flask/docs/quickstart.rst

.. _quickstart:

Quickstart
==========

Eager to get started?  This page gives a good introduction in how to gets
started with Flask.  This 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 that::

    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/

Head over to `http://127.0.0.1:5000/ <http://127.0.0.1:5000/>`_, 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 (like here)
   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
   on that, have a look at the :class:`~flask.Flask` documentation.
2. next we create an instance of it.  We pass it the name of the module /
   package.  This is needed so that Flask knows where it should look for
   templates, static files and so on.
3. Then we use the :meth:`~flask.Flask.route` decorator to tell Flask
   what URL should trigger our function.
4. The function then has a name which is also used to generate URLs to
   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 available
   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.

   Just change 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 a public IP.


Debug Mode
----------

Now that :meth:`~flask.Flask.run` method is nice to start a local
development server, but you would have to restart it manually after each
change you do to code.  That is not very nice and Flask can do better.  If
you enable the debug support the server will reload itself on code changes
and 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 to run::

    app.run(debug=True)

Both will have exactly the same effect.

.. admonition:: Attention

   The interactive debugger however does not work in forking environments
   which makes it nearly impossible to use on production servers but the
   debugger still allows the execution of arbitrary code which makes it a
   major security risk and **must never be used on production machines**
   because of that.

Screenshot of the debugger in action:

.. image:: _static/debugger.png
   :align: center
   :class: screenshot
   :alt: screenshot of debugger in action


Routing
-------

As you have seen above, the :meth:`~flask.Flask.route` decorator is used
to bind a function to a URL.  But there is more to it!  You can make
certain parts of the URL dynamic and attach multiple rules to a function.

Here some examples::

    @app.route('/')
    def index():
        return 'Index Page'

    @app.route('/hello')
    def hello():
        return 'Hello World'


Variable Rules
``````````````

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 he will like the page and come back next time.

To add variable parts to a URL you can mark these special sections as
``<variable_name>``.  Such a part is then passed as keyword argument to
your function.  Optionally a converter can be specified by specifying a
rule with ``<converter:variable_name>``.  Here some nice examples::

    @app.route('/user/<username>')
    def show_user_profile(username):
        # show the user profile for that user
        pass

    @app.route('/post/<int:post_id>')
    def show_post(post_id):
        # show the post with the given id, the id is an integer
        pass

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 nice looking and also unique URLs based
   on behaviour Apache and earlier servers coined.

   Take these two rules::

        @app.route('/projects/')
        def projects():
            pass

        @app.route('/about')
        def about():
            pass

   They look rather similar, the difference is the trailing slash in the
   URL *definition*.  In the first case, the canonical URL for the
   `projects` endpoint has a trailing slash.  It's similar to a folder in
   that sense.  Accessing it without a trailing slash will cause Flask to
   redirect to the canonical URL with the trailing slash.

   However in the second case the URL is defined without a slash so it
   behaves similar to a file and accessing the URL with a trailing slash
   will be a 404 error.

   Why is this?  This allows relative URLs to continue working if users
   access the page when they forget a trailing slash.  This behaviour is
   also consistent with how Apache and other servers work.  Also, the URLs
   will stay unique which helps search engines not indexing the same page
   twice.


.. _url-building:

URL Building
````````````

If it can match URLs, can it also generate them?  Of course you 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
parameter.  Here 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/<username>')
... 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 basically tells flask to think we are handling a
request even though we are not, we are in an interactive Python shell.
Have a look at the explanation below. :ref:`context-locals`).

Why would you want to build URLs instead of hardcoding them in your
templates?  There are three good reasons for this:

1. reversing is often more descriptive than hardcoding the URLs.  Also and
   more importantly you can change URLs in one go without having to change
   the URLs all over the place.
2. URL building will handle escaping of special characters and Unicode
   data transparently for you, you don't have to deal with that.
3. If your application is placed outside the URL root (so 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
to access 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 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 like the `HTTP RFC`_ (the document describing the HTTP
protocol) demands, so you can completely ignore that part of the HTTP
specification.

You have no idea what an HTTP method is?  Worry not, here quick
introduction in 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: just *get* me the information stored on
    that page and send them to me.  This is probably the most common
    method.

`HEAD`
    The Browser tells the server: get me the information, but I am 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 not deliver the actual contents.  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 are usually
    transmitting 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 any useful, but there are some good
    reasons to do that.  Consider the connection is lost during
    transmission, in that situation a system between the browser and the
    server might sent 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 that the given location.

