pact-python

Python version of Pact. Enables consumer driven contract testing, providing a mock service and DSL for the consumer project, and interaction playback and verification for the service provider project.

For more information about what Pact is, and how it can help you test your code more efficiently, check out the Pact documentation.

How to use pact-python

Installation

pip install pact-python

Writing a Pact

Creating a complete contract is a two step process:

1. Create a test on the consumer side that declares the expectations it has of the provider
2. Create a provider state that allows the contract to pass when replayed against the provider

Writing the Consumer Test

If we have a method that communicates with one of our external services, which we'll call Provider, and our product, Consumer is hitting an endpoint on Provider at /users/<user> to get information about a particular user.

If the code to fetch a user looked like this:

import requests


def user(user_name):
    """Fetch a user object by user_name from the server."""
    uri = 'http://localhost:1234/users/' + user_name
    return requests.get(uri).json()

Then Consumer's contract test might look something like this:

import atexit
import unittest

from pact import Consumer, Provider


pact = Consumer('Consumer').has_pact_with(Provider('Provider'))
pact.start_service()
atexit.register(pact.stop_service)


class GetUserInfoContract(unittest.TestCase):
  def test_get_user(self):
    expected = {
      'username': 'UserA',
      'id': 123,
      'groups': ['Editors']
    }
      
    (pact
     .given('UserA exists and is not an administrator')
     .upon_receiving('a request for UserA')
     .with_request('get', '/users/UserA')
     .will_respond_with(200, body=expected))

    with pact:
      result = user('UserA')
    
    self.assertEqual(result, expected)

This does a few important things:

• Defines the Consumer and Provider objects that describe our product and our service under test
• Uses given to define the setup criteria for the Provider UserA exists and is not an administrator
• Defines what the request that is expected to be made by the consumer will contain
• Defines how the server is expected to respond

Using the Pact object as a context manager, we call our method under test which will then communicate with the Pact mock service. The mock service will respond with the items we defined, allowing us to assert that the method processed the response and returned the expected value. If you want more control over when the mock service is configured and the interactions verified, use the setup and verify methods, respectively:

   (pact
     .given('UserA exists and is not an administrator')
     .upon_receiving('a request for UserA')
     .with_request('get', '/users/UserA')
     .will_respond_with(200, body=expected))

    pact.setup()
    # Some additional steps before running the code under test
    result = user('UserA')
    # Some additional steps before verifying all interactions have occurred
    pact.verify()

The default hostname and port for the Pact mock service will be localhost:1234 but you can adjust this during Pact creation:

from pact import Consumer, Provider
pact = Consumer('Consumer').has_pact_with(
    Provider('Provider'), host_name='mockservice', port=8080)

This can be useful if you need to run to create more than one Pact for your test because your code interacts with two different services. It is important to note that the code you are testing with this contract must contact the mock service. So in this example, the user method could accept an argument to specify the location of the server, or retrieve it from an environment variable so you can change its URI during the test.

The mock service offers you several important features when building your contracts:

• It provides a real HTTP server that your code can contact during the test and provides the responses you defined.
• You provide it with the expectations for the request your code will make and it will assert the contents of the actual requests made based on your expectations.
• If a request is made that does not match one you defined or if a request from your code is missing it will return an error with details.
• Finally, it will record your contracts as a JSON file that you can store in your repository or publish to a Pact broker.

Expecting Variable Content

The above test works great if that user information is always static, but what happens if the user has a last updated field that is set to the current time every time the object is modified? To handle variable data and make your tests more robust, there are 3 helpful matchers:

Term(matcher, generate)

Asserts the value should match the given regular expression. You could use this to expect a timestamp with a particular format in the request or response where you know you need a particular format, but are unconcerned about the exact date:

from pact import Term
...
body = {
    'username': 'UserA',
    'last_modified': Term('\d+-\d+-\d+T\d+:\d+:\d+', '2016-12-15T20:16:01')
}

(pact
 .given('UserA exists and is not an administrator')
 .upon_receiving('a request for UserA')
 .with_request('get', '/users/UserA/info')
 .will_respond_with(200, body=body))

When you run the tests for the consumer, the mock service will return the value you provided as generate, in this case 2016-12-15T20:16:01. When the contract is verified on the provider, the regex will be used to search the response from the real provider service and the test will be considered successful if the regex finds a match in the response.

