The installation method is different depending on the version of Python.

CPython Installation

For use with standard Python (CPython) projects, Microdot and all of its core extensions are installed with pip:

pip install microdot

MicroPython Installation

For MicroPython, the recommended approach is to manually copy the necessary source files from the GitHub repository into your device, ideally after compiling them to .mpy files. These source files can also be frozen and incorporated into a custom MicroPython firmware.

Use the following guidelines to know what files to copy:

  • For a minimal setup with only the base web server functionality, copy into your project.

  • For a configuration that includes one or more optional extensions, create a microdot directory in your device and copy the following files:

Getting Started

This section describes the main features of Microdot in an informal manner.

For detailed reference information, consult the API Reference.

If you are familiar with releases of Microdot before 2.x, review the Migration Guide.

A Simple Microdot Web Server

The following is an example of a simple web server:

from microdot import Microdot

app = Microdot()

async def index(request):
    return 'Hello, world!'

The script imports the Microdot class and creates an application instance from it.

The application instance provides a 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.

When the function is called, it is passed a 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.

Microdot is an asynchronous framework that uses the asyncio package. Route handler functions can be defined as async def or def functions, but async def functions are recommended for performance.

The run() method starts the application’s web server on port 5000 by default. This method blocks while it waits for connections from clients.

Running with CPython

Required Microdot source files

Required external dependencies



When using CPython, you can start the web server by running the script that has the call at the bottom:


After starting the script, open a web browser and navigate to http://localhost:5000/ to access the application at 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

Required Microdot source files

Required external dependencies



When using MicroPython, you can upload a file containing the web server code to your device, along with the required Microdot files, as defined in the MicroPython Installation section.

MicroPython will automatically run 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.


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.

Web Server Configuration

The run() method supports a few arguments to configure the web server.

  • port: The port number to listen on. Pass the desired port number in this argument to use a port different than the default of 5000. For example:
  • host: The IP address of the network interface to listen on. By default the server listens on all available interfaces. To listen only on the local loopback interface, pass '' as value for this argument.

  • debug: when set to True, the server ouputs logging information to the console. The default is False.

  • ssl: an SSLContext instance that configures the server to use TLS encryption, or None to disable TLS use. The default is None. The following example demonstrates how to configure the server with an SSL certificate stored in cert.pem and key.pem files:

    import ssl
    # ...
    sslctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
    sslctx.load_cert_chain('cert.pem', 'key.pem'), debug=True, ssl=sslctx)


When using CPython, the certificate and key files must be given in PEM format. When using MicroPython, these files must be given in DER format.

Defining Routes

The 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:

async 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 Request object. The return value of the function is the response that is sent to the client.

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

async 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, which are the default. 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'])
async def invoices(request):
    if request.method == 'GET':
        return 'get invoices'
    elif request.method == 'POST':
        return 'create an invoice'

As an alternative to the example above, in which a single function is used to handle multiple HTTP methods, sometimes 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'])
async def get_invoices(request):
    return 'get invoices'

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

Microdot provides the get(), post(), put(), patch(), and delete() decorators as shortcuts for the corresponding HTTP methods. The two example routes above can be written more concisely with them:

async def get_invoices(request):
    return 'get invoices''/invoices')
async 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:

async def get_user(request, username):
    return 'User: ' + username

As shown in the example, a path component that is enclosed in angle brackets is considered a placeholder. Microdot accepts any values for that portion 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:

async 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:

async 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. The difference between an argument of type path and one of type string is that the latter stops capturing when a / appears in the URL:

async 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:

async def get_user(request, username):
    return 'User: ' + username


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 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:

async 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 route.

After-request handlers registered with the 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:

async def start_timer(request):
    request.g.start_time = time.time()

async 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, and they can return a modified response object to replace the original. If no value is returned from an after-request handler, then the original response object is used.

The after-request handlers are only invoked for successful requests. The after_error_request() decorator can be used to register a function that is called after an error occurs. The function receives the request and the error response and is expected to return an updated response object after performing any necessary cleanup.


The request.g object used in many of the above examples is a special object that allows the before- and after-request handlers, as well as 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 unknown.

  • 405 for URLs that are known, but not implemented for the requested HTTP method.

  • 413 for requests that are larger than the allowed size.

  • 500 when the application raises an unhandled exception.

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

async 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 invoke the handler when an unhandled exception that is an instance of the given class is raised. The next example provides a custom response for division by zero errors:

async def division_by_zero(request, exception):
    return {'error': 'division by zero'}, 500

When the raised exception class does not have an error handler defined, but one or more of its parent classes do, Microdot makes an attempt to invoke the most specific handler.

