@ -26,7 +26,7 @@ So what did that code do?
class will be our WSGI application. class will be our WSGI application.
2. Next we create an instance of this class. The first argument is the name of 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 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 started as application or imported as module the name will be different
(`` '__main__' `` versus the actual import name). This is needed so that (`` '__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 Flask knows where to look for templates, static files, and so on. For more
@ -37,19 +37,32 @@ So what did that code do?
particular function, and returns the message we want to display in the particular function, and returns the message we want to display in the
user's browser. user's browser.
Just save it as :file: `hello.py` (or something similar) and run it with your Python Just save it as :file: `hello.py` or something similar. Make sure to not call
interpreter. Make sure to not call your application :file: `flask.py` because thisyour application :file: `flask.py` because this would conflict with Flask
would conflict with Flask itself.itself.
To run the application you can either use the :command: `flask` command or To run the application you can either use the :command: `flask` command or
python's :option: `-m` switch with Flask:: python's `` -m `` switch with Flask. Before you can do that you need
to tell your terminal the application to work with by exporting the
`` FLASK_APP `` environment variable::
$ flask -a hello run $ export FLASK_APP=hello.py
$ flask run
* Running on http://127.0.0.1:5000/ * Running on http://127.0.0.1:5000/
or alternatively:: If you are on Windows, the environment variable syntax depends on command line
interpreter. On Command Prompt::
$ python -m flask -a hello run 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
* Running on http://127.0.0.1:5000/ * Running on http://127.0.0.1:5000/
This launches a very simple builtin server, which is good enough for testing This launches a very simple builtin server, which is good enough for testing
@ -72,7 +85,7 @@ should see your hello world greeting.
you can make the server publicly available simply by adding you can make the server publicly available simply by adding
`` --host=0.0.0.0 `` to the command line:: `` --host=0.0.0.0 `` to the command line::
flask -a hello run --host=0.0.0.0 flask run --host=0.0.0.0
This tells your operating system to listen on all public IPs. This tells your operating system to listen on all public IPs.
@ -80,35 +93,26 @@ should see your hello world greeting.
What to do if the Server does not Start 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 there are multiple reasons this might be the case. First of all you need
to look at the error message. to look at the error message.
Old Version of Flask Old Version of Flask
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
Versions of Flask older than 1.0 use to have different ways to start the Versions of Flask older than 0.1 1 use to have different ways to start the
application. In short, the :command: `flask` command did not exist, and 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` either upgrade to newer Flask versions or have a look at the :ref: `server`
docs to see the alternative method for running a server. docs to see the alternative method for running a server.
Python older 2.7
`` ` ` ` ` ` ` ` ` ` ` ` ` ``
In case you have a version of Python older than 2.7 `` python -m flask ``
does not work. You can either use :command: `flask` or `` python -m
flask.cli`` as an alternative. This is because Python before 2.7 does no
permit packages to act as executable modules. For more information see
:ref: `cli` .
Invalid Import Name Invalid Import Name
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
The :option: `-a` argument to :command: `flask` is the name of the module to import. In The `` FLASK_APP `` environment variable is the name of the module to import at
case that module is incorrectly named you will get an import error upo n :command: `flask run` . In case that module is incorrectly named you will get an
start (or if debug is enabled when you navigate to the application). It import error upon start (or if debug is enabled when you navigate to the
will tell you what it tried to import and why it failed. 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 The most common reason is a typo or because you did not actually create an
`` app `` object.`` app `` object.
@ -118,16 +122,21 @@ The most common reason is a typo or because you did not actually create an
Debug Mode Debug Mode
---------- ----------
(Want to just log errors and stack traces? See :ref: `application-errors` )
The :command: `flask` script is nice to start a local development server, but The :command: `flask` script is nice to start a local development server, but
you would have to restart it manually after each change to your code. 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 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 support the server will reload itself on code changes, and it will also
provide you with a helpful debugger if things go wrong. provide you with a helpful debugger if things go wrong.
There are different ways to enable the debug mode. The most obvious one To enable debug mode you can export the `` FLASK_DEBUG `` environment variable
is the :option: `--debug` parameter to the :command: `flask` command:: before running the server::
$ export FLASK_DEBUG=1
$ flask run
flask --debug -a hello run (On Windows you need to use `` set `` instead of `` export `` ).
This does the following things: This does the following things:
@ -151,20 +160,22 @@ Screenshot of the debugger in action:
:class: screenshot :class: screenshot
:alt: screenshot of debugger in action :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` . Have another debugger in mind? See :ref: `working-with-debuggers` .
Routing Routing
------- -------
Modern web applications have beautiful URLs. This helps people remember Modern web applications use meaningful URLs to help users. Users are more
the URLs, which is especially handy for applications that are used from likely to like a page and come back if the page uses a meaningful URL they can
mobile devices with slower network connections. If the user can directly remember and use to directly visit a page.
