How can I perform a server-side dry run using create_namespaced_custom_object in the Python Kubernetes client?

Below is an explanation with an example on how to perform a server-side dry run using the Python Kubernetes client:

Explanation

The Kubernetes API server exposes a query parameter named dryRun that accepts a string value. For performing a server-side dry run, you need to pass the value "All" for this parameter. This tells the API server to validate your custom resource definition (CRD) without actually persisting it.

The Python client’s function create_namespaced_custom_object supports the dry_run parameter. Even if you only see two possible values (one for a normal create and the other for a dry run), the only valid value for triggering a server-side dry run is "All". In contrast, omitting this parameter or passing None will cause the object to be persisted if there are no other errors.

Example Code

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from kubernetes import client, config

# Load kube-config from your environment
config.load_kube_config()

# Create an instance of the CustomObjectsApi
api_instance = client.CustomObjectsApi()

# Define your custom resource parameters
group = "example.com"          # The group of your CRD
version = "v1"                 # The version of your CRD
namespace = "default"          # Namespace in which to create the custom resource
plural = "mycustomresources"   # Plural name of the CRD

# Your custom resource object (dictionary)
body = {
    "apiVersion": f"{group}/{version}",
    "kind": "MyCustomResource",
    "metadata": {"name": "example-resource"},
    "spec": {
        # Your resource spec details here
    }
}

# Perform a server-side dry run (no object will be persisted)
try:
    response = api_instance.create_namespaced_custom_object(
        group=group,
        version=version,
        namespace=namespace,
        plural=plural,
        body=body,
        dry_run="All"   # <-- This triggers the dry run
    )
    print("Dry run successful. Server response:")
    print(response)
except client.exceptions.ApiException as e:
    print("Exception when calling CustomObjectsApi->create_namespaced_custom_object:")
    print(e)

Summary

• Server-Side Dry Run: Use the query parameter dry_run="All".
• Behavior: The API server will validate your object as if it were being created but without saving any changes.
• Python Client Limitation: The Python Kubernetes client reflects the underlying API behavior, which only accepts "All" as a valid value for a server-side dry run on creation operations.

This approach ensures your CRD and the custom resource object are validated by the API server before any changes are applied to your cluster.