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API
===
.. module:: 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
------------------
.. autoclass:: Flask
:members:
:inherited-members:
Blueprint Objects
-----------------
.. autoclass:: Blueprint
:members:
:inherited-members:
Incoming Request Data
---------------------
.. autoclass:: Request
:members:
:inherited-members:
:exclude-members: json_module
.. attribute:: request
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 :ref:`notes-on-proxies` for more information.
The request object is an instance of a :class:`~flask.Request`.
Response Objects
----------------
.. autoclass:: flask.Response
:members:
:inherited-members:
:exclude-members: json_module
Sessions
--------
If you have set :attr:`Flask.secret_key` (or configured it from
:data:`SECRET_KEY`) you can use sessions in Flask applications. A session makes
it possible to remember information from one request to another. The way Flask
does this is by using a signed cookie. The user can look at the session
contents, but can't 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 :class:`session` object:
.. class:: session
The session object works pretty much like an ordinary dict, with the
difference that it keeps track of modifications.
This is a proxy. See :ref:`notes-on-proxies` for more information.
The following attributes are interesting:
.. attribute:: new
``True`` if the session is new, ``False`` otherwise.
.. attribute:: 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
.. attribute:: permanent
If set to ``True`` the session lives for
:attr:`~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
-----------------
.. versionadded:: 0.8
The session interface provides a simple way to replace the session
implementation that Flask is using.
.. currentmodule:: flask.sessions
.. autoclass:: SessionInterface
:members:
.. autoclass:: SecureCookieSessionInterface
:members:
.. autoclass:: SecureCookieSession
:members:
.. autoclass:: NullSession
:members:
.. autoclass:: SessionMixin
:members:
.. admonition:: Notice
The :data:`PERMANENT_SESSION_LIFETIME` config can be an integer or ``timedelta``.
The :attr:`~flask.Flask.permanent_session_lifetime` attribute is always a
``timedelta``.
Test Client
-----------
.. currentmodule:: flask.testing
.. autoclass:: FlaskClient
:members:
Test CLI Runner
---------------
.. currentmodule:: flask.testing
.. autoclass:: FlaskCliRunner
:members:
Application Globals
-------------------
.. currentmodule:: 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 :class:`request` and :class:`session`.
.. data:: g
A namespace object that can store data during an
:doc:`application context </appcontext>`. This is an instance of
:attr:`Flask.app_ctx_globals_class`, which defaults to
:class:`ctx._AppCtxGlobals`.
This is a good place to store resources during a request. For
example, a ``before_request`` function could load a user object from
a session id, then set ``g.user`` to be used in the view function.
This is a proxy. See :ref:`notes-on-proxies` for more information.
.. versionchanged:: 0.10
Bound to the application context instead of the request context.
.. autoclass:: flask.ctx._AppCtxGlobals
:members:
Useful Functions and Classes
----------------------------
.. data:: current_app
A proxy to the application handling the current request. This is
useful to access the application without needing to import it, or if
it can't be imported, such as when using the application factory
pattern or in blueprints and extensions.
This is only available when an
:doc:`application context </appcontext>` is pushed. This happens
automatically during requests and CLI commands. It can be controlled
manually with :meth:`~flask.Flask.app_context`.
This is a proxy. See :ref:`notes-on-proxies` for more information.
.. autofunction:: has_request_context
.. autofunction:: copy_current_request_context
.. autofunction:: has_app_context
.. autofunction:: url_for
.. autofunction:: abort
.. autofunction:: redirect
.. autofunction:: make_response
.. autofunction:: after_this_request
.. autofunction:: send_file
.. autofunction:: send_from_directory
.. autofunction:: escape
.. autoclass:: Markup
:members: escape, unescape, striptags
Message Flashing
----------------
.. autofunction:: flash
.. autofunction:: get_flashed_messages
JSON Support
------------
.. module:: flask.json
Flask uses Python's built-in :mod:`json` module for handling JSON by
default. The JSON implementation can be changed by assigning a different
provider to :attr:`flask.Flask.json_provider_class` or
:attr:`flask.Flask.json`. The functions provided by ``flask.json`` will
use methods on ``app.json`` if an app context is active.
Jinja's ``|tojson`` filter is configured to use the app's JSON provider.