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 Use the :meth: `~flask.Flask.route` decorator to bind a function to a URL. ::
bind a function to a URL. Here are some basic examples::
@app.route('/') @app.route('/')
def index(): def index():
@ -174,16 +185,16 @@ bind a function to a URL. Here are some basic examples::
def hello(): def hello():
return 'Hello, World' return 'Hello, World'
But there is more to it! You can make certain parts of the URL dynamic and You can do more! You can make parts of the URL dynamic and attach multiple
attach multiple rules to a function.rules to a function.
Variable Rules Variable Rules
`` ` ` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ` ` ``
To add variable parts to a URL you can mark these special sections as You can add variable sections to a URL by marking sections with
`` <variable_name> `` . Such a part is then passed as a keyword argument to your `` <variable_name> `` . Your function then receives the `` <variable_name> ``
function. Optionally a converter can be used by specifying a rule with as a keyword argument. Optionally, you can use a converter to specify the type
`` <converter:variable_name> `` . Here are some nice examples ::of the argument like `` <converter:variable_name> `` . ::
@app.route('/user/<username>') @app.route('/user/<username>')
def show_user_profile(username): def show_user_profile(username):
@ -195,20 +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 # show the post with the given id, the id is an integer
return 'Post %d' % post_id 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
=========== =============================================== Converter types:
`string`
`int`
`float` `int`
`path`
=========== ===============================================
.. 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 Unique URLs / Redirection Behavior
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:: Take these two rules::
@ -220,84 +234,83 @@ The following converters exist:
def about(): def about():
return 'The about page' return 'The about page'
Though they look rather similar, they differ in their use of the trailingThough they look similar, they differ in their use of the trailing slash in
slash in the URL *definition* . In the first case, the canonical URL for thethe URL. In the first case, the canonical URL for the `` projects `` endpoint
`projects` uses a trailing slash. It's similar to a folder in a file system; if you
a folder on a filesystem. Accessing it without a trailing slash will caus eaccess the URL without a trailing slash, Flask redirects you to th e
Flask to redirect to the canonical URL with the trailing slash.canonical URL with the trailing slash.
In the second case, however, the URL is defined without a trailing slash,In the second case, however, the URL definition lacks a trailing slash,
rather like the pathname of a file on UNIX-like systems. Accessing the URLlike the pathname of a file on UNIX-like systems. Accessing the URL with a
with a trailing slash will produce a 404 "Not Found" error.trailing slash produces a 404 “Not Found” error.
This behavior allows relative URLs to continue working even if the trailing This behavior allows relative URLs to continue working even if the trailing
slash is omitted, consistent with how Apache and other servers work. Also, slash is omitted, consistent with how Apache and other servers work. Also,
the URLs will stay unique, which helps search engines avoid indexing the the URLs will stay unique, which helps search engines avoid indexing the
same page twice. same page twice.
.. _url-building: .. _url-building:
URL Building URL Building
`` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ``
If it can match URLs, can Flask also generate them? Of course it can. To To build a URL to a specific function, use the :func: `~flask.url_for` function.
build a URL to a specific function you can use the :func: `~flask.url_for` It accepts the name of the function as its first argument and any number of
function. It accepts the name of the function as first argument and a number keyword arguments, each corresponding to a variable part of the URL rule.
of keyword arguments, each corresponding to the variable part of the URL rule. Unknown variable parts are appended to the URL as query parameters.
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` ).
Why would you want to build URLs using the URL reversing function Why would you want to build URLs using the URL reversing function
:func: `~flask.url_for` instead of hard-coding them into your templates?: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 1. Reversing is often more descriptive than hard-coding the URLs.
importantly, it allows you to change URLs in one go, without having to 2. You can change your URLs in one go instead of needing to remember to
remember to change URLs all over the place. manually change hard-coded URLs.
2. URL building will handle escaping of special characters and Unicode 3. URL building handles escaping of special characters and Unicode data
data transparently for you, so you don't have to deal with them. transparently.
3. If your application is placed outside the URL root (say, in 4. If your application is placed outside the URL root, for example, in
`` /myapplication `` instead of `` / `` ), :func: `~flask.url_for` will handle `` /myapplication `` instead of `` / `` , :func: `~flask.url_for` properly
that properly for you. 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 Methods
`` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ``
HTTP (the protocol web applications are speaking) knows different methods for Web applications use different HTTP methods when accessing URLs. You should
accessing URLs. By default, a route only answers to `` GET `` requests, but that familiarize yourself with the HTTP methods as you work with Flask. By default,
can be changed by providing the `methods` a route only answers to `` GET `` requests. You can use the `` methods `` argument
:meth: `~flask.Flask.route` decorator. Here are some examples::of the :meth: `~flask.Flask.route` decorator to handle different HTTP methods.
