Request Handling

Connexion validates incoming requests for conformance with the schemas described in swagger specification.

Request parameters will be provided to the handler functions as keyword arguments if they are included in the function’s signature, otherwise body parameters can be accessed from connexion.request.json and query parameters can be accessed from connexion.request.args.

Request Validation

Both the request body and parameters are validated against the specification, using jsonschema.

If the request doesn’t match the specification connexion will return a 400 error.

Automatic Parameter Handling

Connexion automatically maps the parameters defined in your endpoint specification to arguments of your Python views as named parameters and with value casting whenever possible. All you need to do is define the endpoint’s parameters with matching names with your views arguments.

As example 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 will automatically identify that your view function expects an argument named message and will assign the value of the endpoint parameter message to your view function.

Connexion will also use default values if they are provided.

If you want to use a parameter name that collides with a Python built-in, you can enable the pythonic_params option:

With this option enabled, Connexion firstly converts CamelCase names to snake_case. Secondly it looks to see if the name matches a known built-in and if it does it appends an underscore to the name.

As example you have an endpoint specified as:

And the 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.


Please note that when you have a parameter defined as not required at your endpoint and your Python view have a non-named argument, when you call this endpoint WITHOUT the parameter you will get an exception of missing positional argument.

Type casting

Whenever possible Connexion will try to parse your argument values and do type casting to related Python natives 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

In the OpenAPI 2.0 Specification if you use the array type, you can define the collectionFormat to set the deserialization behavior. Connexion currently supports “pipes” and “csv” as collection formats. 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 join them all together. For example, if you provide a URI with the the query string ?letters=a,b,c&letters=d,e,f, connexion will set letters = ['a', 'b', 'c', '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 Swagger2URIParser
options = {'uri_parser_class': Swagger2URIParser}
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)

Nullable parameters

Sometimes your API should explicitly accept nullable parameters. However OpenAPI specification currently does not support officially a way to serve this use case, Connexion adds the x-nullable vendor extension to parameter definitions. Its usage would be:

     - name: name
       in: query
       type: string
       x-nullable: true
       required: true

It is supported by Connexion in all parameter types: body, query, formData, and path. Nullable values are the strings null and None.


Be careful on nullable parameters for sensitive data where the strings “null” or “None” can be valid values.


This extension will be removed as soon as OpenAPI/Swagger Specification provide an official way of supporting nullable values.

Header Parameters

Currently, header parameters are not passed to the handler functions as parameters. But they can be accessed through the underlying connexion.request.headers object which aliases the flask.request.headers object.

def index():
    page_number = connexion.request.headers['Page-Number']

Custom Validators

By default, body and parameters contents are validated against OpenAPI schema via connexion.decorators.validation.RequestBodyValidator or connexion.decorators.validation.ParameterValidator, if you want to change the validation, you can override the defaults with:

validator_map = {
    'body': CustomRequestBodyValidator,
    'parameter': CustomParameterValidator
app = connexion.FlaskApp(__name__)
app.add_api('api.yaml', ..., validator_map=validator_map)

See custom validator example in examples/enforcedefaults.