Skip to content

Writing controllers for Bliss

Bliss put no constrains on controllers classes and developers can start from scratch and define everything. However, there are several generic mechanisms, like loading the controller from a YML configuration (plugin) or managing the controller’s counters and axes, which are already defined in Bliss and which can be inherited while writing a new controller class.

BlissController base class

As a base for the implementation of controllers, Bliss provides the BlissController class. This class already implements the plugin mechanisms and is designed to ease the management of sub-objects under a top controller.

Examples of controllers that should inherit from BlissController class:

  • a controller of axes

  • a controller with counters

  • a controller with axes and counter

  • a top-controller (software) managing other sub-controllers (software/hardware)

Example of the YML structure:

- plugin: generic          <== BlissController works with generic plugin
  module: custom_module    <== module of the custom bliss controller
  class: BCMockup          <== class of the custom bliss controller
  name: bcmock             <== name of the custom bliss controller  (optional)

  param_1: value           <== a parameter for the custom
                               bliss controller (optional)

  section_1:               <== a section where subitems config can be
                               declared (ex: 'counters') (optional)
    - name: subitem_1      <== name of a subitem
    - name: subitem_2      <== name of another subitem of the same type

  section_2:               <== another section for another type of
                               subitems (ex: 'axes') (optional)
    - name: subitem_2      <== name of another subitem type

The signature of a BlissController takes a single argument config. It could be a ConfigNode object or a standard dictionary.

class BlissController(CounterContainer):
    def __init__(self, config):

BlissController and subitems

A BlissController subitem is an object managed by the controller and which could have a name declared under a sub-section of the controller’s configuration. Usually subitems are counters and axes but could be anything else (known by the controller only).

  - name: subitem_1   <== a subitem using the default
                          class (defined by the controller)

  - name: subitem_2                    <== a subitem using a given class
    class:        path (from an absolute path)

  - name: subitem_3       <== a subitem using a given class
    class: myitemclass        name (default path known by the controller)

Subitems can be declared in the controller’s YML configuration if they are expected to be directly imported in a user session. If not declared in the YML, they are still accessible via the controller (see BlissController._get_subitem(name)).

To retrieve the subitems that can be identified as counters or axes, BlissController class implements the @counters and @axes properties.

The BlissController identifies the subitem type thanks to the name of the sub-section where the item was found (aka parent_key).

Also, the controller must provides a default class for each kind of parent_key (see BlissController._get_subitem_default_class_name).


def _get_subitem_default_class_name(self, cfg, parent_key):
    if parent_key == "axes":
        return "Axis"
    elif parent_key == "encoders":
        return "Encoder"
    elif parent_key == "shutters":
        return "Shutter"
    elif parent_key == "switches":
        return "Switch"


def _get_subitem_default_class_name(self, cfg, parent_key):
    if parent_key == "counters":
        tag = cfg["tag"]
        if self._COUNTER_TAGS[tag][1] == "scc":
            return "SamplingCounter"
        elif self._COUNTER_TAGS[tag][1] == "icc":
            return "IntegratingCounter"

The default subitem class can be overridden by specifing the class key in its configuration. The class can be given as an absolute path or as a class name.

If providing a class name the controller tries to find the item class first at its module level, else it uses a default path defined by the controller (see BlissController._get_subitem_default_module).


def _get_subitem_default_module(self, class_name, cfg, parent_key):
    if parent_key == "axes":
        return "bliss.common.axis"

    elif parent_key == "encoders":
        return "bliss.common.encoder"

    elif parent_key == "shutters":
        return "bliss.common.shutter"

    elif parent_key == "switches":
        return "bliss.common.switch"


def _get_subitem_default_module(self, class_name, cfg, parent_key):
    if class_name == "IntegratingCounter":
        return "bliss.common.counter"

Bliss controller plugin

BlissControllers are created from the yml configuration using the generic plugin.

The controlelr class is based on the ConfigItemContainer base class which deals with all the mechanisms of the generic plugin.

Any subitem can be imported in a Bliss session with the command config.get('name').

The bliss controller itself can have a name (optional) and can be imported in the session.

The plugin ensures that the controller and subitems are only created once.

