it from the `flask` module:: from flask import request The current request method is available by using the :attr:`~flask.request.method` attribute. To access form data (data transmitted in a `POST` or `PUT` request) you can use the :attr:`~flask.request.form` attribute. Here is a full example of the two attributes mentioned above:: @app.route('/login', methods=['POST', 'GET']) def login(): error = None if request.method == 'POST': if valid_login(request.form['username'], request.form['password']): return log_the_user_in(request.form['username']) else: error = 'Invalid username/password' # this is executed if the request method was GET or the # credentials were invalid What happens if the key does not exist in the `form` attribute? In that case a special :exc:`KeyError` is raised. You can catch it like a standard :exc:`KeyError` but if you don't do that, a HTTP 400 Bad Request error page is shown instead. So for many situations you don't have to deal with that problem. To access parameters submitted in the URL (``?key=value``) you can use the :attr:`~flask.request.args` attribute:: searchword = request.args.get('q', '') We recommend accessing URL parameters with `get` or by catching the `KeyError` because users might change the URL and presenting them a 400 bad request page in that case is not user friendly. For a full list of methods and attributes of the request object, head over to the :class:`~flask.request` documentation. File Uploads ```````````` You can handle uploaded files with Flask easily. Just make sure not to forget to set the ``enctype="multipart/form-data"`` attribute on your HTML form, otherwise the browser will not transmit your files at all. Uploaded files are stored in memory or at a temporary location on the filesystem. You can access those files by looking at the :attr:`~flask.request.files` attribute on the request object. Each uploaded file is stored in that dictionary. It behaves just like a standard Python :class:`file` object, but it also has a :meth:`~werkzeug.datastructures.FileStorage.save` method that allows you to store that file on the filesystem of the server. Here is a simple example showing how that works:: from flask import request @app.route('/upload', methods=['GET', 'POST']) def upload_file(): if request.method == 'POST': f = request.files['the_file'] f.save('/var/www/uploads/uploaded_file.txt') ... If you want to know how the file was named on the client before it was uploaded to your application, you can access the :attr:`~werkzeug.datastructures.FileStorage.filename` attribute. However please keep in mind that this value can be forged so never ever trust that value. If you want to use the filename of the client to store the file on the server, pass it through the :func:`~werkzeug.utils.secure_filename` function that Werkzeug provides for you:: from flask import request from werkzeug import secure_filename @app.route('/upload', methods=['GET', 'POST']) def upload_file(): if request.method == 'POST': f = request.files['the_file'] f.save('/var/www/uploads/' + secure_filename(f.filename)) ... For some better examples, checkout the :ref:`uploading-files` pattern. Cookies ``````` To access cookies you can use the :attr:`~flask.Request.cookies` attribute. To set cookies you can use the :attr:`~flask.Response.set_cookie` method of response objects. The :attr:`~flask.Request.cookies` attribute of request objects is a dictionary with all the cookies the client transmits. If you want to use sessions, do not use the cookies directly but instead use the :ref:`sessions` in Flask that add some security on top of cookies for you. Reading cookies:: from flask import request @app.route('/') def index(): username = request.cookies.get('username') # use cookies.get(key) instead of cookies[key] to not get a # KeyError if the cookie is missing. Storing cookies:: from flask import make_response @app.route('/') def index(): resp = make_response(render_template(...)) resp.set_cookie('username', 'the username') return resp Note that cookies are set on response objects. Since you normally just return strings from the view functions Flask will convert them into response objects for you. If you explicitly want to do that you can use the :meth:`~flask.make_response` function and then modify it. Sometimes you might want to set a cookie at a point where the response object does not exist yet. This is possible by utilizing the :ref:`deferred-callbacks` pattern. For this also see :ref:`about-responses`. Redirects and Errors -------------------- To redirect a user to somewhere else you can use the :func:`~flask.redirect` function. To abort a request early with an error code use the :func:`~flask.abort` function. Here an example how this works:: from flask import abort, redirect, url_for @app.route('/') def index(): return redirect(url_for('login')) @app.route('/login') def login(): abort(401) this_is_never_executed() This