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Connexion is a framework that automagically handles HTTP requests based on OpenAPI Specification (formerly known as Swagger Spec) of your API described in YAML format. Connexion allows you to write an OpenAPI specification, then maps the endpoints to your Python functions; this makes it unique, as many tools generate the specification based on your Python code. You can describe your REST API in as much detail as you want; then Connexion guarantees that it will work as you specified.

We built Connexion this way in order to:

  • simplify the development process
  • confirm expectations about what your API will look like

Connexion Features:

  • Validates requests and endpoint parameters automatically, based on your specification
  • Provides a Web Swagger Console UI so that the users of your API can have live documentation and even call your API’s endpoints through it
  • Handles OAuth 2 token-based authentication
  • Supports API versioning
  • Supports automatic serialization of payloads. If your specification defines that an endpoint returns JSON, Connexion will automatically serialize the return value for you and set the right content type in the HTTP header.

Why Connexion

With Connexion, you write the spec first. Connexion then calls your Python code, handling the mapping from the specification to the code. This incentivizes you to write the specification so that all of your developers can understand what your API does, even before you write a single line of code.

If multiple teams depend on your APIs, you can use Connexion to easily send them the documentation of your API. This guarantees that your API will follow the specification that you wrote. This is a different process from that offered by frameworks such as Hug, which generates a specification after you’ve written the code. Some disadvantages of generating specifications based on code is that they often end up lacking details or mix your documentation with the code logic of your application.

Other Sources/Mentions

New in Connexion 2.0:

  • App and Api options must be provided through the “options” argument (old_style_options have been removed).
  • You must specify a form content-type in ‘consumes’ in order to consume form data.
  • The Operation interface has been formalized in the AbstractOperation class.
  • The Operation class has been renamed to Swagger2Operation.
  • Array parameter deserialization now follows the Swagger 2.0 spec more closely. In situations when a query parameter is passed multiple times, and the collectionFormat is either csv or pipes, the right-most value will be used. For example, ?q=1,2,3&q=4,5,6 will result in q = [4, 5, 6]. The old behavior is available by setting the collectionFormat to multi, or by importing decorators.uri_parsing.AlwaysMultiURIParser and passing parser_class=AlwaysMultiURIParser to your Api.
  • The spec validator library has changed from swagger-spec-validator to openapi-spec-validator.
  • Errors that previously raised SwaggerValidationError now raise the InvalidSpecification exception. All spec validation errors should be wrapped with InvalidSpecification.
  • Support for nullable/x-nullable, readOnly and writeOnly/x-writeOnly has been added to the standard json schema validator.
  • Custom validators can now be specified on api level (instead of app level).
  • Added support for basic authentication and apikey authentication
  • If unsupported security requirements are defined or x-tokenInfoFunc/x-tokenInfoUrl is missing, connexion now denies requests instead of allowing access without security-check.
  • Accessing connexion.request.user / flask.request.user is no longer supported, use connexion.context['user'] instead

How to Use


Python 3.6+

Installing It

In your command line, type:

$ pip install connexion

Running It

Place your API YAML inside a folder in the root path of your application (e.g swagger/). Then run:

import connexion

app = connexion.App(__name__, specification_dir='swagger/')

See the Connexion Pet Store Example Application for a sample specification.

Now you’re able to run and use Connexion!

OAuth 2 Authentication and Authorization

Connexion supports one of the three OAuth 2 handling methods. (See “TODO” below.) With Connexion, the API security definition must include a ‘x-tokenInfoUrl’ or ‘x-tokenInfoFunc (or set TOKENINFO_URL or TOKENINFO_FUNC env var respectively). ‘x-tokenInfoUrl’ must contain an URL to validate and get the token information and ‘x-tokenInfoFunc must contain a reference to a function used to obtain the token info. When both ‘x-tokenInfoUrl’ and ‘x-tokenInfoFunc’ are used, Connexion will prioritize the function method. Connexion expects to receive the OAuth token in the Authorization header field in the format described in RFC 6750 section 2.1. This aspect represents a significant difference from the usual OAuth flow.

Dynamic Rendering of Your Specification

Connexion uses Jinja2 to allow specification parameterization through the arguments parameter. You can define specification arguments for the application either globally (via the connexion.App constructor) or for each specific API (via the connexion.App#add_api method):

app = connexion.App(__name__, specification_dir='swagger/',
                    arguments={'global': 'global_value'})
app.add_api('my_api.yaml', arguments={'api_local': 'local_value'})

When a value is provided both globally and on the API, the API value will take precedence.

Endpoint Routing to Your Python Views

Connexion uses the operationId from each Operation Object to identify which Python function should handle each URL.