The effective creation of subitems is performed by the BlissController itself and the plugin just ensures that the controller is always created before subitems and only once.

The generic plugin will also manage the resolution order of the references to other objects within the BlissController configuration. It handles external and internal references and allows to use a reference for a subitem name.

Example of an advanced configuration using different kind of references:

- plugin: generic
  module: custom_module       
  class: BCMockup             
  name: bcmock                

  custom_param_1: value       
  custom_param_2: $ref1       <== a referenced object for the controller (optional/authorized)

    - name: sub_item_1        
      tag : item_tag_1        
      sub_param_1: value      
      device: $ref2           <== an external reference for this subitem (optional/authorized)

    - name: sub_item_2        
      tag : item_tag_2        
      input: $sub_item_1      <== an internal reference to another subitem owned by the same controller (optional/authorized)

      sub-section-2-1:        <== nested sub-sections are possible (optional)
        - name: sub_item_21
          tag : item_tag_21

  sub-section-3 :             
    - name: $ref3             <== a subitem as an external reference is possible (optional/authorized)
      something: value

Subitem creation

In order to keep the plugin as generic as possible, all the knowledge specfic to the controller is asked by the plugin to the BlissController.

In particular, when the plugin needs to instantiate a subitem it will call the method BlissController._create_subitem_from_config. This abstract method must be implemented and must return the subitem instance.

To be able to decide which instance should be created, the method receives 4 arguments:

  • name: subitem name
  • cfg: subitem config
  • parent_key: name of the subsection where the item was found (in controller’s config)
  • item_class: class of the subitem (see BlissController and sub-items ).
  • item_obj: the object instance of an item referenced in the config (None if not a reference)

If item_class is None it means that the subitem was given as a reference. In that case the object is already instantiated and is contained in item_obj.


def _create_subitem_from_config(self, name, cfg, parent_key, item_class, item_obj=None):

    if parent_key == "axes":
        if item_class is None:  # it means that item was referenced in config, 
            axis = item_obj     # so just grab the item object provided by 'item_obj'
            axis = item_class(name, self, cfg) # instantiate the item using the given class and decide the correct signature

        # === do anything custom here ================

        self._axes[name] = axis 

        axis_tags = cfg.get("tags")
        if axis_tags:
            for tag in axis_tags.split():
                self._tagged.setdefault(tag, []).append(axis)

        if axis.controller is self:
            set_custom_members(self, axis, self._initialize_axis)
            # reference axis
            return axis

        if axis.controller is self:
            axis_initialized = Cache(axis, "initialized", default_value=0)
            self.__initialized_hw_axis[axis] = axis_initialized
            self.__initialized_axis[axis] = False


        # ====================================================

        return axis   # return the created item

    elif parent_key == "encoders":  # deal with an other kind of items

        encoder = self._encoder_counter_controller.create_counter(
            item_class, name, motor_controller=self, config=cfg

        self._encoders[name] = encoder
        self.__initialized_encoder[encoder] = False

        return encoder


def _create_subitem_from_config(self, name, cfg, parent_key, item_class, item_obj=None):
    if parent_key == "counters":
        name = cfg["name"]
        tag = cfg["tag"]
        mode = cfg.get("mode")
        unit = cfg.get("unit")
        convfunc = cfg.get("convfunc")

        if self._COUNTER_TAGS[tag][1] == "scc":
            cnt = self._counter_controllers["scc"].create_counter(
                item_class, name, unit=unit, mode=mode
            cnt.tag = tag

        elif self._COUNTER_TAGS[tag][1] == "icc":
            cnt = self._counter_controllers["icc"].create_counter(
                item_class, name, unit=unit
            cnt.tag = tag

            raise ValueError(f"cannot identify counter tag {tag}")

        return cnt

    elif parent_key == "operators":
        return item_class(cfg)

    elif parent_key == "axes":
        if item_class is None:  # it is a referenced axis (i.e external axis)
            axis = item_obj  # the axis instance
            tag = cfg[
            ]  # ask for a tag which only concerns this ctrl (local tag)
            self._tag2axis[tag] = name  # store the axis tag
            return axis
            raise ValueError(
                f"{self} only accept referenced axes"
            )  # reject none-referenced axis

Nested BlissControllers

A top-bliss-controller can have multiple sub-bliss-controllers. In that case there are two ways to create the sub-bliss-controllers:

The most simple way to do this is to declare a sub-bliss-controller as an independant object with its own yml config and use a reference to this object into the top-bliss-controller config.

