Seldon Python Component

To create your component to run under Seldon you should create a class that implements the signatures needed for the type of component you are creating.


To wrap your machine learning model create a Class that has a predict method with the following signature:

def predict(self, X: np.ndarray, names: Iterable[str], meta: Dict = None) -> Union[np.ndarray, List, str, bytes]:

Your predict method will receive a numpy array X with iterable set of column names (if they exist in the input features) and optional Dictionary of meta data. It should return the result of the prediction as either:

  • Numpy array

  • List of values

  • String or Bytes

A simple example is shown below:

class MyModel(object):
    Model template. You can load your model parameters in __init__ from a location accessible at runtime

    def __init__(self):
        Add any initialization parameters. These will be passed at runtime from the graph definition parameters defined in your seldondeployment kubernetes resource manifest.

    def predict(self, X, features_names=None):
        Return a prediction.

        X : array-like
        feature_names : array of feature names (optional)
        print("Predict called - will run identity function")
        return X

Returning class names

You can also provide a method to return the column names for your prediction with a method class_names with signature show below:

def class_names(self) -> Iterable[str]:


You can follow various notebook examples.


Seldon Core allows you to create components to transform features either in the input request direction (input transformer) or the output response direction (output transformer). For these components create methods with the signatures below:

def transform_input(self, X: np.ndarray, names: Iterable[str], meta: Dict = None) -> Union[np.ndarray, List, str, bytes]:

def transform_output(self, X: np.ndarray, names: Iterable[str], meta: Dict = None) -> Union[np.ndarray, List, str, bytes]:


Seldon Core allows you to create components that combine responses from multiple models into a single response. To create a class for this add a method with signature below:

def aggregate(self, features_list: List[Union[np.ndarray, str, bytes]], feature_names_list: List) -> Union[np.ndarray, List, str, bytes]:

A simple example that averages a set of responses is shown below:

import numpy as np
logger = logging.getLogger(__name__)

class ImageNetCombiner(object):

    def aggregate(self, Xs, features_names):
        return (np.reshape(Xs[0],(1,-1)) + np.reshape(Xs[1], (1,-1)))/2.0


Routers provide functionality to direct a request to one of a set of child components. For this you should create a method with signature as shown below that returns the id for the child component to route the request to. The id is the index of children connected to the router.

def route(self, features: Union[np.ndarray, str, bytes], feature_names: Iterable[str]) -> int:

To see examples of this you can follow the various example routers that are part of Seldon Core.

Adding Custom Metrics

To return metrics associated with a call create a method with signature as shown below:

def metrics(self) -> List[Dict]:

This method should return a Dictionary of metrics as described in the custom metrics docs.

An illustrative example is shown below:

class ModelWithMetrics(object):

    def __init__(self):

    def predict(self,X,features_names):
        print("Predict called")
        return X

    def metrics(self):
        return [
            {"type": "COUNTER", "key": "mycounter", "value": 1}, # a counter which will increase by the given value
            {"type": "GAUGE", "key": "mygauge", "value": 100},   # a gauge which will be set to given value
            {"type": "TIMER", "key": "mytimer", "value": 20.2},  # a timer which will add sum and count metrics - assumed millisecs

Note: prior to Seldon Core 1.1 custom metrics have always been returned to client. From SC 1.1 you can control this behaviour setting INCLUDE_METRICS_IN_CLIENT_RESPONSE environmental variable to either true or false. Despite value of this environmental variable custom metrics will always be exposed to Prometheus.

Prior to Seldon Core 1.1.0 not implementing custom metrics logs a message at the info level at each predict call. Starting with Seldon Core 1.1.0 this is logged at the debug level. To supress this warning implement a metrics function returning an empty list:

def metrics(self):
    return []

Returning Tags

If we wish to add arbitrary tags to the returned metadata you can provide a tags method with signature as shown below:

def tags(self) -> Dict:

A simple example is shown below:

class ModelWithTags(object):

    def predict(self,X,features_names):
        return X

    def tags(self,X):
        return {"system":"production"}

REST Health Endpoint

If you wish to add a REST health point, you can implement the health_status method with signature as shown below:

def health_status(self) -> Union[np.ndarray, List, str, bytes]:

You can use this to verify that your service can respond to HTTP calls after you have built your docker image and also as kubernetes liveness and readiness probes to verify that your model is healthy.

A simple example is shown below:

class ModelWithHealthEndpoint(object):
    def predict(self, X, features_names):
        return X

    def health_status(self):
        response = self.predict([1, 2], ["f1", "f2"])
        assert len(response) == 2, "health check returning bad predictions" # or some other simple validation
        return response

When you use seldon-core-microservice to start the HTTP server, you can verify that the model is up and running by checking the /health/status endpoint:

$ curl localhost:5000/health/status

Additionally, you can also use the /health/ping endpoint if you want a lightweight call that just checks that the HTTP server is up:

$ curl localhost:5000/health/ping

You can also override the default liveness and readiness probes and use HTTP health endpoints by adding them in your SeldonDeployment YAML. You can modify the parameters for the probes to suit your reliability needs without putting too much stress on the container. Read more about these probes in the kubernetes documentation. An example is shown below:

kind: SeldonDeployment
  name: my-app
  - componentSpecs:
    - spec:
        - image: my-app-image:version
          name: classifier
            failureThreshold: 3
            initialDelaySeconds: 60
            periodSeconds: 5
            successThreshold: 1
              path: /health/status
              port: http
              scheme: HTTP
            timeoutSeconds: 1
            failureThreshold: 3
            initialDelaySeconds: 20
            periodSeconds: 5
            successThreshold: 1
              path: /health/status
              port: http
              scheme: HTTP
            timeoutSeconds: 1