Now the interesting part is that in HTML4 and XHTML1, the only methods a
form might submit to the server are `GET` and `POST`.  But with JavaScript
and future HTML standards you can use other methods as well.  Furthermore
HTTP became quite popular lately and there are more things than browsers
that are speaking HTTP.  (Your revision control system for instance might
speak HTTP)

.. _HTTP RFC: http://www.ietf.org/rfc/rfc2068.txt

Static Files
------------

Dynamic web applications need static files as well.  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 to 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
<http://jinja.pocoo.org/2/>`_ template engine for you automatically.

To render a template you can use the :func:`~flask.render_template`
method.  All you have to do is to 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/<name>')
    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, that 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 `Jinja2 Template Documentation
<http://jinja.pocoo.org/2/documentation/templates>`_ for more information.

Here an example template:

.. sourcecode:: html+jinja

    <!doctype html>
    <title>Hello from Flask</title>
    {% if name %}
      <h1>Hello {{ name }}!</h1>
    {% else %}
      <h1>Hello World!</h1>
    {% 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 (because for example 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 a basic introduction in how the :class:`~jinja2.Markup` class works:

>>> from flask import Markup
>>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>'
Markup(u'<strong>Hello &lt;blink&gt;hacker&lt;/blink&gt;!</strong>')
>>> Markup.escape('<blink>hacker</blink>')
Markup(u'&lt;blink&gt;hacker&lt;/blink&gt;')
>>> Markup('<em>Marked up</em> &raquo; HTML').striptags()
u'Marked up \xbb HTML'

.. [#] Unsure what that :class:`~flask.g` object is? It's something you
   can store information on yourself, 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 are 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 just a standard
global object, but actually a proxy to an object that is 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
webserver decides to spawn a new thread (or something else, the
underlying object is capable of dealing with other concurrency systems
than threads as well).  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 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 unittesting or something different.  You
will notice that code that 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
unittesting is by using 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 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`), but just mention some of
the most common operations.  First of all you have to import it from the
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 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 a bit user unfriendly.

For a full list of methods and attributes on that object, head over to the
:class:`~flask.request` documentation.


File Uploads
````````````

Obviously you can handle uploaded files with Flask just as easy.  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.FileStorage.save` method that allows you to store that
file on the filesystem of the server.  Here a simple example 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.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.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.  Again this 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.


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 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 he 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.

.. _sessions:

Sessions
--------

Besides the request object there is also a second object called
:class:`~flask.session` that 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 he knows 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 '''
            <form action="" method="post">
                <p><input type=text name=username>
                <p><input type=submit value=Login>
            </form>
        '''

    @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 here mentioned :func:`~flask.escape` does escaping for you if you are
not using the template engine (like in this example).

.. admonition:: How to generate good Secret Keys

   The problem with random is that it's hard to judge what random is.  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<!\xd5\xa2\xa0\x9fR"\xa1\xa8'

   Just take that thing and copy/paste it into your code and you're done.

Message Flashing
----------------

Good applications and user interfaces are all about feedback.  If the user
does not get enough feedback he will probably end up hating the
application.  Flask provides a really simple way to give feedback to a
user with the flashing system.  The flashing system basically makes it
possible to record a message at the end of a request and access it next
request and only next request.  This is usually combined with a layout
template that does this.

To flash a message use the :func:`~flask.flash` method, to get hold of the
messages you can use :func:`~flask.get_flashed_messages` which is also
available in the templates.  Check out the :ref:`message-flashing-pattern`
for a full example.

Logging
-------

.. versionadded:: 0.3

Sometimes you might be in the situation where you deal with data that
should be correct, but actually is not.  For example you have some client
side code that sends an HTTP request to the server, and it's obviously
malformed.  This might be caused by a user tempering with the data, or the
client code failed.  Most the time, it's okay to reply with ``400 Bad
Request`` in that situation, but other times it is not and the code has to
continue working.

Yet you want to log that something fishy happened.  This is where loggers
come in handy.  As of Flask 0.3 a logger is preconfigured for you to use.

Here are some example log calls::

    app.logger.debug('A value for debugging')
    app.logger.warning('A warning occurred (%d apples)', 42)
    app.logger.error('An error occurred')

The attached :attr:`~flask.Flask.logger` is a standard logging
:class:`~logging.Logger`, so head over to the official stdlib
documentation for more information.