Unit Testing Assets

When deploying infrastructure, we want to ensure that what we’re deploying matches our expectations. One way to do so is via unit testing. We’ve talked about this concept in previous posts, such as in this overview and this post on deployments with .NET.

Often, when we’re creating cloud resources, we want to ensure that a resource’s underlying assets match certain properties. For example, the entrypoint or handler for a cloud function should be an executable function. Similarly, objects we’ll serve as static assets on a website should not exceed a certain size. We can use Pulumi’s unit testing framework along with language-specific constructs such as introspection or filesystem calls to ensure this type of correctness. We’ll walk through some examples to show just how easy it is to do so.

The examples below are in Python, but you can easily achieve the same types of tests using introspection/reflection in Node, .NET, and Go.

Check that Functions are Executable

When creating a Lambda function, we’re expected to pass in a function handler that gets called upon invocation. Typically, the value is something like mod.handler, where mod is the module name and handler is a function. A basic check would be to make sure that the module exists and that the function is callable.

We can easily construct this unit test like so:


import unittest
import pulumi

class MyMocks(pulumi.runtime.Mocks):
    def new_resource(self, type_, name, inputs, provider, id_):
        return [name + '_id', inputs]
    def call(self, token, args, provider):
        return {}


import infra

class TestInfrastructure(unittest.TestCase):

    # Test that the function entrypoint is actually an executable function.
    def test_function_handler_callable(self):
        def check_entrypoint(handler):
            import importlib
            from types import ModuleType
            from inspect import signature, Parameter

            # The handler should be of the form <module_name>.<function>
            module_name, function_handler = handler.split('.', 1)

            # Check that we can load the module.
            module = importlib.import_module(module_name)
            self.assertIsInstance(module, ModuleType)

            # Check that the function handler is actually callable.
            fn = getattr(module, function_handler)

            # Check that it has two parameters: event and context
            sig = signature(fn)
            self.assertListEqual(['event', 'context'], list(sig.parameters.keys()))

        return infra.lambda_function.handler.apply(check_entrypoint)

Here, we use Python’s introspection capabilities to import a module based on a string that we obtain from the Lambda function definition. Similarly, after we import the module, we can verify that the function is callable and has the appropriate arguments for a Lambda function. At this point, we’ve confirmed that we’ve correctly “wired in” a function handler. Separately, we would expect another set of unit tests that verifies the correctness of the handler itself.

Here’s the corresponding infrastructure code to make the test pass, assuming there’s also a file called lambda_function.py, which contains a function handler(event, context).


import pulumi
import pulumi_aws as aws

lambda_role = aws.iam.Role('example-role', assume_role_policy=f"""{{
        "Effect": "Allow",
                        "Service": "lambda.amazonaws.com"
        "Action": "sts:AssumeRole"

attach_lambda_role_execution = aws.iam.RolePolicyAttachment('example-attach-execute',

module_name = 'lambda_function'
module_file = f'{module_name}.py'
lambda_function = aws.lambda_.Function('example-function',
        module_file: pulumi.FileAsset(module_file)

If lambda_function.py doesn’t exist or there’s no function handler within that module, the test will fail.

File Assets

Another example of checking for underlying asset correctness is when providing files to a resource. For example, we might want to check that the content type we provide to an object matches that of the resource:


# Test that the bucket object matches the correct mimetype
def test_bucket_object_content_type(self):
    def check_filetype(args):
        source, mimetype = args
        import filetype
        kind = filetype.guess(source.path)
        self.assertEqual(kind.mime, mimetype)

    return pulumi.Output.all(infra.bucket_obj.source, infra.bucket_obj.content_type).apply(check_filetype)

Here, we use the filetype module to guess at the type based on reading the file itself. We then check that the mime type specified on the bucket object matches.

For example, if the file represented by image.png isn’t a PNG, this test would fail:


bucket = aws.s3.Bucket('my-bucket')

bucket_obj = aws.s3.BucketObject('my-bucket-obj',


When working with secrets, we want to ensure that provided values are secret and that corresponding outputs are secret. This helps ensure that our state never contains secrets as plaintext values. We can write a simple test to check this:


# Test that the secret input is marked as secret
def test_secret_input_and_output(self):

If we try to write something like the following in our code, it’ll fail this test:


secret_password = aws.secretsmanager.Secret('example-secret')
secret_password_v1 = aws.secretsmanager.SecretVersion('example-secret-version',

If we change secret_string='example-string' to secret_string=pulumi.Output.secret('example-string') our test will pass.


There are many other examples of resources where it’s useful to check the underlying properties of the input assets. For example, checking the validity of certificates, keys, files, and container images are all assets that benefit from making sure we’re wiring in resources correctly to our cloud infrastructure. We hope you take this opportunity to use Pulumi’s unit testing framework to check your infrastructure’s underlying assets for correctness. Please visit the unit testing guide to learn more.

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