Developer guide


What is a task ?

A task is a fundamental object used in any Brick. It is one of the main object of the workflow that you can find in the playground.
A task is a special function that works with Constellab object. It contains a run method to execute any code. It takes resources as inputs and resources as outputs. It also takes a configuration object.
For example, the object TableTransposer is a task that takes a Table as input, transpose it rand return the transposed table as output.
Tasks can be connected to each other to be executed, this is what we called a protocol.

How to create a task ?

To create a new task in your brick, you need to create a new class that:

  • Extend the Task class (import from gws_core)
  • Is decorated with the @task_decorator (import from gws_core)
  • Define the input_specs, output_specs and config_specs

Here is an example of our RobotMove task. This task takes a Robot resource as input, update its positions based on the configuration and return the Robot.

from gws_core import (ConfigParams, FloatParam, InputSpec, OutputSpec,
                      StrParam, Task, TaskInputs, TaskOutputs, task_decorator)
from .robot_resource import Robot  # local import

@task_decorator("RobotMove", human_name="Move robot",
                short_description="This task emulates a short moving step of the robot", hide=True)
class RobotMove(Task):
    input_specs = {'robot': InputSpec(Robot, human_name="Robot",  short_description="The robot to move")}
    output_specs = {'robot': OutputSpec(Robot)}
    config_specs = {
                   'moving_step': FloatParam(default_value=0.1, short_description="The moving step of the robot"),
                   'direction': StrParam( default_value="north", allowed_values=["north", "south", "east", "west"], short_description="The                   moving direction")

    def run(self, params: ConfigParams, inputs: TaskInputs) -> TaskOutputs:
        print(f"Moving {params.get_value('moving_step')}", flush=True)
        robot: Robot = inputs['robot']
        robot.move(direction=params.get_value('direction'), moving_step=params.get_value('moving_step'))
        return {'robot': robot}

WARNING: the unique_name parameters must never be changed once the brick is released. It must be unique in the brick and it is the only way to retrieve the task. If you change it, it will be considered as a new task type and the previous unique_name will no longer exist (the system won't be able to retrieve tasks generated with the old unique_name). You can still update other parameters including python class name, human name or short description.
We recommend that one python file contains only one task and the name of the file is the name of the task with undercores. Here the file is named robot_move.py

Run method

The run method is the method executed when the system runs the task. You must right your code logic for this task in this method.
Some specific tasks have different methods with different parameters like Transformers, Importer, Exporter, ShellTask. See the corresponding section for more information about theses specific tasks.
The run method has 2 parameters

  • params of type ConfigParams : this is a dict object that contains the values of the configuration provided by the user. See Configuration for more information.
  • inputs of type TaskInputs : this is a dict object that contains all the resources provided as input of the task. See Input & Outputs for more information.

The run method must return a TaskOutputs which is a dictionary of resources.
The task will be executed in a protocol, so the outputs of one task can be connected to the one or more inputs of the next task. With this you can pass resources that a task generated to another task. 

Inputs & Outputs

When we create a task we need to define what are the task IO (inputs and outputs) and the type of each IO. A task takes only Resource as IO so the type provided in the IO must be Resource. When the task is run the inputs are provided in the run method.
To define the specifications of the task IO we must define the input_specs and output_specs.


The input_specs object is a dictionary of InputSpec. The keys of the dictionary are important because it helps you retrieve the input in the run method. In the above example we retrieve the Robot resource by calling inputs['robot'] robot Is the key define in the input_specs.
The InputSpec object help you to define your inputs. It takes multiple parameters:

  • resource_types: a resource type or a List of resource types. If 1 type is provided (recommended), the system only accepts this type and sub-classes for this input. If multiple types are provided, the system will accept any of this classes and sub-classes. Before running the task, the system checks the inputs and if 1 input is not compatible the task is not run.
  • human_name: pretty name that will be displayed in the playground.
  • short_description: small description to add information about this input (also for the playground).
  • is_optional: this input might not be connected to another task output and the task will still be executed. If the input is connected, the system will wait for the input to be provided before running the task. Also tells that None value is allowed as input.  

The system will not run the task unless all the input are connected (to a previous task’s outputs) and each input received a resource (see the is_optional options to see how you can adjust that). So you know that the inputs exist when a task is run.


The output_specs object is a dictionary of OutputSpec. The keys of the dictionary are important because it must match the keys of the returned object in the run method. For example in the RobotMove task, we returned : return {'robot': robot} which means we return the resource robot with the key robot. The robot key is the same as defined in the output_specs of the task.
The OutputSpec object help you to define your outputs. It takes multiple parameters (some like InputSpec):

  • resource_types: a resource type or a List of resource types. If 1 type is provided (recommended), it tells the system that this task returns a specific type(and sub-classes) for this output. If multiple it tells that the task will return one of the provided type (and sub-classes) for this output. 
  • human_name: pretty name that will be displayed in the playground.
  • short_description: small description to add information about this output (also for the playground).
  • is_optional: tell that this output may return None or not being provided.  
  • is_constant (for expert): When true, it tells the system that the output resource was not modified from the input resource. The system will not need to create a new resource after the task.
  • sub_class (for expert): When true, it tells that the resource_types are compatible with any child class of the provided resource type (useful for task that work with generic resource type).

