api.rst

API

flask

This part of the documentation covers all the interfaces of Flask. For parts where Flask depends on external libraries, we document the most important right here and provide links to the canonical documentation.

Application Object

Flask

Blueprint Objects

Blueprint

Incoming Request Data

Request

form

A ~werkzeug.datastructures.MultiDict with the parsed form data from POST or PUT requests. Please keep in mind that file uploads will not end up here, but instead in the files attribute.

args

A ~werkzeug.datastructures.MultiDict with the parsed contents of the query string. (The part in the URL after the question mark).

values

A ~werkzeug.datastructures.CombinedMultiDict with the contents of both form and args.

cookies

A dict with the contents of all cookies transmitted with the request.

stream

If the incoming form data was not encoded with a known mimetype the data is stored unmodified in this stream for consumption. Most of the time it is a better idea to use data which will give you that data as a string. The stream only returns the data once.

headers

The incoming request headers as a dictionary like object.

data

Contains the incoming request data as string in case it came with a mimetype Flask does not handle.

files

A ~werkzeug.datastructures.MultiDict with files uploaded as part of a POST or PUT request. Each file is stored as ~werkzeug.datastructures.FileStorage object. It basically behaves like a standard file object you know from Python, with the difference that it also has a ~werkzeug.datastructures.FileStorage.save function that can store the file on the filesystem.

environ

The underlying WSGI environment.

method

The current request method (POST, GET etc.)

path

script_root

url

base_url

url_root

Provides different ways to look at the current URL. Imagine your application is listening on the following URL:

http://www.example.com/myapplication

And a user requests the following URL:

http://www.example.com/myapplication/page.html?x=y

In this case the values of the above mentioned attributes would be the following:

path	/page.html
script_root	/myapplication
base_url	http://www.example.com/myapplication/page.html
url	http://www.example.com/myapplication/page.html?x=y
url_root	http://www.example.com/myapplication/

is_xhr

True if the request was triggered via a JavaScript XMLHttpRequest. This only works with libraries that support the X-Requested-With header and set it to XMLHttpRequest. Libraries that do that are prototype, jQuery and Mochikit and probably some more.

To access incoming request data, you can use the global request object. Flask parses incoming request data for you and gives you access to it through that global object. Internally Flask makes sure that you always get the correct data for the active thread if you are in a multithreaded environment.

This is a proxy. See notes-on-proxies for more information.

The request object is an instance of a ~werkzeug.wrappers.Request subclass and provides all of the attributes Werkzeug defines. This just shows a quick overview of the most important ones.

Response Objects

flask.Response

headers

A Headers object representing the response headers.

status

A string with a response status.

status_code

The response status as integer.

Sessions

If you have the Flask.secret_key set you can use sessions in Flask applications. A session basically makes it possible to remember information from one request to another. The way Flask does this is by using a signed cookie. So the user can look at the session contents, but not modify it unless they know the secret key, so make sure to set that to something complex and unguessable.

To access the current session you can use the session object:

The session object works pretty much like an ordinary dict, with the difference that it keeps track on modifications.

This is a proxy. See notes-on-proxies for more information.

The following attributes are interesting:

new

True if the session is new, False otherwise.

modified

True if the session object detected a modification. Be advised that modifications on mutable structures are not picked up automatically, in that situation you have to explicitly set the attribute to True yourself. Here an example:

# this change is not picked up because a mutable object (here
# a list) is changed.
session['objects'].append(42)
# so mark it as modified yourself
session.modified = True

permanent

If set to True the session lives for ~flask.Flask.permanent_session_lifetime seconds. The default is 31 days. If set to False (which is the default) the session will be deleted when the user closes the browser.

Session Interface

0.8

The session interface provides a simple way to replace the session implementation that Flask is using.

flask.sessions

SessionInterface

SecureCookieSessionInterface

NullSession

SessionMixin

Notice

The PERMANENT_SESSION_LIFETIME config key can also be an integer starting with Flask 0.8. Either catch this down yourself or use the ~flask.Flask.permanent_session_lifetime attribute on the app which converts the result to an integer automatically.

Test Client

flask.testing

FlaskClient

Application Globals

flask

To share data that is valid for one request only from one function to another, a global variable is not good enough because it would break in threaded environments. Flask provides you with a special object that ensures it is only valid for the active request and that will return different values for each request. In a nutshell: it does the right thing, like it does for request and session.

g

Just store on this whatever you want. For example a database connection or the user that is currently logged in.

This is a proxy. See notes-on-proxies for more information.

Useful Functions and Classes

current_app

Points to the application handling the request. This is useful for extensions that want to support multiple applications running side by side.

This is a proxy. See notes-on-proxies for more information.

has_request_context

url_for

abort(code)

Raises an ~werkzeug.exceptions.HTTPException for the given status code. For example to abort request handling with a page not found exception, you would call abort(404).

param code

the HTTP error code.

redirect

make_response

send_file

send_from_directory

safe_join

escape

Markup

Message Flashing

flash

get_flashed_messages

Returning JSON

jsonify

json

If JSON support is picked up, this will be the module that Flask is using to parse and serialize JSON. So instead of doing this yourself:

try:
    import simplejson as json
except ImportError:
    import json

You can instead just do this:

from flask import json

For usage examples, read the json documentation.

The ~json.dumps function of this json module is also available as filter called |tojson in Jinja2. Note that inside script tags no escaping must take place, so make sure to disable escaping with |safe if you intend to use it inside script tags:

html+jinja

<script type=text/javascript>

doSomethingWith({{ user.usernamesafe }});

</script>

Note that the |tojson filter escapes forward slashes properly.

