@ -26,7 +26,7 @@ So what did that code do?
class will be our WSGI application.
2. Next we create an instance of this class. The first argument is the name of
the application's module or package. If you are using a single module (as
in this example), you should use `__name__`
in this example), you should use `` __name__ ``
started as application or imported as module the name will be different
(`` '__main__' `` versus the actual import name). This is needed so that
Flask knows where to look for templates, static files, and so on. For more
@ -42,17 +42,24 @@ your application :file:`flask.py` because this would conflict with Flask
itself.
To run the application you can either use the :command: `flask` command or
python's :option: `-m `
python's `` -m ` `
to tell your terminal the application to work with by exporting the
`FLASK_APP`
`` FLASK_APP ``
$ export FLASK_APP=hello.py
$ flask run
* Running on http://127.0.0.1:5000/
If you are on Windows you need to use `set` `export`
If you are on Windows, the environment variable syntax depends on command line
interpreter. On Command Prompt::
Alternatively you can use `python -m flask`
C:\path\to\app>set FLASK_APP=hello.py
And on PowerShell::
PS C:\path\to\app> $env:FLASK_APP = "hello.py"
Alternatively you can use :command: `python -m flask` ::
$ export FLASK_APP=hello.py
$ python -m flask run
@ -86,7 +93,7 @@ should see your hello world greeting.
What to do if the Server does not Start
---------------------------------------
In case the `` python -m flask ` `:command: `flask` does not exist,
In case the :command: `python -m flask `:command: `flask` does not exist,
there are multiple reasons this might be the case. First of all you need
to look at the error message.
@ -95,16 +102,16 @@ Old Version of Flask
Versions of Flask older than 0.11 use to have different ways to start the
application. In short, the :command: `flask` command did not exist, and
neither did `` python -m flask ` `
neither did :command: `python -m flask `
either upgrade to newer Flask versions or have a look at the :ref: `server`
docs to see the alternative method for running a server.
Invalid Import Name
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
The :option: `-a` argument to :command: `flask` is the name of the module to
import. In case that module is incorrectly named you will get an import
error upon start (or if debug is enabled when you navigate to the
The `` FLASK_APP `` environment variable is the name of the module to import at
:command: `flask run` .
import error upon start (or if debug is enabled when you navigate to the
application). It will tell you what it tried to import and why it failed.
The most common reason is a typo or because you did not actually create an
@ -123,13 +130,13 @@ 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.
To enable debug mode you can export the `FLASK_DEBUG`
To enable debug mode you can export the `` FLASK_DEBUG ``
before running the server::
$ export FLASK_DEBUG=1
$ flask run
(On Windows you need to use `set` `export`
(On Windows you need to use `` set `` instead of `` export ``
This does the following things:
@ -153,20 +160,22 @@ Screenshot of the debugger in action:
:class: screenshot
:alt: screenshot of debugger in action
More information on using the debugger can be found in the `Werkzeug
documentation`_.
.. _Werkzeug documentation: http://werkzeug.pocoo.org/docs/debug/#using-the-debugger
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.
Modern web applications use meaningful URLs to help users. Users are more
likely to like a page and come back if the page uses a meaningful URL they can
remember and use to directly visit a page.
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::
Use the :meth: `~flask.Flask.route` decorator to bind a function to a URL. ::
@app.route('/')
def index():
@ -176,16 +185,16 @@ bind a function to a URL. Here are some basic examples::
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.
You can do more! You can make 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
`` <variable_name> `` . Such a part is then passed as a keyword argument to your
function. Optionally a converter can be used by specifying a rule with
`` <converter:variable_name> `` . Here are some nice examples ::
You can add variable sections to a URL by marking sections with
`` <variable_name> `` . Your function then receives the `` <variable_name> ``
as a keyword argument. Optionally, you can use a converter to specify the type
of the argument like `` <converter:variable_name> `` . ::
@app.route('/user/<username>')
def show_user_profile(username):
@ -197,22 +206,23 @@ function. Optionally a converter can be used by specifying a rule with
# show the post with the given id, the id is an integer
return 'Post %d' % post_id
The following converters exist:
@app.route('/path/<path:subpath>')
def show_subpath(subpath):
# show the subpath after /path/
return 'Subpath %s' % subpath
=========== ===============================================
`string`
`int`
`float` `int`
`path`
`any`
`uuid`
=========== ===============================================
Converter types:
.. admonition :: Unique URLs / Redirection Behavior
========== ==========================================
`` string `` (default) accepts any text without a slash
`` int `` accepts positive integers
`` float `` accepts positive floating point values
`` path `` like `` string `` but also accepts slashes
`` uuid `` accepts UUID strings
========== ==========================================
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.
Unique URLs / Redirection Behavior
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
Take these two rules::
@ -224,84 +234,83 @@ The following converters exist:
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`
a folder on a filesystem. Accessing it without a trailing slash will caus e
Flask to redirect to the canonical URL with the trailing slash.
Though they look similar, they differ in their use of the trailing slash in
the URL. In the first case, the canonical URL for the `` projects `` endpoint
uses a trailing slash. It's similar to a folder in a file system; if you
access the URL without a trailing slash, Flask redirects you to th e
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.
In the second case, however, the URL definition lacks a trailing slash,
like the pathname of a file on UNIX-like systems. Accessing the URL with a
trailing slash produces a 404 “Not Found” error.