Else, if a sub-bliss-controller has no reason to exist independently from the top-bliss-controller, then the top-bliss-controller will create and manage its sub-bliss-controllers from the knowledge of the top-bliss-controller configuration only.

In the second case, some items declared in the top-bliss-controller are, in fact, managed by one of the sub-bliss-controllers. Then, the author of the top-bliss-controller class must overload the _get_item_owner method and specify which is the sub-bliss-controller that manages which items.


Consider a top-bliss-controller which has internally another sub-bliss-controller that manages pseudo axes. (self._motor_controller = AxesBlissController(...))

- plugin: generic    
  module: custom_module       
  class: BCMockup             
  name: bcmock                


    - name: $xrot
      tags: real xrot

    - name: $yrot
      tags: real yrot

    - name: axis_1        
      tag : theta

The top-bliss-controller configuration declares the axes subitems but those items are in fact managed by the motors controller (self._motor_controller).

In that case, developers can override the self._get_item_owner method to specify the subitems that are managed by self._motor_controller instead of self.

def _get_item_owner(self, name, cfg, pkey):
    """ Return the controller that owns the items declared in the config.
        By default, this controller is the owner of all config items.
        However if this controller has sub-controllers that are the real owners 
        of some items, this method should use to specify which sub-controller is
        the owner of which item (identified with name and pkey). 
    if pkey == "axes":
        return self._motor_controller
        return self

The method receives the item name and the parent_key. So self._motor_controller can be associated to all subitems under the axes parent_key (instead of doing it for each subitem name).

Note: it would have been possible to not override self._get_item_owner and handle the axes items in the top-controller methods but it is not recommended as the code is already in the sub-bliss-controller that handles motors.

Direct instantiation

A BlissController can be instantiated directly (i.e. not instantiated by the plugin) providing a configuration as a dictionary.

In that case, users must call the method self._initialize_config() just after the controller instantiation to ensure that the controller is initialized in the same way as the plugin does.

The config dictionary should be structured like a YML file (i.e: nested dict and list) and references replaced by their corresponding object instances.

Example: bctrl = BlissController( config_dict ) => bctrl._initialize_config()

BlissController and default chain

The DEFAULT_CHAIN can be customized with DEFAULT_CHAIN.set_settings (see Default chain).

The devices introduced in the chain must be of the type Counter, CounterController or BlissController.

While introducing a BlissController in the default chain, the method BlissController._get_default_chain_counter_controller is called to obtain the CounterController object that should be used. By default this method is not implemented.

Other tips

@autocomplete_property decorator

In many controllers, the @property decorator is heavily used to protect certain attributes of the instance or to limit the access to read-only. When using the bliss command line interface the autocompletion will not suggest any completion based on the return value of the method underneath the property.

This is a wanted behavior e.g. in case this would trigger hardware communication. There are however also use cases where a deeper autocompletion is wanted.


“↹” represents the action of pressing the “Tab” key of the keyboard.

Example: the .counter namespace of a controller. If implemented as @property:

BLISS [1]: lima_simulator.counters. ↹

Would not show any autocompletion suggestions. To enable deeper autocompletion a special decorator called @autocomplete_property must be used.

from bliss.common.utils import autocomplete_property

class Lima(object):
    def counters(self):
        all_counters = [self.image]

Using this decorator would result in autocompletion suggestions:

BLISS [1]: lima_simulator.counters. ↹

The __info__() method for Bliss shell


  • Any Bliss controller that is visible to the user in the command line should have an __info__() function implemented!
  • The return type of __info__() must be str, otherwhise it fails and __repr__() is used as fallback!
  • As a rule of thumb: the return value of a custom __repr__() implementation should not contain \n and should be inspired by the standard implementation of __repr__() in python.

