Serving WSGI Applications¶
There are many ways to serve a WSGI application. While you’re developing it, you usually don’t want to have a full-blown webserver like Apache up and running, but instead a simple standalone one. Because of that Werkzeug comes with a builtin development server.
The easiest way is creating a small
start-myproject.py file that runs the
application using the builtin server:
from werkzeug.serving import run_simple from myproject import make_app app = make_app(...) run_simple('localhost', 8080, app, use_reloader=True)
You can also pass it the extra_files keyword argument with a list of additional files (like configuration files) you want to observe.
- werkzeug.serving.run_simple(hostname, port, application, use_reloader=False, use_debugger=False, use_evalex=True, extra_files=None, exclude_patterns=None, reloader_interval=1, reloader_type='auto', threaded=False, processes=1, request_handler=None, static_files=None, passthrough_errors=False, ssl_context=None)¶
Start a development server for a WSGI application. Various optional features can be enabled.
Do not use the development server when deploying to production. It is intended for use only during local development. It is not designed to be particularly efficient, stable, or secure.
hostname (str) – The host to bind to, for example
'localhost'. Can be a domain, IPv4 or IPv6 address, or file path starting with
unix://for a Unix socket.
port (int) – The port to bind to, for example
0tells the OS to pick a random free port.
application (WSGIApplication) – The WSGI application to run.
use_reloader (bool) – Use a reloader process to restart the server process when files are changed.
use_debugger (bool) – Use Werkzeug’s debugger, which will show formatted tracebacks on unhandled exceptions.
use_evalex (bool) – Make the debugger interactive. A Python terminal can be opened for any frame in the traceback. Some protection is provided by requiring a PIN, but this should never be enabled on a publicly visible server.
reloader_interval (int) – How often the reloader tries to check for changes.
reloader_type (str) – The reloader to use. The
'stat'reloader is built in, but may require significant CPU to watch files. The
'watchdog'reloader is much more efficient but requires installing the
threaded (bool) – Handle concurrent requests using threads. Cannot be used with
processes (int) – Handle concurrent requests using up to this number of processes. Cannot be used with
passthrough_errors (bool) – Don’t catch unhandled exceptions at the server level, let the serve crash instead. If
use_debuggeris enabled, the debugger will still catch such errors.
ssl_context (Optional[Union[ssl.SSLContext, Tuple[str, Optional[str]], te.Literal['adhoc']]]) – Configure TLS to serve over HTTPS. Can be an
(cert_file, key_file)tuple to create a typical context, or the string
'adhoc'to generate a temporary self-signed certificate.
- Return type:
Changed in version 2.1: Instructions are shown for dealing with an “address already in use” error.
Changed in version 2.1: Running on
::shows the loopback IP in addition to a real IP.
Changed in version 2.1: The command-line interface was removed.
Changed in version 2.0: Running on
::shows a real IP address that was bound as well as a warning not to run the development server in production.
Changed in version 2.0: The
exclude_patternsparameter was added.
Changed in version 0.15: Bind to a Unix socket by passing a
hostnamethat starts with
Changed in version 0.10: Improved the reloader and added support for changing the backend through the
Changed in version 0.9: A command-line interface was added.
Changed in version 0.8:
ssl_contextcan be a tuple of paths to the certificate and private key files.
Changed in version 0.6: The
ssl_contextparameter was added.
Changed in version 0.5: The
passthrough_errorsparameters were added.
Check if the server is running as a subprocess within the Werkzeug reloader.
New in version 0.10.
- Return type:
- werkzeug.serving.make_ssl_devcert(base_path, host=None, cn=None)¶
Creates an SSL key for development. This should be used instead of the
'adhoc'key which generates a new cert on each server start. It accepts a path for where it should store the key and cert and either a host or CN. If a host is given it will use the CN
For more information see
New in version 0.9.
The development server is not intended to be used on production systems. It was designed especially for development purposes and performs poorly under high load. For deployment setups have a look at the Deploying to Production pages.
Changed in version 0.10.
The Werkzeug reloader constantly monitors modules and paths of your web application, and restarts the server if any of the observed files change.
Since version 0.10, there are two backends the reloader supports:
statbackend simply checks the
mtimeof all files in a regular interval. This is sufficient for most cases, however, it is known to drain a laptop’s battery.
watchdogbackend uses filesystem events, and is much faster than
stat. It requires the watchdog module to be installed. The recommended way to achieve this is to add
Werkzeug[watchdog]to your requirements file.
watchdog is installed and available it will automatically be used
instead of the builtin
To switch between the backends you can use the reloader_type parameter of the
'stat' sets it to the default stat based
'watchdog' forces it to the watchdog backend.
Some edge cases, like modules that failed to import correctly, are not handled by the stat reloader for performance reasons. The watchdog reloader monitors such files too.