The filter marks the output with ``|safe``. Use it to render data inside
HTML ``<script>`` tags.
.. sourcecode:: html+jinja
<script>
const names = {{ names|tosjon }};
renderChart(names, {{ axis_data|tojson }});
</script>
.. autofunction:: jsonify
.. autofunction:: dumps
.. autofunction:: dump
.. autofunction:: loads
.. autofunction:: load
.. autoclass:: flask.json.provider.JSONProvider
:members:
:member-order: bysource
.. autoclass:: flask.json.provider.DefaultJSONProvider
:members:
:member-order: bysource
.. autoclass:: JSONEncoder
:members:
.. autoclass:: JSONDecoder
:members:
.. automodule:: flask.json.tag
Template Rendering
------------------
.. currentmodule:: flask
.. autofunction:: render_template
.. autofunction:: render_template_string
.. autofunction:: stream_template
.. autofunction:: stream_template_string
.. autofunction:: get_template_attribute
Configuration
-------------
.. autoclass:: Config
:members:
Stream Helpers
--------------
.. autofunction:: stream_with_context
Useful Internals
----------------
.. autoclass:: flask.ctx.RequestContext
:members:
.. data:: flask.globals.request_ctx
The current :class:`~flask.ctx.RequestContext`. If a request context
is not active, accessing attributes on this proxy will raise a
``RuntimeError``.
This is an internal object that is essential to how Flask handles
requests. Accessing this should not be needed in most cases. Most
likely you want :data:`request` and :data:`session` instead.
.. autoclass:: flask.ctx.AppContext
:members:
.. data:: flask.globals.app_ctx
The current :class:`~flask.ctx.AppContext`. If an app context is not
active, accessing attributes on this proxy will raise a
``RuntimeError``.
This is an internal object that is essential to how Flask handles
requests. Accessing this should not be needed in most cases. Most
likely you want :data:`current_app` and :data:`g` instead.
.. autoclass:: flask.blueprints.BlueprintSetupState
:members:
.. _core-signals-list:
Signals
-------
.. versionadded:: 0.6
.. data:: signals.signals_available
``True`` if the signaling system is available. This is the case
when `blinker`_ is installed.
The following signals exist in Flask:
.. data:: 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`).
Example subscriber::
def log_template_renders(sender, template, context, **extra):
sender.logger.debug('Rendering template "%s" with context %s',
template.name or 'string template',
context)
from flask import template_rendered
template_rendered.connect(log_template_renders, app)
.. data:: flask.before_render_template
:noindex:
This signal is sent before template rendering process. The
signal is invoked with the instance of the template as `template`
and the context as dictionary (named `context`).
Example subscriber::
def log_template_renders(sender, template, context, **extra):
sender.logger.debug('Rendering template "%s" with context %s',
template.name or 'string template',
context)
from flask import before_render_template
before_render_template.connect(log_template_renders, app)
.. data:: request_started
This signal is sent when the request context is set up, before
any request processing happens. Because the request context is already
bound, the subscriber can access the request with the standard global
proxies such as :class:`~flask.request`.
Example subscriber::
def log_request(sender, **extra):
sender.logger.debug('Request context is set up')
from flask import request_started
request_started.connect(log_request, app)
.. data:: 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`.
Example subscriber::
def log_response(sender, response, **extra):
sender.logger.debug('Request context is about to close down. '
'Response: %s', response)
from flask import request_finished
request_finished.connect(log_response, app)
.. data:: got_request_exception
This signal is sent when an unhandled exception happens during
request processing, including when debugging. The exception is
passed to the subscriber as ``exception``.
This signal is not sent for
:exc:`~werkzeug.exceptions.HTTPException`, or other exceptions that
have error handlers registered, unless the exception was raised from
an error handler.
This example shows how to do some extra logging if a theoretical
``SecurityException`` was raised:
.. code-block:: python
from flask import got_request_exception
def log_security_exception(sender, exception, **extra):
if not isinstance(exception, SecurityException):
return
security_logger.exception(
f"SecurityException at {request.url!r}",
exc_info=exception,
)
got_request_exception.connect(log_security_exception, app)
.. data:: request_tearing_down
This signal is sent when the request is tearing down. This is always
called, even if an exception is caused. Currently functions listening
to this signal are called after the regular teardown handlers, but this
is not something you can rely on.