SomethingLike(matcher)

Asserts the element's type matches the matcher. For example:

from pact import SomethingLike
SomethingLike(123)  # Matches if the value is an integer
SomethingLike('hello world')  # Matches if the value is a string
SomethingLike(3.14)  # Matches if the value is a float

The argument supplied to SomethingLike will be what the mock service responds with.

EachLike(matcher, minimum=None, maximum=None)

Asserts the value is an array type that consists of elements like the ones passed in. It can be used to assert simple arrays:

from pact import EachLike
EachLike(1)  # All items are integers
EachLike('hello')  # All items are strings

Or other matchers can be nested inside to assert more complex objects:

from pact import EachLike, SomethingLike, Term
EachLike({
    'username': Term('[a-zA-Z]+', 'username'),
    'id': SomethingLike(123),
    'groups': EachLike('administrators')
})

Note, you do not need to specify everything that will be returned from the Provider in a JSON response, any extra data that is received will be ignored and the tests will still pass.

For more information see Matching

Verifying Pacts Against a Service

In addition to writing Pacts for Python consumers, you can also verify those Pacts against a provider of any language. After installing pact-python a pact-verifier application should be available. To get details about its use you can call it with the help argument:

pact-verifier --help

The simplest example is verifying a server with locally stored Pact files and no provider states:

pact-verifier --provider-base-url=http://localhost:8080 --pact-urls=./pacts/consumer-provider.json

Which will immediately invoke the Pact verifier, making HTTP requests to the server located at http://localhost:8080 based on the Pacts in ./pacts/consumer-provider.json and reporting the results.

There are several options for configuring how the Pacts are verified:

--provider-base-url

Required. Defines the URL of the server to make requests to when verifying the Pacts.

--pact-urls

Required. The location of the Pact files you want to verify. This can be a URL to a Pact Broker or one or more local paths, separated by a comma.

--provider-states-url

The URL where your provider application will produce the list of available provider states. The verifier calls this URL to ensure the Pacts specify valid states before making the HTTP requests.

--provider-states-setup-url

The URL which should be called to setup a specific provider state before a Pact is verified.

--pact-broker-username

The username to use when contacting the Pact Broker.

--pact-broker-password

The password to use when contacting the Pact Broker. You can also specify this value as the environment variable PACT_BROKER_PASSWORD.

Provider States

In many cases, your contracts will need very specific data to exist on the provider to pass successfully. If you are fetching a user profile, that user needs to exist, if querying a list of records, one or more records needs to exist. To support decoupling the testing of the consumer and provider, Pact offers the idea of provider states to communicate from the consumer what data should exist on the provider.

When setting up the testing of a provider you will also need to setup the management of these provider states. The Pact verifier does this by making additional HTTP requests to the provider_states_url and provider_states_setup_url you provide. These URLs could be on the provider application or a separate one. Some strategies for managing state include:

• Having endpoints in your application that are not active in production that create and delete your datastore state
• A separate application that has access to the same datastore to create and delete, like a separate App Engine module or Docker container pointing to the same datastore
• A standalone application that can start and stop the other server with different datastore states

For more information about provider states, refer to the Pact documentation on Provider States.

Development

Please read CONTRIBUTING.md

To setup a development environment:

1. If you want to run tests for all Python versions, install 2.7, 3.3, 3.4, 3.5, and 3.6 from source or using a tool like pyenv
2. Its recommended to create a Python virtualenv for the project

The setup the environment, run tests, and package the application, run: make release

If you are just interested in packaging pact-python so you can install it using pip:

make package

This creates a dist/pact-python-N.N.N.tar.gz file, where the Ns are the current version. From there you can use pip to install it:

pip install ./dist/pact-python-N.N.N.tar.gz

Testing

This project has unit and end to end tests, which can both be run from make:

Unit: make test

End to end: make e2e