In Bliss, __info__() is used by the command line interface (Bliss shell or Bliss repl) to enquire information of the internal state of any object / controller in case it is available.

That way, a user can get information how to use the object, detailed from the user perspective. This is in contrast to the built-in python function __repr__(), which should return a short summary of the concerned object from the developer perspective. The Protocol that is put in place in the Bliss shell is the following:

  • if the return value of a statement entered into the Bliss shel is a python object with __info__() implemented this __info__() function will be called by the Bliss shell to display the output. As a fallback option (__info__() not implemented) the standard behavior of the interactive python interpreter involving __repr__ is used. (For details about __repr__ see next section.)

Here is an example for the lima controller that is using __info__:

LIMA_TEST_SESSION [3]: lima_simulator
              Out [3]: Simulator - Generator (Simulator) - Lima Simulator

                       bin = [1 1]
                       flip = [False False]
                       height = 1024
                       roi = <0,0> <1024 x 1024>
                       rotation = rotation_enum.NONE
                       sizes = [   0    4 1024 1024]
                       type = Bpp32
                       width = 1024

                       expo_time = 1.0
                       mode = mode_enum.SINGLE
                       nb_frames = 1
                       status = Ready
                       status_fault_error = No error
                       trigger_mode = trigger_mode_enum.INTERNAL_TRIGGER

                       ROI Counters:

                       Name  ROI (<X, Y> <W x H>)
                       ----  ------------------
                         r1  <0, 0> <100 x 200>

The information given above is usefull from a user point of view. As a developer one might want to work in the Bliss shell with live object e.g.

LIMA [4]: my_detectors = {'my_lima':lima_simulator,'my_mca':simu1}
LIMA [5]: my_detectors
 Out [5]: {'my_lima': <Lima Controller for Simulator (Lima Simulator)>,
                        'my_mca': <bliss.controllers.mca.simulation.SimulatedMCA
                                   object at 0x7f2f535b5f60>}

In this case, it is desirable that the python objects themselves are clearly represented, which is exactly the role of __repr__ (in this example the lima_simulator has a custom __repr__ while in simu1 there is no __repr__ implemented so the bulid in python implementation is used).

The signature of __info__() should be def __info__(self): the return value must be a string.

BLISS [1]: class A(object):
      ...:     def __repr__(self):
      ...:         return "my repl"
      ...:     def __str__(self):
      ...:         return "my str"
      ...:     def __info__(self):
      ...:         return "my info"

BLISS [2]: a=A()

BLISS [3]: a
  Out [3]: my info

BLISS [4]: [a]
  Out [4]: [my repl]


If, for any reason, there is an exception raised inside __info__, the fallback option will be used and __repr__ is evaluated in this case.

And this will hide the error. So, any error must be treated before returning.


    def __info__(self):
        info_str = "bla \n"
        info_str += "bli \n"

        return info_str

The equivalent of repr(obj) or str(obj) is also available in as info(obj) which can be used also outside the Bliss shell.

Python 3.7.3 (default, Mar 27 2019, 22:11:17)
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> from import info

>>> class A(object):
...     def __repr__(self):
...          return "my repl"
...     def __info__(self):
...          return "my info"
>>> info(A())
'my info'

>>> class B(object):
...     def __repr__(self):
...          return "my repl"

>>> info(B())
'my repl'

__str__() and __repr__()

If implemented in a Python class, __repr__ and __str__ methods are build-in functions Python to return information about an object instantiating this class.

  • __str__ should print a readable message
  • __repr__ should print a short message about the object that is unambiguous (e.g. name of an identifier, class name, etc.).

  • __str__ is called:

    • when the object is passed to the print() function (e.g. print(my_obj)).
    • wheh the object is used in string operations (e.g. str(my_obj) or '{}'.format(my_obj) or f'some text {my_obj}')
  • __repr__ method is called:
    • when user type the name of the object in an interpreter session (a python shell).
    • when displaying containers like lists and dicts (the result of __repr__ is used to represent the objects they contain)
    • when explicitly asking for it in the print() function. (e.g. print("%r" % my_object))

By default when no __str__ or __repr__ methods are defined, the __repr__ returns the name of the class (Length) and __str__ calls __repr__.