Template Rendering

render_template

render_template_string

get_template_attribute

Configuration

Config

Extensions

flask.ext

This module acts as redirect import module to Flask extensions. It was added in 0.8 as the canonical way to import Flask extensions and makes it possible for us to have more flexibility in how we distribute extensions.

If you want to use an extension named “Flask-Foo” you would import it from ~flask.ext as follows:

from flask.ext import foo

0.8

Useful Internals

flask.ctx.RequestContext

_request_ctx_stack

The internal ~werkzeug.local.LocalStack that is used to implement all the context local objects used in Flask. This is a documented instance and can be used by extensions and application code but the use is discouraged in general.

The following attributes are always present on each layer of the stack:

app

the active Flask application.

url_adapter

the URL adapter that was used to match the request.

request

the current request object.

session

the active session object.

g

an object with all the attributes of the flask.g object.

flashes

an internal cache for the flashed messages.

Example usage:

from flask import _request_ctx_stack

def get_session():
    ctx = _request_ctx_stack.top
    if ctx is not None:
        return ctx.session

flask.blueprints.BlueprintSetupState

Signals

0.6

signals_available

True if the signalling system is available. This is the case when blinker is installed.

template_rendered

This signal is sent when a template was successfully rendered. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

request_started

This signal is sent before any request processing started but when the request context was set up. Because the request context is already bound, the subscriber can access the request with the standard global proxies such as ~flask.request.

request_finished

This signal is sent right before the response is sent to the client. It is passed the response to be sent named response.

got_request_exception

This signal is sent when an exception happens during request processing. It is sent before the standard exception handling kicks in and even in debug mode, where no exception handling happens. The exception itself is passed to the subscriber as exception.

request_tearing_down

This signal is sent when the application is tearing down the request. This is always called, even if an error happened. No arguments are provided.

None

An alias for blinker.base.Namespace if blinker is available, otherwise a dummy class that creates fake signals. This class is available for Flask extensions that want to provide the same fallback system as Flask itself.

signal(name, doc=None)

Creates a new signal for this namespace if blinker is available, otherwise returns a fake signal that has a send method that will do nothing but will fail with a RuntimeError for all other operations, including connecting.

Class-Based Views

0.7

None

flask.views.View

flask.views.MethodView

URL Route Registrations

Generally there are three ways to define rules for the routing system:

1. You can use the flask.Flask.route decorator.
2. You can use the flask.Flask.add_url_rule function.
3. You can directly access the underlying Werkzeug routing system which is exposed as flask.Flask.url_map.

Variable parts in the route can be specified with angular brackets (/user/<username>). By default a variable part in the URL accepts any string without a slash however a different converter can be specified as well by using <converter:name>.

Variable parts are passed to the view function as keyword arguments.

The following converters are available:

string	accepts any text without a slash (the default)
int	accepts integers
float	like int but for floating point values
path	like the default but also accepts slashes

Here are some examples:

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

@app.route('/<username>')
def show_user(username):
    pass

@app.route('/post/<int:post_id>')
def show_post(post_id):
    pass

An important detail to keep in mind is how Flask deals with trailing slashes. The idea is to keep each URL unique so the following rules apply:

1. If a rule ends with a slash and is requested without a slash by the user, the user is automatically redirected to the same page with a trailing slash attached.
2. If a rule does not end with a trailing slash and the user requests the page with a trailing slash, a 404 not found is raised.

This is consistent with how web servers deal with static files. This also makes it possible to use relative link targets safely.

You can also define multiple rules for the same function. They have to be unique however. Defaults can also be specified. Here for example is a definition for a URL that accepts an optional page:

@app.route('/users/', defaults={'page': 1})
@app.route('/users/page/<int:page>')
def show_users(page):
    pass

This specifies that /users/ will be the URL for page one and /users/page/N will be the URL for page N.

Here are the parameters that ~flask.Flask.route and ~flask.Flask.add_url_rule accept. The only difference is that with the route parameter the view function is defined with the decorator instead of the view_func parameter.

rule	the URL roule as string
endpoint	the endpoint for the registered URL rule. Flask itself assumes that the name of the view function is the name of the endpoint if not explicitly stated.
view_func	the function to call when serving a request to the provided endpoint. If this is not provided one can specify the function later by storing it in the ~flask.Flask.view_functions dictionary with the endpoint as key.
defaults	A dictionary with defaults for this rule. See the example above for how defaults work.
subdomain	specifies the rule for the subdomain in case subdomain matching is in use. If not specified the default subdomain is assumed.
**options	the options to be forwarded to the underlying ~werkzeug.routing.Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling. They have to be specified as keyword arguments.

View Function Options

For internal usage the view functions can have some attributes attached to customize behavior the view function would normally not have control over. The following attributes can be provided optionally to either override some defaults to ~flask.Flask.add_url_rule or general behavior:

• `__name__`: The name of a function is by default used as endpoint. If endpoint is provided explicitly this value is used. Additionally this will be prefixed with the name of the blueprint by default which cannot be customized from the function itself.
• `methods`: If methods are not provided when the URL rule is added, Flask will look on the view function object itself is an methods attribute exists. If it does, it will pull the information for the methods from there.
• `provide_automatic_options`: if this attribute is set Flask will either force enable or disable the automatic implementation of the HTTP OPTIONS response. This can be useful when working with decorators that want to customize the OPTIONS response on a per-view basis.

Full example:

def index():
    if request.method == 'OPTIONS':
        # custom options handling here
        ...
    return 'Hello World!'
index.provide_automatic_options = False
index.methods = ['GET', 'OPTIONS']

app.add_url_rule('/', index)

0.8 The provide_automatic_options functionality was added.