Mounting a Sub-Application

Small Microdot applications can be written as a single source file, but this is not the best option for applications that past a 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. For developers familiar with the Flask framework, this is a similar concept to Flask’s blueprints.

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

from microdot import Microdot

customers_app = Microdot()

async def get_customers(request):
    # return all customers'/')
async def new_customer(request):
    # create a new customer

Similar to the above, the sub-application implements operations on customer orders:

from microdot import Microdot

orders_app = Microdot()

async def get_orders(request):
    # return all orders'/')
async def new_order(request):
    # create a new order

Now the main application, which is stored in, can import and mount the sub-applications to build the larger 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()

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


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 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:

async def shutdown(request):
    return 'The server is shutting down...'

The request that invokes the shutdown() method will complete, and then the server will not accept any new requests and stop once any remaining requests complete. At this point the call will return.

The Request Object

The 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:

  • method: The HTTP method of the request.

  • path: The path of the request.

  • args: The query string parameters of the request, as a MultiDict object.

  • headers: The headers of the request, as a dictionary.

  • cookies: The cookies that the client sent with the request, as a dictionary.

  • content_type: The content type specified by the client, or None if no content type was specified.

  • content_length: The content length of the request, or 0 if no content length was specified.

  • client_addr: The network address of the client, as a tuple (host, port).

  • app: The application instance that created the request.

  • g: The g object, where handlers can store request-specific data to be shared among handlers. See The “g” Object for details.

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 json attribute. The following example shows how to use this attribute:'/customers')
async def create_customer(request):
    customer = request.json
    # do something with customer
    return {'success': True}


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 form attribute, which presents the form data as a MultiDict object. Example:

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


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 body request attribute returns the entire body of the request as a byte sequence.

If the expected body is too large to fit safely in memory, the application can use the stream request attribute to read the body contents as a file-like object. The max_body_length attribute of the request object defines the size at which bodies are streamed instead of loaded into memory.


Cookies that are sent by the client are made available through the 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- and after-request handlers, the route function and any error handlers. The request object provides the 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:

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

async 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 after_request decorator. Request-specific after-request handlers are called by Microdot after the route function returns and all the application-wide 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:'/logout')
async def logout(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:

  • 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.

  • max_body_length: The maximum size that is loaded in the 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 stream attribute. The default is also 16KB.

  • 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 in size, but prevents requests that are larger than 8KB from being loaded into memory:

from microdot import Request

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


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:

async def index(request):
    return 'Hello, World!'

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

The application can provide its own status code as a second value returned from the route to override the 200 default. The example below returns a 202 status code:

async 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 included by Microdot. The next example returns an HTML response, instead of a default text response:

async 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 status code:

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

The application can also return a 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.


async def index(request):
    return {'hello': 'world'}


A Content-Type header set to application/json is automatically added to the response.


The redirect function is a helper that creates redirect responses:

from microdot import redirect

async def index(request):
    return redirect('/about')

File Responses

The send_file function builds a response object for a file:

from microdot import send_file

async def index(request):
    return send_file('/static/index.html')

A suggested caching duration can be returned to the client in the max_age argument:

from microdot import send_file

async def image(request):
    return send_file('/static/image.jpg', max_age=3600)  # in seconds


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:

async def static(request, path):
    if '..' in path:
        # directory traversal is not allowed
        return 'Not found', 404
    return send_file('static/' + path, max_age=86400)

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 Python generator. The example below returns all the numbers in the fibonacci sequence below 100:

async def fibonacci(request):
    async def generate_fibonacci():
        a, b = 0, 1
        while a < 100:
            yield str(a) + '\n'
            a, b = b, a + b

    return generate_fibonacci()


Under CPython, the generator function can be a def or async def function, as well as a class-based generator.

Under MicroPython, asynchronous generator functions are not supported, so only def generator functions can be used. Asynchronous class-based generators are supported.

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 default_content_type attribute of the 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 applications 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 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 request-specific after-request handler.


async def index(request):
    async 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:

async def index(request):
    response = Response('Hello, World!')
    response.set_cookie('name', 'value')
    return response


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 session extension implements signed cookies that prevent tampering by malicious actors.


Microdot implements concurrency through the asyncio package. Applications must ensure their handlers do not block, as this will prevent other concurrent requests from being handled.

When running under CPython, async def handler functions run as native asyncio tasks, while def handler functions are executed in a thread executor to prevent them from blocking the asynchronous loop.

Under MicroPython the situation is different. Most microcontroller boards implementing MicroPython do not have threading support or executors, so def handler functions in this platform can only run in the main and only thread. These functions will block the asynchronous loop when they take too long to complete so async def handlers properly written to allow other handlers to run in parallel should be preferred.