# API

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

## Blueprint Objects

## Incoming Request Data

class `flask.``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 Notes On Proxies for more information.

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

## Response Objects

## 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:

class `flask.``session`

  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 `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

New in version 0.8.

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

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 `permanent_session_lifetime` attribute on the app which converts the result to an integer automatically.

## Test Client

## Application Globals

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`.

`flask.``g`

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

  Starting with Flask 0.10 this is stored on the application context and no longer on the request context which means it becomes available if only the application context is bound and not yet a request. This is especially useful when combined with the Faking Resources and Context pattern for testing.

  Additionally as of 0.10 you can use the `get()` method to get an attribute or `None` (or the second argument) if it’s not set. These two usages are now equivalent:

      user = getattr(flask.g, 'user', None)
      user = flask.g.get('user', None)

  It’s now also possible to use the `in` operator on it to see if an attribute is defined and it yields all keys on iteration.

  As of 0.11 you can use `pop()` and `setdefault()` in the same way you would use them on a dictionary.

  This is a proxy. See Notes On Proxies for more information.

## Useful Functions and Classes

`flask.``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 powered by the application context and not by the request context, so you can change the value of this proxy by using the `app_context()` method.

  This is a proxy. See Notes On Proxies for more information.

## Message Flashing

## JSON Support

Flask uses `simplejson` for the JSON implementation. Since simplejson is provided by both the standard library as well as extension, Flask will try simplejson first and then fall back to the stdlib json module. On top of that it will delegate access to the current application’s JSON encoders and decoders for easier customization.

So for starters instead of doing:

    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 in the standard library. The following extensions are by default applied to the stdlib’s JSON module:

1. `datetime` objects are serialized as RFC 822 strings.
2. Any object with an `__html__` method (like `Markup`) will have that method called and then the return value is serialized as string.

The `htmlsafe_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 unless you are using Flask 0.10 which implies that:

    <script type=text/javascript>
        doSomethingWith({{ user.username|tojson|safe }});
    </script>

Auto-Sort JSON Keys

The configuration variable `JSON_SORT_KEYS` (Configuration Handling) can be set to false to stop Flask from auto-sorting keys. By default sorting is enabled and outside of the app context sorting is turned on.

Notice that disabling key sorting can cause issues when using content based HTTP caches and Python’s hash randomization feature.

## Template Rendering

## Configuration

## 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 `ext` as follows:

      from flask.ext import foo

  New in version 0.8.

## Stream Helpers

## Useful Internals

`flask.``_request_ctx_stack`

  The internal `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.``_app_ctx_stack`

  Works similar to the request context but only binds the application. This is mainly there for extensions to store data.

  New in version 0.9.

## Signals

New in version 0.6.

`signals.``signals_available`

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

The following signals exist in Flask:

`flask.``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)

`flask.``before_render_template`

  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)

`flask.``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 `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)

`flask.``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)

`flask.``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.

  Example subscriber:

      def log_exception(sender, exception, **extra):
          sender.logger.debug('Got exception during processing: %s', exception)

      from flask import got_request_exception
      got_request_exception.connect(log_exception, app)

`flask.``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.

`flask.``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.

`flask.``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'

  New in version 0.10.

`flask.``appcontext_popped`

  This signal is sent when an application context is popped. The sender is the application. This usually falls in line with the `appcontext_tearing_down` signal.

  New in version 0.10.

`flask.``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)

  New in version 0.10.

class `signals.``Namespace`

  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

New in version 0.7.

## 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
any | matches one of the items provided
uuid | accepts UUID strings

Custom converters can be defined using `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.

Here are the parameters that `route()` and `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 `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 `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 `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)

New in version 0.8: The provide_automatic_options functionality was added.

## Command Line Interface