microdot/docs/intro.rst

Installation
------------

For standard Python (CPython) projects, Microdot and all of its core extensions
can be installed with ``pip``::

    pip install microdot

For MicroPython, you can install it with ``upip`` if that option is available,
but the recommended approach is to manually copy *microdot.py* and any
desired optional extension source files from the
`GitHub repository <https://github.com/miguelgrinberg/microdot/tree/main/src>`_
into your device, possibly after
`compiling <https://docs.micropython.org/en/latest/reference/mpyfiles.html>`_
them to *.mpy* files. These source files can also be
`frozen <https://docs.micropython.org/en/latest/develop/optimizations.html?highlight=frozen#frozen-bytecode>`_
and incorporated into a custom MicroPython firmware.

Getting Started
---------------

This section describes the main features of Microdot in an informal manner. For
detailed reference information, consult the :ref:`API Reference`.

A Simple Microdot Web Server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The following is an example of a simple web server::

    from microdot import Microdot

    app = Microdot()

    @app.route('/')
    def index(request):
        return 'Hello, world!'

    app.run()

The script imports the :class:`Microdot <microdot.Microdot>` class and creates
an application instance from it.

The application instance provides a :func:`route() <microdot.Microdot.route>`
decorator, which is used to define one or more routes, as needed by the
application.

The ``route()`` decorator takes the path portion of the URL as an
argument, and maps it to the decorated function, so that the function is called
when the client requests the URL. The function is passed a
:class:`Request <microdot.Request>` object as an argument, which provides
access to the information passed by the client. The value returned by the
function is sent back to the client as the response.

The :func:`run() <microdot.Microdot.run>` method starts the application's web
server on port 5000 (or the port number passed in the ``port`` argument). This
method blocks while it waits for connections from clients.

Running with CPython
~~~~~~~~~~~~~~~~~~~~

.. list-table::
   :align: left

   * - Required Microdot source files
     - | `microdot.py <https://github.com/miguelgrinberg/microdot/tree/main/src/microdot.py>`_

   * - Required external dependencies
     - | None

When using CPython, you can start the web server by running the script that
defines and runs the application instance::

    python main.py

While the script is running, you can open a web browser and navigate to
*http://localhost:5000/*, which is the default address for the Microdot web
server. From other computers in the same network, use the IP address or
hostname of the computer running the script instead of ``localhost``.

Running with MicroPython
~~~~~~~~~~~~~~~~~~~~~~~~

.. list-table::
   :align: left

   * - Required Microdot source files
     - | `microdot.py <https://github.com/miguelgrinberg/microdot/tree/main/src/microdot.py>`_

   * - Required external dependencies
     - | None

When using MicroPython, you can upload a *main.py* file containing the web
server code to your device along with *microdot.py*. MicroPython will
automatically run *main.py* when the device is powered on, so the web server
will automatically start. The application can be accessed on port 5000 at the
device's IP address. As indicated above, the port can be changed by passing the
``port`` argument to the ``run()`` method.

.. note::
   Microdot does not configure the network interface of the device in which it
   is running. If your device requires a network connection to be made in
   advance, for example to a Wi-Fi access point, this must be configured before
   the ``run()`` method is invoked.