Explicit Routing:

      operationId: myapp.api.hello_world

If you provide this path in your specification POST requests to http://MYHOST/hello_world, it will be handled by the function hello_world in the myapp.api module. Optionally, you can include x-swagger-router-controller (or x-openapi-router-controller) in your operation definition, making operationId relative:

      x-swagger-router-controller: myapp.api
      operationId: hello_world

Keep in mind that Connexion follows how HTTP methods work in Flask and therefore HEAD requests will be handled by the operationId specified under GET in the specification. If both methods are supported, connexion.request.method can be used to determine which request was made.

Automatic Routing

To customize this behavior, Connexion can use alternative Resolvers–for example, RestyResolver. The RestyResolver will compose an operationId based on the path and HTTP method of the endpoints in your specification:

from connexion.resolver import RestyResolver

app = connexion.App(__name__)
app.add_api('swagger.yaml', resolver=RestyResolver('api'))
       # Implied operationId: api.get
       # Implied operationId:
       # Implied operationId:

       # Implied operationId:
       # Implied operationId:
       # Implied operationId:
       # Implied operationId:

RestyResolver will give precedence to any operationId encountered in the specification. It will also respect x-router-controller. You can import and extend connexion.resolver.Resolver to implement your own operationId (and function) resolution algorithm.

Automatic Parameter Handling

Connexion automatically maps the parameters defined in your endpoint specification to arguments of your Python views as named parameters, and, whenever possible, with value casting. Simply define the endpoint’s parameters with the same names as your views arguments.

As an example, say you have an endpoint specified as:

      operationId: api.foo_get
        - name: message
          description: Some message.
          in: query
          type: string
          required: true

And the view function:

# file

def foo_get(message):
    # do something
    return 'You send the message: {}'.format(message), 200

In this example, Connexion automatically recognizes that your view function expects an argument named message and assigns the value of the endpoint parameter message to your view function.


In the OpenAPI 3.x.x spec, the requestBody does not have a name. By default it will be passed in as ‘body’. You can optionally provide the x-body-name parameter in your requestBody schema to override the name of the parameter that will be passed to your handler function.


When you define a parameter at your endpoint as not required, and this argument does not have default value in your Python view, you will get a “missing positional argument” exception whenever you call this endpoint WITHOUT the parameter. Provide a default value for a named argument or use **kwargs dict.

Type casting

Whenever possible, Connexion will try to parse your argument values and do type casting to related Python native values. The current available type castings are:

OpenAPI Type Python Type
integer int
string str
number float
boolean bool
array list
null None
object dict

If you use the array type In the Swagger definition, you can define the collectionFormat so that it won’t be recognized. Connexion currently supports collection formats “pipes” and “csv”. The default format is “csv”.

Connexion is opinionated about how the URI is parsed for array types. The default behavior for query parameters that have been defined multiple times is to use the right-most value. For example, if you provide a URI with the the query string ?letters=a,b,c&letters=d,e,f, connexion will set letters = ['d', 'e', 'f'].

You can override this behavior by specifying the URI parser in the app or api options.

from connexion.decorators.uri_parsing import AlwaysMultiURIParser
options = {'uri_parser_class': AlwaysMultiURIParser}
app = connexion.App(__name__, specification_dir='swagger/', options=options)

You can implement your own URI parsing behavior by inheriting from connexion.decorators.uri_parsing.AbstractURIParser.

There are a handful of URI parsers included with connection.

OpenAPIURIParser default: OpenAPI 3.0 This parser adheres to the OpenAPI 3.x.x spec, and uses the style parameter. Query parameters are parsed from left to right, so if a query parameter is defined twice, then the right-most definition will take precedence. For example, if you provided a URI with the query string ?letters=a,b,c&letters=d,e,f, and style: simple, then connexion will set letters = ['d', 'e', 'f']. For additional information see OpenAPI 3.0 Style Values.
Swagger2URIParser default: OpenAPI 2.0 This parser adheres to the Swagger 2.0 spec, and will only join together multiple instance of the same query parameter if the collectionFormat is set to multi. Query parameters are parsed from left to right, so if a query parameter is defined twice, then the right-most definition wins. For example, if you provided a URI with the query string ?letters=a,b,c&letters=d,e,f, and collectionFormat: csv, then connexion will set letters = ['d', 'e', 'f']
FirstValueURIParser This parser behaves like the Swagger2URIParser, except that it prefers the first defined value. For example, if you provided a URI with the query string ?letters=a,b,c&letters=d,e,f and collectionFormat: csv hen connexion will set letters = ['a', 'b', 'c']
AlwaysMultiURIParser This parser is backwards compatible with Connexion 1.x. It joins together multiple instances of the same query parameter.

Parameter validation

Connexion can apply strict parameter validation for query and form data parameters. When this is enabled, requests that include parameters not defined in the swagger spec return a 400 error. You can enable it when adding the API to your application:

app.add_api('my_apy.yaml', strict_validation=True)

API Versioning and basePath

Setting a base path is useful for versioned APIs. An example of a base path would be the 1.0 in http://MYHOST/1.0/hello_world.

