Assertions

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End-of-Support

AWS CDK v1 has reached End-of-Support on 2023-06-01. This package is no longer being updated, and users should migrate to AWS CDK v2.

For more information on how to migrate, see the Migrating to AWS CDK v2 guide.

If you’re migrating from the old assert library, the migration guide can be found in our GitHub repository.

Functions for writing test asserting against CDK applications, with focus on CloudFormation templates.

The Template class includes a set of methods for writing assertions against CloudFormation templates. Use one of the Template.fromXxx() static methods to create an instance of this class.

To create Template from CDK stack, start off with:

from aws_cdk.core import Stack
from aws_cdk.assertions import Template

stack = Stack()
# ...
template = Template.from_stack(stack)

Alternatively, assertions can be run on an existing CloudFormation template -

template_json = "{ "Resources": ... }" # The CloudFormation template as JSON serialized string.
template = Template.from_string(template_json)

Full Template Match

The simplest assertion would be to assert that the template matches a given template.

template.template_matches({
    "Resources": {
        "BarLogicalId": {
            "Type": "Foo::Bar",
            "Properties": {
                "Baz": "Qux"
            }
        }
    }
})

By default, the templateMatches() API will use the an ‘object-like’ comparison, which means that it will allow for the actual template to be a superset of the given expectation. See Special Matchers for details on how to change this.

Snapshot testing is a common technique to store a snapshot of the output and compare it during future changes. Since CloudFormation templates are human readable, they are a good target for snapshot testing.

The toJSON() method on the Template can be used to produce a well formatted JSON of the CloudFormation template that can be used as a snapshot.

See Snapshot Testing in Jest and Snapshot Testing in Java.

Counting Resources

This module allows asserting the number of resources of a specific type found in a template.

template.resource_count_is("Foo::Bar", 2)

Resource Matching & Retrieval

Beyond resource counting, the module also allows asserting that a resource with specific properties are present.

The following code asserts that the Properties section of a resource of type Foo::Bar contains the specified properties -

template.has_resource_properties("Foo::Bar", {
    "Foo": "Bar",
    "Baz": 5,
    "Qux": ["Waldo", "Fred"]
})

Alternatively, if you would like to assert the entire resource definition, you can use the hasResource() API.

template.has_resource("Foo::Bar", {
    "Properties": {"Foo": "Bar"},
    "DependsOn": ["Waldo", "Fred"]
})

Beyond assertions, the module provides APIs to retrieve matching resources. The findResources() API is complementary to the hasResource() API, except, instead of asserting its presence, it returns the set of matching resources.

By default, the hasResource() and hasResourceProperties() APIs perform deep partial object matching. This behavior can be configured using matchers. See subsequent section on special matchers.

Output and Mapping sections

The module allows you to assert that the CloudFormation template contains an Output that matches specific properties. The following code asserts that a template contains an Output with a logicalId of Foo and the specified properties -

expected = {
    "Value": "Bar",
    "Export": {"Name": "ExportBaz"}
}
template.has_output("Foo", expected)

If you want to match against all Outputs in the template, use * as the logicalId.

template.has_output("*", {
    "Value": "Bar",
    "Export": {"Name": "ExportBaz"}
})

findOutputs() will return a set of outputs that match the logicalId and props, and you can use the '*' special case as well.

result = template.find_outputs("*", {"Value": "Fred"})
expect(result.Foo).to_equal({"Value": "Fred", "Description": "FooFred"})
expect(result.Bar).to_equal({"Value": "Fred", "Description": "BarFred"})

The APIs hasMapping(), findMappings(), hasCondition(), and hasCondtions() provide similar functionalities.

Special Matchers

The expectation provided to the hasXxx(), findXxx() and templateMatches() APIs, besides carrying literal values, as seen in the above examples, also accept special matchers.

They are available as part of the Match class.

Object Matchers

The Match.objectLike() API can be used to assert that the target is a superset object of the provided pattern. This API will perform a deep partial match on the target. Deep partial matching is where objects are matched partially recursively. At each level, the list of keys in the target is a subset of the provided pattern.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": {
#           "Wobble": "Flob",
#           "Bob": "Cat"
#         }
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.object_like({
        "Wobble": "Flob"
    })
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.object_like({
        "Brew": "Coffee"
    })
})

The Match.objectEquals() API can be used to assert a target as a deep exact match.

Presence and Absence

The Match.absent() matcher can be used to specify that a specific value should not exist on the target. This can be used within Match.objectLike() or outside of any matchers.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": {
#           "Wobble": "Flob",
#         }
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.object_like({
        "Bob": Match.absent()
    })
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.object_like({
        "Wobble": Match.absent()
    })
})

The Match.anyValue() matcher can be used to specify that a specific value should be found at the location. This matcher will fail if when the target location has null-ish values (i.e., null or undefined).