Note: By default all the ressources returned by a task are considered as new resources and will be saved after the task was executed and before the next task. So, all the input resources of a task are already saved in the database. You can modify an input resource, its representation in the database will not be affected and a new resource will be created with the modified information after the task. This is what is happening when we call robot.move in the RobotMove task exemple above. 


The config_specs attribute allows you to expose configuration to the playground. You can define multiple parameters with different type and this will generate a form available in the playground when the user uses the task.
The config_specs object is a dictionary of Param. The keys are important because it helps you retrieve the parameters value from the params object (first parameter of the run method). In the above example we retrieve the two parameters’ values direction and moving_step by calling params.get_value('direction') moving_step=params.get_value('moving_step'). The params object is a python dict with additional methods.
There are different parameter types (int, float, bool, str..) but they all have common options when we define a new parameter:

  • human_name: pretty name that will be displayed in the playground when configuring the task.
  • short_description: small description to add information about this parameter (also for the playground).
  • default_value: default value to use when the user did not provide a value for this parameter. If a default value is provided, it automatically set the optional to True. If not provided and the optional is set to False, the parameter is mandatory. If you want to set a default value to None, set optional to True.
  • optional: define the parameters as optional. If default_value is not provided and the user did not define this parameter, the value will be None
  • visibility
    • public : main param visible in the first config section in the playground
    • protected : considered as advanced param, it will be in the advanced section in the playground. It must have a default value or be optional.
    • private (for expert) : it will not be visible in the playground. It must have a default value or be optional)
  • allowed_values: list of allowed value for the parameter. It will be rendered as a select in the playground
  • unit: define the measure unit nit of this parameter

Here is the list of all the basic param types with additional options for some of them:

  • StrParam: string param. It will show a basic text input in the playground. Use allowed value to restrict possible values
    • min_length: minimal length for the provided text
    • max_length: maximal length for the provided text
  • IntParam & FloatParam : numeric param. It will show an input with only number possible.
    • min_value: minimum value for the param
    • max_value: maximum value for the param
  • BoolParam: Boolean param. It will show a checkbox.
  • ListParam: List param. Show a text input in the playground where the user can provide multiple values. The value for this param will be a list of strings. It also works with allowed_value, to allow user to select multiple values from a list of choices.
  • DictParam (for expert) : only to use for hidden parameters (it has no representation in the playground).
  • TagsParams : Show a special input to select multiple tags. The value of the parameter will be a list of tags. 


Use to define a group of parameters that can be added multiple times. The ParamSet contains a dictionary of parameters. Use this when there are 2 or more configurations that are related and you want the user to be able to provided multiple combinations of theses parameters.
When not optional, the user will need to provide at least one value (a combination) in the playground.
Use the max_number_of_occurence attribute to limit the number of combinations the user can provide.
The value of this parameter will be a list of dictionaires.

Task Special methods

Log messages

There are multiple method that exist to write logs during the task. The logs are available in the playground and are update in real time. The logs are really usefull when the task takes some time and it is also possible to log progress.
Here are the log methods:

  • update_progress_value: log a progress message and update the progress value (value from 0 to 100). Ex: update_progress_value(10, 'Reached 10%')
  • log_info_message: log an information message
  • log_success_message: log a success message
  • log_warning_message: log a warning message
  • log_error_message: log an error message
  • log_message: log a message with the type in parameter

You can call theses methods in the run methods like this : self.log_info_message('Hello world').
If 2 messages are logs almost instantly, the 2 messages are merges.


This method can be overwritten to perform custom check before running task. It must return a CheckBeforeTaskResult that contains 2 objects :

  • result: boolean
    • If True, everything is ok
    • If False, the task will not be executed after this check it might be run later if they are some SKippableIn inputs. If all the input values were provided and the check retuns False. the task will endup in error because it won't be run
  • message: Optional[str]
    • If False, a message can be provided to log the error message if the task will not be called


This method can be overwritten to perform action after the task run. This method is called after the resource save. This method is useful to delete temporary objects (like files) to clear the server after the task is run.