Defining Routes
~~~~~~~~~~~~~~~

The :func:`route() <microdot.Microdot.route>` decorator is used to associate an
application URL with the function that handles it. The only required argument
to the decorator is the path portion of the URL.

The following example creates a route for the root URL of the application::

    @app.route('/')
    def index(request):
        return 'Hello, world!'

When a client requests the root URL (for example, *http://localhost:5000/*),
Microdot will call the ``index()`` function, passing it a
:class:`Request <microdot.Request>` object. The return value of the function
is the response that is sent to the client.

Below is a another example, this one with a route for a URL with two components
in its path::

    @app.route('/users/active')
    def active_users(request):
        return 'Active users: Susan, Joe, and Bob'

The complete URL that maps to this route is
*http://localhost:5000/users/active*.

An application can include multiple routes. Microdot uses the path portion of
the URL to determine the correct route function to call for each incoming
request.

Choosing the HTTP Method
^^^^^^^^^^^^^^^^^^^^^^^^

All the example routes shown above are associated with ``GET`` requests. But
applications often need to define routes for other HTTP methods, such as
``POST``, ``PUT``, ``PATCH`` and ``DELETE``. The ``route()`` decorator takes a
``methods`` optional argument, in which the application can provide a list of
HTTP methods that the route should be associated with on the given path.

The following example defines a route that handles ``GET`` and ``POST``
requests within the same function::

    @app.route('/invoices', methods=['GET', 'POST'])
    def invoices(request):
        if request.method == 'GET':
            return 'get invoices'
        elif request.method == 'POST':
            return 'create an invoice'

In cases like the above, where a single URL is used to handle multiple HTTP
methods, it may be desirable to write a separate function for each HTTP method.
The above example can be implemented with two routes as follows::

    @app.route('/invoices', methods=['GET'])
    def get_invoices(request):
        return 'get invoices'

    @app.route('/invoices', methods=['POST'])
    def create_invoice(request):
        return 'create an invoice'

Microdot provides the :func:`get() <microdot.Microdot.get>`,
:func:`post() <microdot.Microdot.post>`, :func:`put() <microdot.Microdot.put>`,
:func:`patch() <microdot.Microdot.patch>`, and
:func:`delete() <microdot.Microdot.delete>` decorator shortcuts as well. The
two example routes above can be written more concisely with them::

    @app.get('/invoices')
    def get_invoices(request):
        return 'get invoices'

    @app.post('/invoices')
    def create_invoice(request):
        return 'create an invoice'

Including Dynamic Components in the URL Path
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The examples shown above all use hardcoded URL paths. Microdot also supports
the definition of routes that have dynamic components in the path. For example,
the following route associates all URLs that have a path following the pattern
*http://localhost:5000/users/<username>* with the ``get_user()`` function::

    @app.get('/users/<username>')
    def get_user(request, username):
        return 'User: ' + username

As shown in the example, a path components that is enclosed in angle brackets
is considered dynamic. Microdot accepts any values for that section of the URL
path, and passes the value received to the function as an argument after
the request object.

Routes are not limited to a single dynamic component. The following route shows
how multiple dynamic components can be included in the path::

    @app.get('/users/<firstname>/<lastname>')
    def get_user(request, firstname, lastname):
        return 'User: ' + firstname + ' ' + lastname

Dynamic path components are considered to be strings by default. An explicit
type can be specified as a prefix, separated from the dynamic component name by
a colon. The following route has two dynamic components declared as an integer
and a string respectively::

    @app.get('/users/<int:id>/<string:username>')
    def get_user(request, id, username):
        return 'User: ' + username + ' (' + str(id) + ')'

If a dynamic path component is defined as an integer, the value passed to the
route function is also an integer. If the client sends a value that is not an
integer in the corresponding section of the URL path, then the URL will not
match and the route will not be called.

A special type ``path`` can be used to capture the remainder of the path as a
single argument::

    @app.get('/tests/<path:path>')
    def get_test(request, path):
        return 'Test: ' + path

For the most control, the ``re`` type allows the application to provide a
custom regular expression for the dynamic component. The next example defines
a route that only matches usernames that begin with an upper or lower case
letter, followed by a sequence of letters or numbers::

    @app.get('/users/<re:[a-zA-Z][a-zA-Z0-9]*:username>')
    def get_user(request, username):
        return 'User: ' + username

.. note::
   Dynamic path components are passed to route functions as keyword arguments,
   so the names of the function arguments must match the names declared in the
   path specification.

Before and After Request Handlers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is common for applications to need to perform one or more actions before a
request is handled. Examples include authenticating and/or authorizing the
client, opening a connection to a database, or checking if the requested
resource can be obtained from a cache. The
:func:`before_request() <microdot.Microdot.before_request>` decorator registers