This matcher can be combined with any of the other matchers.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": {
#           "Wobble": ["Flob", "Flib"],
#         }
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": {
        "Wobble": [Match.any_value(), Match.any_value()]
    }
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": {
        "Wimble": Match.any_value()
    }
})

Array Matchers

The Match.arrayWith() API can be used to assert that the target is equal to or a subset of the provided pattern array. This API will perform subset match on the target.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": ["Flob", "Cat"]
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.array_with(["Flob"])
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", Match.object_like({
    "Fred": Match.array_with(["Wobble"])
}))

Note: The list of items in the pattern array should be in order as they appear in the target array. Out of order will be recorded as a match failure.

Alternatively, the Match.arrayEquals() API can be used to assert that the target is exactly equal to the pattern array.

String Matchers

The Match.stringLikeRegexp() API can be used to assert that the target matches the provided regular expression.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Template": "const includeHeaders = true;"
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Template": Match.string_like_regexp("includeHeaders = (true|false)")
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Template": Match.string_like_regexp("includeHeaders = null")
})

Not Matcher

The not matcher inverts the search pattern and matches all patterns in the path that does not match the pattern specified.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": ["Flob", "Cat"]
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Fred": Match.not(["Flob"])
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", Match.object_like({
    "Fred": Match.not(["Flob", "Cat"])
}))

Serialized JSON

Often, we find that some CloudFormation Resource types declare properties as a string, but actually expect JSON serialized as a string. For example, the BuildSpec property of AWS::CodeBuild::Project, the Definition property of AWS::StepFunctions::StateMachine, to name a couple.

The Match.serializedJson() matcher allows deep matching within a stringified JSON.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Baz": "{ "Fred": ["Waldo", "Willow"] }"
#       }
#     }
#   }
# }

# The following will NOT throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Baz": Match.serialized_json({
        "Fred": Match.array_with(["Waldo"])
    })
})

# The following will throw an assertion error
template.has_resource_properties("Foo::Bar", {
    "Baz": Match.serialized_json({
        "Fred": ["Waldo", "Johnny"]
    })
})

Capturing Values

The matcher APIs documented above allow capturing values in the matching entry (Resource, Output, Mapping, etc.). The following code captures a string from a matching resource.

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": ["Flob", "Cat"],
#         "Waldo": ["Qix", "Qux"],
#       }
#     }
#   }
# }

fred_capture = Capture()
waldo_capture = Capture()
template.has_resource_properties("Foo::Bar", {
    "Fred": fred_capture,
    "Waldo": ["Qix", waldo_capture]
})

fred_capture.as_array() # returns ["Flob", "Cat"]
waldo_capture.as_string()

With captures, a nested pattern can also be specified, so that only targets that match the nested pattern will be captured. This pattern can be literals or further Matchers.

# Given a template -
# {
#   "Resources": {
#     "MyBar1": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": ["Flob", "Cat"],
#       }
#     }
#     "MyBar2": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": ["Qix", "Qux"],
#       }
#     }
#   }
# }

capture = Capture(Match.array_with(["Cat"]))
template.has_resource_properties("Foo::Bar", {
    "Fred": capture
})

capture.as_array()

When multiple resources match the given condition, each Capture defined in the condition will capture all matching values. They can be paged through using the next() API. The following example illustrates this -

# Given a template -
# {
#   "Resources": {
#     "MyBar": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": "Flob",
#       }
#     },
#     "MyBaz": {
#       "Type": "Foo::Bar",
#       "Properties": {
#         "Fred": "Quib",
#       }
#     }
#   }
# }

fred_capture = Capture()
template.has_resource_properties("Foo::Bar", {
    "Fred": fred_capture
})

fred_capture.as_string() # returns "Flob"
fred_capture.next() # returns true
fred_capture.as_string()

Asserting Annotations

In addition to template matching, we provide an API for annotation matching. Annotations can be added via the Aspects API. You can learn more about Aspects here.

Say you have a MyAspect and a MyStack that uses MyAspect:

import aws_cdk.core as cdk
from constructs import Construct, IConstruct

class MyAspect(cdk.IAspect):
    def visit(self, node):
        if node instanceof cdk.CfnResource && node.cfn_resource_type == "Foo::Bar":
            self.error(node, "we do not want a Foo::Bar resource")

    def error(self, node, message):
        cdk.Annotations.of(node).add_error(message)

class MyStack(cdk.Stack):
    def __init__(self, scope, id):
        super().__init__(scope, id)

        stack = cdk.Stack()
        cdk.CfnResource(stack, "Foo",
            type="Foo::Bar",
            properties={
                "Fred": "Thud"
            }
        )
        cdk.Aspects.of(stack).add(MyAspect())

We can then assert that the stack contains the expected Error:

# import { Annotations } from '@aws-cdk/assertions';

Annotations.from_stack(stack).has_error("/Default/Foo", "we do not want a Foo::Bar resource")

Here are the available APIs for Annotations:

hasError(), hasNoError(), and findError()
hasWarning(), hasNoWarning(), and findWarning()
hasInfo(), hasNoInfo(), and findInfo()

The corresponding findXxx() API is complementary to the hasXxx() API, except instead of asserting its presence, it returns the set of matching messages.

In addition, this suite of APIs is compatable with Matchers for more fine-grained control. For example, the following assertion works as well:

Annotations.from_stack(stack).has_error("/Default/Foo",
    Match.string_like_regexp(".*Foo::Bar.*"))