::
from flask import request
@app.route('/login', methods=['GET', 'POST']) @app.route('/login', methods=['GET', 'POST'])
def login(): def login():
@ -306,64 +319,11 @@ can be changed by providing the `methods` argument to the
else: else:
show_the_login_form() show_the_login_form()
If `` GET `` is present, `` HEAD `` will be added automatically for you. You If `` GET `` is present, Flask automatically adds support for the `` HEAD `` method
don't have to deal with that. It will also make sure that `` HEAD `` requests and handles `` HEAD `` requests according to the the `HTTP RFC`_ . Likewise,
are handled as the `HTTP RFC`_ (the document describing the HTTP `` OPTIONS `` is automatically implemented for you.
protocol) demands, so you can completely ignore that part of the HTTP
specification. Likewise, as of Flask 0.6, `` OPTIONS `` is implemented for you .. _HTTP RFC: https://www.ietf.org/rfc/rfc2068.txt
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 systems
use it.
.. _HTTP RFC: http://www.ietf.org/rfc/rfc2068.txt
Static Files Static Files
------------ ------------
@ -442,14 +402,14 @@ know how that works, head over to the :ref:`template-inheritance` pattern
documentation. Basically template inheritance makes it possible to keep documentation. Basically template inheritance makes it possible to keep
certain elements on each page (like header, navigation and footer). 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 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 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 markup to HTML) you can mark it as safe by using the
:class: `~jinja2.Markup` class or by using the `` |safe `` filter in the:class: `~jinja2.Markup` class or by using the `` |safe `` filter in the
template. Head over to the Jinja 2 documentation for more examples. 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 >>> from flask import Markup
>>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>' >>> Markup('<strong>Hello %s!</strong>') % '<blink>hacker</blink>'
@ -534,16 +494,16 @@ The Request Object
`` ` ` ` ` ` ` ` ` ` ` ` ` ` ` `` `` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ``
The request object is documented in the API section and we will not cover 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 some of the most common operations. First of all you have to import it from
the `flask` the `` flask ``
from flask import request from flask import request
The current request method is available by using the 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 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:: attributes mentioned above::
@app.route('/login', methods=['POST', 'GET']) @app.route('/login', methods=['POST', 'GET'])
@ -559,14 +519,14 @@ attributes mentioned above::
# was GET or the credentials were invalid # was GET or the credentials were invalid
return render_template('login.html', error=error) 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 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 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 error page is shown instead. So for many situations you don't have to
deal with that problem. deal with that problem.
To access parameters submitted in the URL (`` ?key=value `` ) you can use the 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', '') searchword = request.args.get('key', '')
@ -575,7 +535,7 @@ We recommend accessing URL parameters with `get` or by catching the
bad request page in that case is not user friendly. bad request page in that case is not user friendly.
For a full list of methods and attributes of the request object, head over 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 File Uploads
@ -721,17 +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 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, information. Such tuples have to be in the form `` (response, status,
headers)`` or ` ` (response, headers) `` where at least one item has headers)`` or ` ` (response, headers) `` where at least one item has
to be in the tuple. The `status` to be in the tuple. The `` status ``
and `headers` and `` headers ``
4. If none of that works, Flask will assume the return value is a 4. If none of that works, Flask will assume the return value is a
valid WSGI application and convert that into a response object. valid WSGI application and convert that into a response object.
If you want to get hold of the resulting response object inside the view If you want to get hold of the resulting response object inside the view
you can use the :func: `~flask.make_response` function. you can use the :func: `~flask.make_response` function.
Imagine you have a view like this: Imagine you have a view like this::
.. sourcecode :: python
@app.errorhandler(404) @app.errorhandler(404)
def not_found(error): def not_found(error):
@ -739,9 +697,7 @@ Imagine you have a view like this:
You just need to wrap the return expression with You just need to wrap the return expression with
:func: `~flask.make_response` and get the response object to modify it, then:func: `~flask.make_response` and get the response object to modify it, then
return it: return it::
.. sourcecode :: python
@app.errorhandler(404) @app.errorhandler(404)
def not_found(error): def not_found(error):
@ -780,7 +736,7 @@ sessions work::
session['username'] = request.form['username'] session['username'] = request.form['username']
return redirect(url_for('index')) return redirect(url_for('index'))
return ''' return '''
<form action="" method="post"> <form method="post">
<p><input type=text name=username> <p><input type=text name=username>
<p><input type=submit value=Login> <p><input type=submit value=Login>
</form> </form>
@ -803,7 +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 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 a secret key should be as random as possible. Your operating system
has ways to generate pretty random stuff based on a cryptographic 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 >>> import os
>>> os.urandom(24) >>> os.urandom(24)
@ -817,6 +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 not getting a clear error message, check the size of the cookie in your page
responses compared to the size supported by web browsers. 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 Message Flashing
---------------- ----------------
Kenneth Reitz
7 years ago