a function to be called before the request is dispatched to the route function.

The following example registers a before request handler that ensures that the
client is authenticated before the request is handled::

    @app.before_request
    def authenticate(request):
        user = authorize(request)
        if not user:
            return 'Unauthorized', 401
        request.g.user = user

Before request handlers receive the request object as an argument. If the
function returns a value, Microdot sends it to the client as the response, and
does not invoke the route function. This gives before request handlers the
power to intercept a request if necessary. The example above uses this
technique to prevent an unauthorized user from accessing the requested
resource.

After request handlers registered with the
:func:`after_request() <microdot.Microdot.after_request>` decorator are called
after the route function returns a response. Their purpose is to perform any
common closing or cleanup tasks. The next example shows a combination of before
and after request handlers that print the time it takes for a request to be
handled::

    @app.before_request
    def start_timer(request):
        request.g.start_time = time.time()

    @ap.after_request
    def end_timer(request, response):
        duration = time.time() - request.g.start_time
        print(f'Request took {duration:0.2f} seconds')

After request handlers receive the request and response objects as arguments.
The function can return a modified response object to replace the original. If
the function does not return a value, then the original response object is
used.

.. note::
   The :ref:`request.g <The "g" Object>` object is a special object that allows
   the before and after request handlers, as well sa the route function to
   share data during the life of the request.

Error Handlers
^^^^^^^^^^^^^^

When an error occurs during the handling of a request, Microdot ensures that
the client receives an appropriate error response. Some of the common errors
automatically handled by Microdot are:

- 400 for malformed requests.
- 404 for URLs that are not defined.
- 405 for URLs that are defined, but not for the requested HTTP method.
- 413 for requests that are larger than the allowed size.
- 500 when the application raises an exception.

While the above errors are fully complaint with the HTTP specification, the
application might want to provide custom responses for them. The
:func:`errorhandler() <microdot.Microdot.errorhandler>` decorator registers
a functions to respond to specific error codes. The following example shows a
custom error handler for 404 errors::

    @app.errorhandler(404)
    def not_found(request):
        return {'error': 'resource not found'}, 404

The ``errorhandler()`` decorator has a second form, in which it takes an
exception class as an argument. Microdot will then invoke the handler when an
exception of that class is raised. The next example provides a custom response
for division by zero errors::

    @app.errorhandler(ZeroDivisionError)
    def division_by_zero(request):
        return {'error': 'division by zero'}, 500

Mounting a Sub-Application
^^^^^^^^^^^^^^^^^^^^^^^^^^

Small Microdot applications can be written an a single source file, but this
is not the best option for applications that past certain size. To make it
simpler to write large applications, Microdot supports the concept of
sub-applications that can be "mounted" on a larger application, possibly with
a common URL prefix applied to all of its routes.

Consider, for example, a *customers.py* sub-application that implements
operations on customers::

    from microdot import Microdot

    customers_app = Microdot()

    @customers_app.get('/')
    def get_customers(request):
        # return all customers

    @customers_app.post('/')
    def new_customer(request):
        # create a new customer

In the same way, the *orders.py* sub-application implements operations on
customer orders::

    from microdot import Microdot

    orders_app = Microdot()

    @orders_app.get('/')
    def get_orders(request):
        # return all orders

    @orders_app.post('/')
    def new_order(request):
        # create a new order

Now the main application, which is stored in *main.py*, can import and mount
the sub-applications to build the combined application::

    from microdot import Microdot
    from customers import customers_app
    from orders import orders_app

    def create_app():
        app = Microdot()
        app.mount(customers_app, url_prefix='/customers')
        app.mount(orders_app, url_prefix='/orders')
        return app

    app = create_app()
    app.run()

The resulting application will have the customer endpoints available at
*/customers/* and the order endpoints available at */orders/*.

.. note::
   Before request, after request and error handlers defined in the
   sub-application are also copied over to the main application at mount time.
   Once installed in the main application, these handlers will apply to the
   whole application and not just the sub-application in which they were
   created.

Shutting Down the Server
^^^^^^^^^^^^^^^^^^^^^^^^

Web servers are designed to run forever, and are often stopped by sending them
an interrupt signal. But having a way to gracefully stop the server is
sometimes useful, especially in testing environments. Microdot provides a
:func:`shutdown() <microdot.Microdot.shutdown>` method that can be invoked
during the handling of a route to gracefully shut down the server when that
request completes. The next example shows how to use this feature::

    @app.get('/shutdown')
    def shutdown(request):
        request.app.shutdown()
        return 'The server is shutting down...'

The Request Object
~~~~~~~~~~~~~~~~~~