Low level Methods

If you want more control you can provide a low-level methods that will provide as input the raw proto buffer payloads. The signatures for these are shown below for release sedon_core>=

def predict_raw(self, msg: prediction_pb2.SeldonMessage) -> prediction_pb2.SeldonMessage:

def send_feedback_raw(self, feedback: prediction_pb2.Feedback) -> prediction_pb2.SeldonMessage:

def transform_input_raw(self, msg: prediction_pb2.SeldonMessage) -> prediction_pb2.SeldonMessage:

def transform_output_raw(self, msg: prediction_pb2.SeldonMessage) -> prediction_pb2.SeldonMessage:

def route_raw(self, msg: prediction_pb2.SeldonMessage) -> prediction_pb2.SeldonMessage:

def aggregate_raw(self, msgs: prediction_pb2.SeldonMessageList) -> prediction_pb2.SeldonMessage:

def health_status_raw(self) -> prediction_pb2.SeldonMessage:

User Defined Exceptions

If you want to handle custom exceptions define a field model_error_handler as shown below:

model_error_handler = flask.Blueprint('error_handlers', __name__)

An example is as follows:

Model Template
class MyModel(Object):

    The field is used to register custom exceptions
    model_error_handler = flask.Blueprint('error_handlers', __name__)

    Register the handler for an exception
    def handleCustomError(error):
        response = jsonify(error.to_dict())
        response.status_code = error.status_code
        return response

    def __init__(self, metrics_ok=True, ret_nparray=False, ret_meta=False):

    def predict(self, X, features_names, **kwargs):
        raise UserCustomException('Test-Error-Msg',1402,402)
        return X
User Defined Exception
class UserCustomException(Exception):

    status_code = 404

    def __init__(self, message, application_error_code,http_status_code):
        self.message = message
        if http_status_code is not None:
            self.status_code = http_status_code
        self.application_error_code = application_error_code

    def to_dict(self):
        rv = {"status": {"status": self.status_code, "message": self.message,
                         "app_code": self.application_error_code}}
        return rv

Multi-value numpy arrays

By default, when using the data ndarray parameter, the conversion to ndarray (by default) converts all inner types into the same type. With models that may take as input arrays with different value types, you will be able to do so by overriding the predict_raw function yourself which gives you access to the raw request, and creating the numpy array as follows:

import numpy as np

class Model:
    def predict_raw(self, request):
        data = request.get("data", {}).get("ndarray")
        if data:
            mult_types_array = np.array(data, dtype=object)

        # Handle other data types as required + your logic

Gunicorn and load

If the wrapped python class is served under Gunicorn then as part of initialization of each gunicorn worker a load method will be called on your class if it has it. You should use this method to load and initialise your model. This is important for Tensorflow models which need their session created in each worker process. The Tensorflow MNIST example does this.

import tensorflow as tf
import numpy as np
import os

class DeepMnist(object):
    def __init__(self):
        self.loaded = False
        self.class_names = ["class:{}".format(str(i)) for i in range(10)]

    def load(self):
        print("Loading model",os.getpid())
        self.sess = tf.Session()
        saver = tf.train.import_meta_graph("model/deep_mnist_model.meta")
        graph = tf.get_default_graph()
        self.x = graph.get_tensor_by_name("x:0")
        self.y = graph.get_tensor_by_name("y:0")
        self.loaded = True
        print("Loaded model")

    def predict(self,X,feature_names):
        if not self.loaded:
        predictions =,feed_dict={self.x:X})
        return predictions.astype(np.float64)

Integer numbers

The json package in Python, parses numbers with no decimal part as integers. Therefore, a tensor containing only numbers without a decimal part will get parsed as an integer tensor.

To illustrate the above, we can consider the following example:

  "data": {
    "ndarray": [0, 1, 2, 3]

By default, the json package will parse the array in the data.ndarray field as an array of Python Integer values. Since there are no floating-point values, numpy will then create a tensor with dtype = np.int32.

If we want to force a different behaviour, we can use the underlying predict_raw() method to control the deserialisation of the input payload. As an example, using the example above, we could force the resulting tensor to always using dtype = np.float64 by implementing predict_raw() as:

import numpy as np

class Model:
    def predict_raw(self, request):
        data = request.get("data", {}).get("ndarray")
        if data:
            float_array = np.array(data, dtype=np.float64)

        # Make predictions using float_array

Incubating features

REST Metadata Endpoint

The python wrapper will automatically expose a /metadata endpoint to return metadata about the loaded model. It is up to the developer to implement a metadata method in their class to provide a dict back containing the model metadata.

See metadata documentation for more details.

Example format:

class Model:

    def init_metadata(self):

        meta = {
            "name": "model-name",
            "versions": ["model-version"],
            "platform": "platform-name",
            "inputs": [{"name": "input", "datatype": "BYTES", "shape": [1]}],
            "outputs": [{"name": "output", "datatype": "BYTES", "shape": [1]}],

        return meta


Output of developer-defined metadata method will be validated to follow the kfserving dataplane proposal protocol, see this GitHub issue for details:

$metadata_model_response =
  "name" : $string,
  "versions" : [ $string, ... ] #optional,
  "platform" : $string,
  "inputs" : [ $metadata_tensor, ... ],
  "outputs" : [ $metadata_tensor, ... ]


$metadata_tensor =
  "name" : $string,
  "datatype" : $string,
  "shape" : [ $number, ... ]

If validation fails server will reply with 500 response MICROSERVICE_BAD_METADATA when requested for metadata.

Next Steps

After you have created the Component you need to create a Docker image that can be managed by Seldon Core. Follow the documentation to do this with s2i or Docker.