This behavior allows relative URLs to continue working even if the trailing
slash is omitted, consistent 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/<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 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` ).
To build a URL to a specific function, use the :func: `~flask.url_for` function.
It accepts the name of the function as its first argument and any number of
keyword arguments, each corresponding to a variable part of the URL rule.
Unknown variable parts are appended to the URL as query parameters.
Why would you want to build URLs using the URL reversing function
:func: `~flask.url_for` 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.
1. Reversing is often more descriptive than hard-coding the URLs.
2. You can change your URLs in one go instead of needing to remember to
manually change hard-coded URLs.
3. URL building handles escaping of special characters and Unicode data
transparently.
4. If your application is placed outside the URL root, for example, in
`` /myapplication `` instead of `` / `` , :func: `~flask.url_for` properly
handles that for you.
For example, here we use the :meth: `~flask.Flask.test_request_context` method
to try out :func: `~flask.url_for` . :meth: `~flask.Flask.test_request_context`
tells Flask to behave as though it's handling a request even while we use a
Python shell. See :ref: `context-locals` . ::
from flask import Flask, url_for
app = Flask(__name__)
@app.route('/')
def index():
return 'index'
@app.route('/login')
def login():
return 'login'
@app.route('/user/<username>')
def profile(username):
return '{}'s profile'.format(username)
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
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`
:meth: `~flask.Flask.route` decorator. Here are some examples::
from flask import request
Web applications use different HTTP methods when accessing URLs. You should
familiarize yourself with the HTTP methods as you work with Flask. By default,
a route only answers to `` GET `` requests. You can use the `` methods `` argument
of the :meth: `~flask.Flask.route` decorator to handle different HTTP methods.
::
@app.route('/login', methods=['GET', 'POST'])
def login():
@ -310,64 +319,11 @@ can be changed by providing the `methods` argument to the
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
client 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 systems
use it.
.. _HTTP RFC: http://www.ietf.org/rfc/rfc2068.txt
If `` GET `` is present, Flask automatically adds support for the `` HEAD `` method
and handles `` HEAD `` requests according to the the `HTTP RFC`_ . Likewise,
`` OPTIONS `` is automatically implemented for you.
.. _HTTP RFC: https://www.ietf.org/rfc/rfc2068.txt
Static Files
------------
@ -446,14 +402,14 @@ 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`
Automatic escaping is enabled, so if `` name ``
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:
Here is a basic introduction to how the :class: `~jinja2.Markup` class works::
>>> from flask import Markup
>>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>'
@ -538,16 +494,16 @@ The Request Object
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
The request object is documented in the API section and we will not cover
it here in detail (see :class: `~flask.r equest` ). Here is a broad overview of
it here in detail (see :class: `~flask.R equest` ). Here is a broad overview of
some of the most common operations. First of all you have to import it from
the `flask`
the `` flask ``
from flask import request
The current request method is available by using the
:attr: `~flask.r equest.method` attribute. To access form data (data
:attr: `~flask.R equest.method` attribute. To access form data (data
transmitted in a `` POST `` or `` PUT `` request) you can use the
:attr: `~flask.r equest.form` attribute. Here is a full example of the two
:attr: `~flask.R equest.form` attribute. Here is a full example of the two
attributes mentioned above::
@app.route('/login', methods=['POST', 'GET'])
@ -563,14 +519,14 @@ attributes mentioned above::
# was GET or the credentials were invalid
return render_template('login.html', error=error)
What happens if the key does not exist in the `form`
What happens if the key does not exist in the `` form ``
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.r equest.args` attribute::
:attr: `~flask.R equest.args` attribute::
searchword = request.args.get('key', '')
@ -579,7 +535,7 @@ We recommend accessing URL parameters with `get` or by catching the
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.r equest` documentation.
to the :class: `~flask.R equest` documentation.
File Uploads
@ -725,17 +681,15 @@ converting return values into response objects is as follows:
3. If a tuple is returned the items in the tuple can provide extra
information. Such tuples have to be in the form `` (response, status,
headers)`` or ` ` (response, headers) `` where at least one item has
to be in the tuple. The `status`
and `headers`
to be in the tuple. The `` status ``
and `` headers ``
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
Imagine you have a view like this::
@app.errorhandler(404)
def not_found(error):
@ -743,9 +697,7 @@ Imagine you have a view like this:
You just need to wrap the return expression with
:func: `~flask.make_response` and get the response object to modify it, then
return it:
.. sourcecode :: python
return it::
@app.errorhandler(404)
def not_found(error):
@ -784,7 +736,7 @@ sessions work::
session['username'] = request.form['username']
return redirect(url_for('index'))
return '''
<form action="" method="post">
<form method="post">
<p><input type=text name=username>
<p><input type=submit value=Login>
</form>
@ -807,7 +759,7 @@ not using the template engine (as in this example).
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:
random generator which can be used to get such a key::
>>> import os
>>> os.urandom(24)
@ -821,6 +773,9 @@ values do not persist across requests, cookies are indeed enabled, and you are
not getting a clear error message, check the size of the cookie in your page
responses compared to the size supported by web browsers.
Besides the default client-side based sessions, if you want to handle
sessions on the server-side instead, there are several
Flask extensions that support this.
Message Flashing
----------------
David Lord
8 years ago