The :class:`Request <microdot.Request>` object encapsulates all the information
passed by the client. It is passed as an argument to route handlers, as well as
to before request, after request and error handlers.

Request Attributes
^^^^^^^^^^^^^^^^^^

The request object provides access to the request attributes, including:

- :attr:`method <microdot.Request.method>`: The HTTP method of the request.
- :attr:`path <microdot.Request.path>`: The path of the request.
- :attr:`args <microdot.Request.args>`: The query string parameters of the
  request, as a :class:`MultiDict <microdot.MultiDict>` object.
- :attr:`headers <microdot.Request.headers>`: The headers of the request, as a
  dictionary.
- :attr:`cookies <microdot.Request.cookies>`: The cookies that the client sent
  with the request, as a dictionary.
- :attr:`content_type <microdot.Request.content_type>`: The content type
  specified by the client, or ``None`` if no content type was specified.
- :attr:`content_length <microdot.Request.content_length>`: The content
  length of the request, or 0 if no content length was specified.
- :attr:`client_addr <microdot.Request.client_addr>`: The network address of
  the client, as a tuple (host, port).
- :attr:`app <microdot.Request.app>`: The application instance that created the
  request.

JSON Payloads
^^^^^^^^^^^^^

When the client sends a request that contains JSON data in the body, the
application can access the parsed JSON data using the
:attr:`json <microdot.Request.json>` attribute. The following example shows how
to use this attribute::

    @app.post('/customers')
    def create_customer(request):
        customer = request.json
        # do something with customer
        return {'success': True}

.. note::
   The client must set the ``Content-Type`` header to ``application/json`` for
   the ``json`` attribute of the request object to be populated.

URLEncoded Form Data
^^^^^^^^^^^^^^^^^^^^

The request object also supports standard HTML form submissions through the
:attr:`form <microdot.Request.form>` attribute, which presents the form data
as a :class:`MultiDict <microdot.MultiDict>` object. Example::

    @app.route('/', methods=['GET', 'POST'])
    def index(req):
        name = 'Unknown'
        if req.method == 'POST':
            name = req.form.get('name')
        return f'Hello {name}'

.. note::
   Form submissions are only parsed when the ``Content-Type`` header is set by
   the client to ``application/x-www-form-urlencoded``. Form submissions using
   the ``multipart/form-data`` content type are currently not supported.

Accessing the Raw Request Body
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For cases in which neither JSON nor form data is expected, the
:attr:`body <microdot.Request.body>` request attribute returns the entire body
of the request as a byte sequence. 

If the expected body is too large to fit in memory, the application can use the
:attr:`stream <microdot.Request.stream>` request attribute to read the body
contents as a file-like object.

Cookies
^^^^^^^

Cookies that are sent by the client are made available throught the
:attr:`cookies <microdot.Request.cookies>` attribute of the request object in
dictionary form.

The "g" Object
^^^^^^^^^^^^^^

Sometimes applications need to store data during the lifetime of a request, so
that it can be shared between the before or after request handlers and the
route function. The request object provides the :attr:`g <microdot.Request.g>`
attribute for that purpose.

In the following example, a before request handler
authorizes the client and stores the username so that the route function can
use it::

    @app.before_request
    def authorize(request):
        username = authenticate_user(request)
        if not username:
            return 'Unauthorized', 401
        request.g.username = username

    @app.get('/')
    def index(request):
        return f'Hello, {request.g.username}!'

Request-Specific After Request Handlers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes applications need to perform operations on the response object,
before it is sent to the client, for example to set or remove a cookie. A good
option to use for this is to define a request-specific after request handler
using the :func:`after_request <microdot.Microdot.after_request>` decorator.
Request-specific after request handlers are called by Microdot after the route
function returns and all the application's after request handlers have been
called.

The next example shows how a cookie can be updated using a request-specific
after request handler defined inside a route function::

    @app.post('/logout')
    def logout(request):
        @request.after_request
        def reset_session(request, response):
            response.set_cookie('session', '', http_only=True)
            return response

        return 'Logged out'

Request Limits
^^^^^^^^^^^^^^

To help prevent malicious attacks, Microdot provides some configuration options
to limit the amount of information that is accepted:

- :attr:`max_content_length <microdot.Microdot.max_content_length>`: The
  maximum size accepted for the request body, in bytes. When a client sends a
  request that is larger than this, the server will respond with a 413 error.
  The default is 16KB.
- :attr:`max_body_length <microdot.Microdot.max_body_length>`: The maximum
  size that is loaded in the :attr:`body <microdot.Request.body>` attribute, in
  bytes. Requests that have a body that is larger than this size but smaller
  than the size set for ``max_content_length`` can only be accessed through the
  :attr:`stream <microdot.Request.stream>` attribute. The default is also 16KB.
- :attr:`max_readline <microdot.Microdot.max_readline>`: The maximum allowed
  size for a request line, in bytes. The default is 2KB.