The development server highlights the request logs in different colors based on the status code. On Windows, Colorama must be installed as well to enable this.
Many web applications utilize multiple subdomains. This can be a bit tricky to simulate locally. Fortunately there is the hosts file that can be used to assign the local computer multiple names.
This allows you to call your local computer yourapplication.local and api.yourapplication.local (or anything else) in addition to localhost.
You can find the hosts file on the following location:
Linux / OS X
You can open the file with your favorite text editor and add a new name after localhost:
127.0.0.1 localhost yourapplication.local api.yourapplication.local
Save the changes and after a while you should be able to access the
development server on these host names as well. You can use the
URL Routing system to dispatch between different hosts or parse
Shutting Down The Server¶
In some cases it can be useful to shut down a server after handling a request. For example, a local command line tool that needs OAuth authentication could temporarily start a server to listen for a response, record the user’s token, then stop the server.
One method to do this could be to start a server in a
multiprocessing process, then terminate the process after a value
is passed back to the parent.
import multiprocessing from werkzeug import Request, Response, run_simple def get_token(q: multiprocessing.Queue) -> None: @Request.application def app(request: Request) -> Response: q.put(request.args["token"]) return Response("", 204) run_simple("localhost", 5000, app) if __name__ == "__main__": q = multiprocessing.Queue() p = multiprocessing.Process(target=get_token, args=(q,)) p.start() print("waiting") token = q.get(block=True) p.terminate() print(token)
That example uses Werkzeug’s development server, but any production
server that can be started as a Python process could use the same
technique and should be preferred for security. Another method could be
to start a
subprocess process and send the value back over
On operating systems that support ipv6 and have it configured such as modern Linux systems, OS X 10.4 or higher as well as Windows Vista some browsers can be painfully slow if accessing your local server. The reason for this is that sometimes “localhost” is configured to be available on both ipv4 and ipv6 sockets and some browsers will try to access ipv6 first and then ipv4.
At the current time the integrated webserver does not support ipv6 and ipv4 at the same time and for better portability ipv4 is the default.
If you notice that the web browser takes ages to load the page there are two ways around this issue. If you don’t need ipv6 support you can disable the ipv6 entry in the hosts file by removing this line:
Alternatively you can also disable ipv6 support in your browser. For example
if Firefox shows this behavior you can disable it by going to
and disabling the network.dns.disableIPv6 key. This however is not
recommended as of Werkzeug 0.6.1!
Starting with Werkzeug 0.6.1, the server will now switch between ipv4 and
ipv6 based on your operating system’s configuration. This means if that
you disabled ipv6 support in your browser but your operating system is
preferring ipv6, you will be unable to connect to your server. In that
situation, you can either remove the localhost entry for
explicitly bind the hostname to an ipv4 address (127.0.0.1)
New in version 0.6.
The builtin server supports SSL for testing purposes. If an SSL context is provided it will be used. That means a server can either run in HTTP or HTTPS mode, but not both.
The easiest way to do SSL based development with Werkzeug is by using it to generate an SSL certificate and private key and storing that somewhere and to then put it there. For the certificate you need to provide the name of your server on generation or a CN.
Generate an SSL key and store it somewhere:
>>> from werkzeug.serving import make_ssl_devcert >>> make_ssl_devcert('/path/to/the/key', host='localhost') ('/path/to/the/key.crt', '/path/to/the/key.key')
Now this tuple can be passed as
run_simple('localhost', 4000, application, ssl_context=('/path/to/the/key.crt', '/path/to/the/key.key'))
You will have to acknowledge the certificate in your browser once then.
Loading Contexts by Hand¶
You can use a
ssl.SSLContext object instead of a tuple for full
control over the TLS configuration.
import ssl ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER) ctx.load_cert_chain('ssl.cert', 'ssl.key') run_simple('localhost', 4000, application, ssl_context=ctx)
A key and certificate can be created in advance using the openssl tool
instead of the
make_ssl_devcert(). This requires that you have
the openssl command installed on your system:
$ openssl genrsa 1024 > ssl.key $ openssl req -new -x509 -nodes -sha1 -days 365 -key ssl.key > ssl.cert
The easiest way to enable SSL is to start the server in adhoc-mode. In that case Werkzeug will generate an SSL certificate for you:
run_simple('localhost', 4000, application, ssl_context='adhoc')
The downside of this of course is that you will have to acknowledge the certificate each time the server is reloaded. Adhoc certificates are discouraged because modern browsers do a bad job at supporting them for security reasons.
This feature requires the cryptography library to be installed.
The dev server can bind to a Unix socket instead of a TCP socket.
run_simple() will bind to a Unix socket if the
parameter starts with
from werkzeug.serving import run_simple run_simple('unix://example.sock', 0, app)