Frequently Asked Questions

This page answers some of the often asked questions about Schemathesis.

Usage & Configuration

What kind of data does Schemathesis generate?

Schemathesis generates random test data that conforms to the given API schema. This data consists of all possible data types from the JSON schema specification in various combinations and different nesting levels.

We can’t guarantee that the generated data will always be accepted by the application under test since there could be validation rules not covered by the API schema. If you found that Schemathesis generated something that doesn’t fit the API schema, consider reporting a bug

What kind errors Schemathesis is capable to find?

The main two groups of problems that Schemathesis targets are server-side errors and nonconformity to the behavior described in the API schema.

What parts of the application is Schemathesis targeting during its tests?

It depends. The test data that Schemathesis generates is random. Input validation is, therefore, more frequently examined than other parts.

Since Schemathesis generates data that fits the application’s API schema, it can reach the app’s business logic, but it depends on the architecture of each particular application.

What if my application doesn’t have an API schema?

As the first step, you can use schema generators like flasgger for Python, GrapeSwagger for Ruby, or Swashbuckle for ASP.Net. Then, running Schemathesis against the generated API schema will help you to refine its definitions.

How is Schemathesis different from Dredd?

Schemathesis focuses on finding inputs that result in application crash, but it shares the goal of keeping the API documentation up to date with Dredd. Both tools can generate requests to the API under test, but they approach it differently.

Schemathesis uses Property-Based Testing to infer all input values and uses examples defined in the API schema as separate test cases. Dredd uses examples described in the API schema as the primary source of inputs (and requires them to work) and generates data only in some situations.

By using Hypothesis as the underlying testing framework, Schemathesis benefits from all its features like test case reduction and stateful testing. Dredd works more in a way that requires you to write some sort of example-based tests when Schemathesis requires only a valid API schema and will generate tests for you.

There are a lot of features that Dredd has are Schemathesis has not (e.g., API Blueprint support, that powerful hook system, and many more) and probably vice versa. Definitely, Schemathesis can learn a lot from Dredd and if you miss any feature that exists in Dredd but doesn’t exist in Schemathesis, let us know.

How should I run Schemathesis?

There are two main ways to run it — as a part of Python’s test suite, and as a command-line tool.

If you wrote a Python application and you want to utilize the features of an existing test suite, then the in-code option will best suit your needs.

If you wrote your application in a language other than Python, you should use the built-in CLI. Please keep in mind that you will need to have a running application where you can run Schemathesis against.

Should I always have my application running before starting the test suite?

Only in some workflows! In CLI, you can test your AioHTTP / ASGI / WSGI apps with the --app CLI option. For the pytest integration, there is schemathesis.from_pytest_fixture loader where you can postpone API schema loading and start the test application as a part of your test setup. See more information in the Writing Python tests section.

How long does it usually take for Schemathesis to test an app?

It depends on many factors, including the API’s complexity under test, the network connection speed, and the Schemathesis configuration. Usually, it takes from a few seconds to a few minutes to run all the tests. However, there are exceptions where it might take an hour and more.

Can I exclude particular data from being generated?

Yes. Schemathesis’s hooks mechanism allows you to adapt its behavior and generate data that better fits your use case.

Also, if your application fails on some input early in the code, then it’s often a good idea to exclude this input from the next test run so you can explore deeper parts of your codebase.

How can I use database objects IDs in tests?

The case object that is injected in each test can be modified, assuming your URL template is /api/users/{user_id} then in tests, it can be done like this:

schema = ...  # Load the API schema here

def test_api(case):
    case.path_parameters["user_id"] = 42

Why does Schemathesis fail to parse my API schema generate by FastAPI?

Because FastAPI uses JSON Draft 7 under the hood (via pydantic), which is not compatible with JSON drafts defined by the Open API 2 / 3.0.x versions. It is a known issue on the FastAPI side. Schemathesis is more strict in schema handling by default, but we provide optional fixups for this case:

import schemathesis

# will install all available compatibility fixups.
# You can also provide a list of fixup names as the first argument
# schemathesis.fixups.install(["fast_api"])

For more information, take a look into the “Compatibility” section.

Why Schemathesis generates uniform data for my API schema?

There might be multiple reasons for that, but usually, this behavior occurs when the API schema is complex or deeply nested. Please, refer to the Data generation section in the documentation for more info. If you think that it is not the case, feel free to open an issue.

Does Schemathesis support Open API discriminators? Schemathesis raises an “Unsatisfiable” error.

The discriminator field does not affect data generation, and Schemathesis work directly with the underlying schemas. Usually, the problem comes from using the oneOf keyword with very permissive sub-schemas. For example:

  propertyName: objectType
  - type: object
      - objectType
        type: string
        type: string
  - type: object
      - objectType
        type: string
        type: string

Here both schemas do not restrict their additional properties, and for this reason, any object that is valid for the first sub-schema is also valid for the second one, which contradicts the definition of the oneOf keyword behavior, where the value should be valid against exactly one sub-schema.

To solve this problem, you can use anyOf or make your sub-schemas less permissive.

Working with API schemas

How to disallow random field names in my schema?

You need to add additionalProperties: false to the relevant object definition. But there is a caveat with emulating inheritance with Open API via allOf.

In this case, it is better to use YAML anchors to share schema parts; otherwise it will prevent valid data from passing the validation.