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 and any desired optional extension source files from the GitHub repository into your device, possibly after compiling them to .mpy files. These source files can also be frozen 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 API Reference.

A Simple Microdot Web Server

The following is an example of a simple web server:

from microdot import Microdot

app = Microdot()

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. The function 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.

The 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

Required Microdot source files

Required external dependencies



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


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

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 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.

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:

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

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 get(), post(), put(), patch(), and delete() decorator shortcuts as well. The two example routes above can be written more concisely with them:

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

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:

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:

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:

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:

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:

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 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:

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

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.

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.


The request.g object 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 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 errorhandler() decorator registers functions to respond to specific error codes. The following example shows a custom error handler for 404 errors:

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 the exception is an instance of the given class is raised. The next example provides a custom response for division by zero errors:

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 base classes do, Microdot makes an attempt to invoke the most specific handler.

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 sub-application that implements operations on customers:

from microdot import Microdot

customers_app = Microdot()

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

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

from microdot import Microdot

orders_app = Microdot()

def get_orders(request):
    # return all orders'/')
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 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:

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

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.

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')
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'])
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 in memory, the application can use the stream request attribute to read the body contents as a file-like object.


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 or after request handlers and the route function. 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:

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

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’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:'/logout')
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 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


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:

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

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

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

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:

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:

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.


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

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

File Responses

The send_file function builds a response object for a file:

from microdot import send_file

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

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:

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

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 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.


def index(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:

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.


By default, Microdot runs in synchronous (single-threaded) mode. However, if the threading module is available, each request will be started on a separate thread and requests will be handled concurrently.

Be aware that most microcontroller boards support a very limited form of multi-threading that is not appropriate for concurrent request handling. For that reason, use of the threading module on microcontroller platforms is not recommended.

The micropython_asyncio extension provides a more robust concurrency option that is supported even on low-end MicroPython boards.