# Mocking clients for Testing

## Test Doubles: The idea behind mocking Tango entities

Suppose we want to test a Tango Device, Device A. In particular, we want to assert that when a certain action is invoked on Device A, it should produce an expected result. This will prove to us that Device A ‘s implementation sufficiently manifests the behaviour we would like it to have.

Now suppose Device A depends on Device B to complete its action. In other words, the result will depend, in part, on the state of Device B. This means that to test this scenario, both devices need to be in a base state that we control.

This might be difficult to achieve when using real devices since it might require a lot of orchestration and manipulation of details irrelevant to the test scenario, i.e. to get Device B into the required state.

A Test Double is a component that can act as a real device but is easier to manipulate and configure into the states that we want during testing. This means that we can replace Device B with its Test Double as long as it conforms to the interface that Device A expects.

What’s more, we can manipulate the Test Double to respond in the way we expect Device B to respond under the various conditions we want to test. A Mock is simply a type of Test Double that might have some conditional logic or behaviour to help in testing.

## Tango’s DeviceProxys

In Tango, the DeviceProxy is an object that allows communication between devices. It can be regarded as the client part of a client-server interaction.

Thus, any Tango device (say, Device A) that depends on a secondary Tango device (Device B) will need to use a DeviceProxy to connect and communicate with the secondary device (Device B).

This invariably means that in the implementation of Device A, it will be instantiating and using a DeviceProxy object.

However, the mechanism by which this happens is a multi-step process which requires communication with a TangoDB DS and an appropriately configured Tango system that contains a deployed Device B.

If we can replace the DeviceProxy object that Device A will use to communicate to Device B with our own Mock object (DeviceProxyMock), we can test the behaviour of Device A while faking the responses it expects to receive from querying Device B. All this without the need for deploying a real Device B, since for all intents and purposes, the DeviceProxyMock would represent the real device.

In other words, mocking the DeviceProxy is sufficient to mock the underlying device it connects to, with reference to how DeviceProxy is used by Device A.

## Mocking the DeviceProxy

In some PyTango devices, such as those in the SKA TMC Prototype, the DeviceProxy object is instantiated during the operation of the Device Under Test (DUT). Also, there isn’t usually an explicit interface to inject a DeviceProxyMock as a replacement for the real class.

As an example, the CentralNode (at v0.1.8) device from the TMC Prototype instantiates all the DeviceProxy objects it needs to connect to its child elements/components in its init_device method:

     1class CentralNode(SKABaseDevice):
     2...
     3  def init_device(self):
     4    ...
     5    self._leaf_device_proxy.append(DeviceProxy(self._dish_leaf_node_devices[name]))
     6    ...
     7    self._csp_master_leaf_proxy = DeviceProxy(self.CspMasterLeafNodeFQDN)
     8    ...
     9    self._sdp_master_leaf_proxy = DeviceProxy(self.SdpMasterLeafNodeFQDN)
    10    ...
    11    subarray_proxy = DeviceProxy(self.TMMidSubarrayNodes[subarray])

Unfortunately, the init_device method does not allow us to provide the class we want the device to use when it needs to instantiate its DeviceProxys.

So we will have to mock the DeviceProxy class that the DUT imports before it instantiates that class.

The diagram below illustrates the relationship between the TestCase, DUT and its transitive import of the DeviceProxy class from the PyTango module:

So, we want to replace the imported DeviceProxy class with our own Fake Constructor that will provide a Mocked Device Proxy for the DUT during tests.

In other words, we want to replace the thing that instantiates the DeviceProxy (i.e. the constructor) with our own callable object that constructs a mocked DeviceProxy object instead of the real one. We want to move from the original implementation to the mocked implementation shown in the diagram below:

## Solution

This can be achieved by using the unittest.mock library that comes with Python 3.

The mock.patch() method allows us to temporarily change the object that a name points to with another one.

We use this mechanism to replace the DeviceProxy class (constructor) with our own fake constructor (a mock) that returns a Mock object:

    1with mock.patch(device_proxy_import_path) as patched_constructor:
    2    patched_constructor.side_effect = lambda device_fqdn: proxies_to_mock.get(device_fqdn, Mock())
    3    patched_module = importlib.reload(sys.modules[device_under_test.__module__])

An alternative to using mock.patch is pytest’s monkeypatch. Its .setattr method provides the same functionality, i.e. allowing you to intercept what an object call would normally do and substituting its full execution with your own specification. There are more examples of its use in the OET implementation which is discussed below.

proxies_to_mock is a dictionary that maps DeviceProxyMock objects to their associated Tango device addresses that we expect the DUT to use when instantiating DeviceProxy objects. A brand new generic Mock() is returned if a specific mapping isn’t provided.

Since the DeviceProxy class is defined at import time, we will need to reload the module that holds the DUT. This is why we explicitly call importlib.reload(…) in the context of mock.patch().

For full details and code that implement this solution, see the following merge requests:

> - https://gitlab.com/ska-telescope/tmc-prototype/-/merge_requests/23

> - https://gitlab.com/ska-telescope/tmc-prototype/-/merge_requests/35

## Moving on

Once we mocked DeviceProxy, then we can use the constructor of this object to return a device that is fake. This can be:

> - a stub device, programmed to behave in a way that suits the tests that we are writing; in this case we are using the stub to inject other inputs to the DUT, under control of the test case;

> - a mock device, a stub device where we can inspect also how the DUT interacted with it, and we can write assertions.

The benefits that we can achieve with the technique described here are:

> 1. ability to test the DUT in isolation

> 2. ability to create tests that are very fast (no network, no databases)

> 3. ability to inject into the DUT indirect inputs

> 4. ability to observe the indirect outputs of the DUT

> 5. ability to observe the interactions that the DUT has with the mock.

## Using pytest and fixtures

The above mocking techniques can be achieved in a very succint way using pytest fixtures. Examples of this can be found in the pytango/examples. And more examples are available in the last section of the Unit testing Tango devices in Python presentation from the Tango 2020 November status update meeting.

## Acknowledgement

Initial content for this page contributed by the Square Kilometre Array.