Example subscriber::
def close_db_connection(sender, **extra):
session.close()
from flask import request_tearing_down
request_tearing_down.connect(close_db_connection, app)
As of Flask 0.9, this will also be passed an `exc` keyword argument
that has a reference to the exception that caused the teardown if
there was one.
.. data:: appcontext_tearing_down
This signal is sent when the app context is tearing down. This is always
called, even if an exception is caused. Currently functions listening
to this signal are called after the regular teardown handlers, but this
is not something you can rely on.
Example subscriber::
def close_db_connection(sender, **extra):
session.close()
from flask import appcontext_tearing_down
appcontext_tearing_down.connect(close_db_connection, app)
This will also be passed an `exc` keyword argument that has a reference
to the exception that caused the teardown if there was one.
.. data:: appcontext_pushed
This signal is sent when an application context is pushed. The sender
is the application. This is usually useful for unittests in order to
temporarily hook in information. For instance it can be used to
set a resource early onto the `g` object.
Example usage::
from contextlib import contextmanager
from flask import appcontext_pushed
@contextmanager
def user_set(app, user):
def handler(sender, **kwargs):
g.user = user
with appcontext_pushed.connected_to(handler, app):
yield
And in the testcode::
def test_user_me(self):
with user_set(app, 'john'):
c = app.test_client()
resp = c.get('/users/me')
assert resp.data == 'username=john'
.. versionadded:: 0.10
.. data:: appcontext_popped
This signal is sent when an application context is popped. The sender
is the application. This usually falls in line with the
:data:`appcontext_tearing_down` signal.
.. versionadded:: 0.10
.. data:: message_flashed
This signal is sent when the application is flashing a message. The
messages is sent as `message` keyword argument and the category as
`category`.
Example subscriber::
recorded = []
def record(sender, message, category, **extra):
recorded.append((message, category))
from flask import message_flashed
message_flashed.connect(record, app)
.. versionadded:: 0.10
.. class:: signals.Namespace
An alias for :class:`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.
.. method:: 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 :exc:`RuntimeError` for all other
operations, including connecting.
.. _blinker: https://pypi.org/project/blinker/
Class-Based Views
-----------------
.. versionadded:: 0.7
.. currentmodule:: None
.. autoclass:: flask.views.View
:members:
.. autoclass:: flask.views.MethodView
:members:
.. _url-route-registrations:
URL Route Registrations
-----------------------
Generally there are three ways to define rules for the routing system:
1. You can use the :meth:`flask.Flask.route` decorator.
2. You can use the :meth:`flask.Flask.add_url_rule` function.
3. You can directly access the underlying Werkzeug routing system
which is exposed as :attr:`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
`any` matches one of the items provided
`uuid` accepts UUID strings
=========== ===============================================
Custom converters can be defined using :attr:`flask.Flask.url_map`.
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``.
If a URL contains a default value, it will be redirected to its simpler
form with a 301 redirect. In the above example, ``/users/page/1`` will
be redirected to ``/users/``. If your route handles ``GET`` and ``POST``
requests, make sure the default route only handles ``GET``, as redirects
can't preserve form data. ::
@app.route('/region/', defaults={'id': 1})
@app.route('/region/<int:id>', methods=['GET', 'POST'])
def region(id):
pass
Here are the parameters that :meth:`~flask.Flask.route` and
:meth:`~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 rule 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
:attr:`~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
:class:`~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 :meth:`~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 if a `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.
- `required_methods`: if this attribute is set, Flask will always add
these methods when registering a URL rule even if the methods were
explicitly overridden in the ``route()`` call.
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)
.. versionadded:: 0.8
The `provide_automatic_options` functionality was added.
Command Line Interface
----------------------
.. currentmodule:: flask.cli
.. autoclass:: FlaskGroup
:members:
.. autoclass:: AppGroup
:members:
.. autoclass:: ScriptInfo
:members:
.. autofunction:: load_dotenv
.. autofunction:: with_appcontext
.. autofunction:: pass_script_info
Marks a function so that an instance of :class:`ScriptInfo` is passed
as first argument to the click callback.
.. autodata:: run_command
.. autodata:: shell_command
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