The following example configures the application to accept requests with
payloads up to 1MB big, but prevents requests that are larger than 8KB from
being loaded into memory::

    Request.max_content_length = 1024 * 1024
    Request.max_body_length = 8 * 1024

Responses
~~~~~~~~~

The value or values that are returned from the route function are used by
Microdot to build the response that is sent to the client. The following
sections describe the different types of responses that are supported.

The Three Parts of a Response
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Route functions can return one, two or three values. The first or only value is
always returned to the client in the response body::

    @app.get('/')
    def index(request):
        return 'Hello, World!'

In the above example, Microdot issues a standard 200 status code response, and
inserts the necessary headers.

The applicaton can provide its own status code as a second value returned from
the route. The example below returns a 202 status code::

    @app.get('/')
    def index(request):
        return 'Hello, World!', 202

The application can also return a third value, a dictionary with additional
headers that are added to, or replace the default ones provided by Microdot.
The next example returns an HTML response, instead of a default text response::

    @app.get('/')
    def index(request):
        return '<h1>Hello, World!</h1>', 202, {'Content-Type': 'text/html'}

If the application needs to return custom headers, but does not need to change
the default status code, then it can return two values, omitting the stauts
code::

    @app.get('/')
    def index(request):
        return '<h1>Hello, World!</h1>', {'Content-Type': 'text/html'}

The application can also return a :class:`Response <microdot.Response>` object
containing all the details of the response as a single value.

JSON Responses
^^^^^^^^^^^^^^

If the application needs to return a response with JSON formatted data, it can
return a dictionary or a list as the first value, and Microdot will
automatically format the response as JSON.

Example::

    @app.get('/')
    def index(request):
        return {'hello': 'world'}

.. note::
   JSON responses are sent with the ``Content-Type`` header set to
   ``application/json``.

Redirects
^^^^^^^^^

The :func:`redirect <microdot.Response.redirect>` function is a helper that
creates redirect responses::

    from microdot import redirect

    @app.get('/')
    def index(request):
        return redirect('/about')

File Responses
^^^^^^^^^^^^^^

The :func:`send_file <microdot.Response.send_file>` function builds a response
object for a file::
    
        from microdot import send_file
    
        @app.get('/')
        def index(request):
            return send_file('/static/index.html')

.. note::
   Unlike other web frameworks, Microdot does not automatically configure a
   route to serve static files. The following is an example route that can be
   added to the application to serve static files from a *static* directory in
   the project::

        @app.route('/static/<path:path>')
        def static(request, path):
            if '..' in path:
                # directory traversal is not allowed
                return 'Not found', 404
            return send_file('static/' + path)

Streaming Responses
^^^^^^^^^^^^^^^^^^^

Instead of providing a response as a single value, an application can opt to
return a response that is generated in chunks by returning a generator. The
example below returns all the numbers in the fibonacci sequence below 100::

    @app.get('/fibonacci')
    def fibonacci(request):
        def generate_fibonacci():
            a, b = 0, 1
            while a < 100:
                yield str(a) + '\n'
                a, b = b, a + b

        return generate_fibonacci()

Changing the Default Response Content Type
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Microdot uses a ``text/plain`` content type by default for responses that do
not explicitly include the ``Content-Type`` header. The application can change
this default by setting the desired content type in the
:attr:`default_content_type <microdot.Response.default_content_type>` attribute
of the :class:`Response <microdot.Response>` class.

The example that follows configures the application to use ``text/html`` as
default content type::

    from microdot import Response

    Response.default_content_type = 'text/html'

Setting Cookies
^^^^^^^^^^^^^^^

Many web application rely on cookies to maintain client state between requests.
Cookies can be set with the ``Set-Cookie`` header in the response, but since
this is such a common practice, Microdot provides the
:func:`set_cookie() <microdot.Response.set_cookie>` method in the response
object to add a properly formatted cookie header to the response.

Given that route functions do not normally work directly with the response
object, the recommended way to set a cookie is to do it in a
:ref:`Request-Specific After Request Handler <Request-Specific After Request Handlers>`.

Example::

    @app.get('/')
    def index(request):
        @request.after_request
        def set_cookie(request, response):
            response.set_cookie('name', 'value')
            return response

        return 'Hello, World!'

Another option is to create a response object directly in the route function::

    @app.get('/')
    def index(request):
        response = Response('Hello, World!'))
        response.set_cookie('name', 'value')
        return response

.. note::
   Standard cookies do not offer sufficient privacy and security controls, so
   never store sensitive information in them unless you are adding additional
   protection mechanisms such as encryption or cryptographic signing. The
   :ref:`session <Maintaing Secure User Sessions>` extension implements signed
   cookies that prevent tampering by malicious actors.