Test the task

There are 2 ways of testing the task and we recommend to do both of them:

  • Write unit test
  • Test in dev environment

Write unit test

Writing a unit test in the best and easiest way to test your task and make sure it still works after some changes (you change it or update a dependency for example).
Create a new test file in the tests folder. The name of the file must start with test_.
We recommend to create 1 test file per task and to name the test file the same name as the task file name. For example we will create the test file test_robot_move.py
Here is a example on how to test the RobotMove task.

from gws_core import TaskRunner
from gws_core.impl.robot.robot_resource import \
    Robot  # local import of the resource to test
from gws_core.impl.robot.robot_tasks import \
    RobotMove  # local import of the task to test
from unittest import TestCase

class TestRobotMove(TestCase):
    def test_robot_move(self):
        # create an empty robot with position [0,0]
        robot_output = Robot.empty()
        # create a task runner and configure it, then run it
        runner = TaskRunner(
            params={'moving_step': 3, 'direction': 'south'},
            inputs={'robot': robot_output},
        outputs = await runner.run()

        # retrieve the robot output
        robot_output: Robot = outputs['robot']

        # check the robot position, y should be -3 as he went south
        self.assertEqual(robot_output.position, [0, -3])

Let's break down this example.
Firstly we declared the class TestRobotMove that extend the TestCase. The test class must extend the TestCase to be considered as a test file by VsCode.
Note: if your tests requires the database tables (with some SQL request) you will have to extend the BaseTestCase class from gws_core. This class will automatically creates all the tables required by the gws environment. The table are created once for the entire class file and are cleaned and the end of the test file (not between each tests). With this class the test environment is almost the same as the real environment. The downside is that the creation of the tables and the loading of the complete gws environment is slow and this will make your test significantly slower. Use this class only if your making advanced tests that requires the database.
We manually create our Robot resource.
The TaskRunner is the main part of this test. This class allows you to run a task manually with custom config and input and retrieve the outputs. Then we create the task runner by passing the task type RobotMove, the configuration we want to test, and the input (the previously created robot).
Note: the import of the RobotMove class is an absolute import and not a relative one. This is because the tests folder is output the scope of the brick package. All the import in you test file must be aboslute from the brick name. VsCode will complain about the absolute import but you can ignore the errors.
Note: keys of the params and inputs (as well as output) must match the keys of the specs defined in the task.
Then we run the task by calling the runner.run() , this method will return a TaskOutputs. We can then retrieve the robot from the outputs and check that the value of the robot is correct. Here we check that the position of the robot was changed.
To learn how to run the test and debug it, see the following link.
Dev environment > debug-tests
You can also test the check_before_run method with the task runner. It may also be necessary to call the run_after_task at the end of your test if your task generated side data that need to be cleared.

Test in dev environment

Once you unit tests are ok, you can test your task in the dev environment. See the following link:
Dev environment > start-dev-server

Document the task

Once your task is developed and tested, we recommend to document it. Here is a list of things you can do to improve the documentation of your task:

  • Define the human_name and short_description in the @task_decorator
  • Define the human_name and short_description in the input_specs, output_specs and config_specs
  • Write comment in the python code of your classe.

You can write complete documentation using python comment. It supports the markdown syntax: (https://www.markdownguide.org/basic-syntax/). This documentation is really important and it will be available in Constellab community and in the playground. Here is an example on how to do it.

class RobotMove(Task):
    This is a documentation that support **markdown** syntax. 

    And this documentation will be available in 
    - the hub 
    - the playground.

Hint: to write your markdown, you can create an .md file in vscode open edit it. VsCode can show you the preview directly. Then copy the content back to your python code.
Hint: you can check your documentation in the dev environment by searching for your task in the lab en opening the documentation. Warning, the documentation of the task that were already added in the playground may not be up to date.

Deploy your task

Once you created your test and write your documentation, you can deploy your task in a new version of your brick. To do so, check the following link:
Deploy a brick version

Use external data

If your task rely on one or more external data (like a database) that needs to be downloaded first you can use the TaskFileDownloader. This class let you download external file resource (using http protocol) and store it in a specific folder for your brick.
To create the TaskFileDownloder from your task simple do the following :

# create the file_downloader from a task.
file_downloader = TaskFileDownloader(MyTaskClass.get_brick_name(), self.message_dispatcher)

# download a file
file_path = file_downloader.download_file_if_missing("my_file_url", "file_name.json")

It is recommended to use MyTaskClass.get_brick_name() to retrieve the task brick name because this defines the destination of the downloader file. With this, the destination will be the same even is your task is overriden by another task (and the file will not be duplicated).
When you download a file you must provide the name of the file once downloaded. This name must be unique for your brick. If a TaskFileDownloader tries to download a file with the same name, it considers that the file has already been downloaded and will not download it. If you want to force the download of a file (new version for example), change the filename (adding v2 for example)
The self.message_dispatcher passes the task message_dispatcher to the file downloader. With this, the logs generated by file downloader will be available in the task.
The external downloaded data can be viewed directly in the lab in the Monitoring > Brick data page.
In this page you can delete manually a data so it will be force to be re-downloaded next time the task is executed.