is a rather pointless example because a user will be redirected from the index to a page they cannot access (401 means access denied) but it shows how that works. By default a black and white error page is shown for each error code. If you want to customize the error page, you can use the :meth:`~flask.Flask.errorhandler` decorator:: from flask import render_template @app.errorhandler(404) def page_not_found(error): return render_template('page_not_found.html'), 404 Note the ``404`` after the :func:`~flask.render_template` call. This tells Flask that the status code of that page should be 404 which means not found. By default 200 is assumed which translates to: all went well. .. _about-responses: About Responses --------------- The return value from a view function is automatically converted into a response object for you. If the return value is a string it's converted into a response object with the string as response body, an ``200 OK`` error code and a ``text/html`` mimetype. The logic that Flask applies to converting return values into response objects is as follows: 1. If a response object of the correct type is returned it's directly returned from the view. 2. If it's a string, a response object is created with that data and the default parameters. 3. If a tuple is returned the response object is created by passing the tuple as arguments to the response object's constructor. 4. If none of that works, Flask will assume the return value is a valid WSGI application and convert that into a response object. If you want to get hold of the resulting response object inside the view you can use the :func:`~flask.make_response` function. Imagine you have a view like this: .. sourcecode:: python @app.errorhandler(404) def not_found(error): return render_template('error.html'), 404 You just need to wrap the return expression with :func:`~flask.make_response` and get the result object to modify it, then return it: .. sourcecode:: python @app.errorhandler(404) def not_found(error): resp = make_response(render_template('error.html'), 404) resp.headers['X-Something'] = 'A value' return resp .. _sessions: Sessions -------- In addition to the request object there is also a second object called :class:`~flask.session` which allows you to store information specific to a user from one request to the next. This is implemented on top of cookies for you and signs the cookies cryptographically. What this means is that the user could look at the contents of your cookie but not modify it, unless they know the secret key used for signing. In order to use sessions you have to set a secret key. Here is how sessions work:: from flask import Flask, session, redirect, url_for, escape, request app = Flask(__name__) @app.route('/') def index(): if 'username' in session: return 'Logged in as %s' % escape(session['username']) return 'You are not logged in' @app.route('/login', methods=['GET', 'POST']) def login(): if request.method == 'POST': session['username'] = request.form['username'] return redirect(url_for('index')) return '''

''' @app.route('/logout') def logout(): # remove the username from the session if its there session.pop('username', None) return redirect(url_for('index')) # set the secret key. keep this really secret: app.secret_key = 'A0Zr98j/3yX R~XHH!jmN]LWX/,?RT' The :func:`~flask.escape` mentioned here does escaping for you if you are not using the template engine (as in this example). .. admonition:: How to generate good secret keys The problem with random is that it's hard to judge what is truly random. And a secret key should be as random as possible. Your operating system has ways to generate pretty random stuff based on a cryptographic random generator which can be used to get such a key: >>> import os >>> os.urandom(24) '\xfd{H\xe5<\x95\xf9\xe3\x96.5\xd1\x01O`_ for more information. Hooking in WSGI Middlewares --------------------------- If you want to add a WSGI middleware to your application you can wrap the internal WSGI application. For example if you want to use one of the middlewares from the Werkzeug package to work around bugs in lighttpd, you can do it like this:: from werkzeug.contrib.fixers import LighttpdCGIRootFix app.wsgi_app = LighttpdCGIRootFix(app.wsgi_app) .. _quickstart_deployment: Deploying to a Web Server ------------------------- Ready to deploy your new Flask app? To wrap up the quickstart, you can immediately deploy to a hosted platform, all of which offer a free plan for small projects: - `Deploying Flask on Heroku `_ - `Deploying Flask on ep.io `_ - `Deploying WSGI on dotCloud `_ with `Flask-specific notes `_ Other places where you can host your Flask app: - `Deploying Flask on Webfaction `_ - `Deploying Flask on Google App Engine `_ - `Sharing your Localhost Server with Localtunnel `_ If you manage your own hosts and would like to host yourself, see the chapter on :ref:`deployment`.