If you are using OpenAPI 3.x.x, you set your base URL path in the servers block of the specification. You can either specify a full URL, or just a relative path.

  - url: https://MYHOST/1.0
    description: full url example
  - url: /1.0
    description: relative path example


If you are using OpenAPI 2.0, you can define a basePath on the top level of your OpenAPI 2.0 specification.

basePath: /1.0


If you don’t want to include the base path in your specification, you can provide it when adding the API to your application:

app.add_api('my_api.yaml', base_path='/1.0')

Swagger JSON

Connexion makes the OpenAPI/Swagger specification in JSON format available from either swagger.json (for OpenAPI 2.0) or openapi.json (for OpenAPI 3.x.x) at the base path of the API. For example, if your base path was 1.0, then your spec would be available at /1.0/openapi.json.

You can disable serving the spec JSON at the application level:

options = {"serve_spec": False}
app = connexion.App(__name__, specification_dir='openapi/',

You can also disable it at the API level:

options = {"serve_spec": False}
app = connexion.App(__name__, specification_dir='openapi/')
app.add_api('my_api.yaml', options=options)

HTTPS Support

When specifying HTTPS as the scheme in the API YAML file, all the URIs in the served Swagger UI are HTTPS endpoints. The problem: The default server that runs is a “normal” HTTP server. This means that the Swagger UI cannot be used to play with the API. What is the correct way to start a HTTPS server when using Connexion?

One way, described by Flask, looks like this:

from OpenSSL import SSL
context = SSL.Context(SSL.SSLv23_METHOD)
context.use_certificate_file('yourserver.crt')'', port='12344',
        debug=False/True, ssl_context=context)

However, Connexion doesn’t provide an ssl_context parameter. This is because Flask doesn’t, either–but it uses **kwargs to send the parameters to the underlying werkzeug server.

The Swagger UI Console

The Swagger UI for an API is available through pip extras. You can install it with pip install connexion[swagger-ui]. It will be served up at {base_path}/ui/ where base_path is the base path of the API.

You can disable the Swagger UI at the application level:

app = connexion.App(__name__, specification_dir='openapi/',
                    options={"swagger_ui": False})

You can also disable it at the API level:

app = connexion.App(__name__, specification_dir='openapi/')
app.add_api('my_api.yaml', options={"swagger_ui": False})

If necessary, you can explicitly specify the path to the directory with swagger-ui to not use the connexion[swagger-ui] distro. In order to do this, you should specify the following option:

options = {'swagger_path': '/path/to/swagger_ui/'}
app = connexion.App(__name__, specification_dir='openapi/', options=options)

If you wish to provide your own swagger-ui distro, note that connexion expects a jinja2 file called swagger_ui/index.j2 in order to load the correct swagger.json by default. Your index.j2 file can use the openapi_spec_url jinja variable for this purpose:

const ui = SwaggerUIBundle({ url: "{{ openapi_spec_url }}"})

Additionally, if you wish to use swagger-ui-3.x.x, it is also provided by installing connexion[swagger-ui], and can be enabled like this:

from swagger_ui_bundle import swagger_ui_3_path
options = {'swagger_path': swagger_ui_3_path}
app = connexion.App(__name__, specification_dir='swagger/', options=options)

Server Backend

By default Connexion uses the Flask server. For asynchronous applications, you can also use Tornado as the HTTP server. To do this, set your server to tornado:

import connexion

app = connexion.App(__name__, specification_dir='swagger/')'tornado', port=8080)

You can use the Flask WSGI app with any WSGI container, e.g. using Flask with uWSGI (this is common):

app = connexion.App(__name__, specification_dir='swagger/')
application = # expose global WSGI application object

You can use the aiohttp framework as server backend as well:

import connexion

app = connexion.AioHttpApp(__name__, specification_dir='swagger/')


Also check aiohttp handler examples.

Set up and run the installation code:

$ sudo pip3 install uwsgi
$ uwsgi --http :8080 -w app -p 16  # use 16 worker processes

See the uWSGI documentation for more information.


Additional information is available at Connexion’s Documentation Page.


A full changelog is maintained on the GitHub releases page.

Contributing to Connexion/TODOs

We welcome your ideas, issues, and pull requests. Just follow the usual/standard GitHub practices.

Unless you explicitly state otherwise in advance, any non trivial contribution intentionally submitted for inclusion in this project by you to the steward of this repository (Zalando SE, Berlin) shall be under the terms and conditions of Apache License 2.0 written below, without any additional copyright information, terms or conditions.


If you’d like to become a more consistent contributor to Connexion, we’d love your help working on these we have a list of issues where we are looking for contributions.


We’d like to thank all of Connexion’s contributors for working on this project, and to Swagger/OpenAPI for their support.


Copyright 2015 Zalando SE

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.