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BLISS sessions

BLISS session

A Session groups BLISS objects defined in configuration under a single entity.

Such a group can reflect geographic characteristics (ex: all devices of EH1) or functional characteristics (ex: Spectrometers motors and counters)

By default, all objects defined in beacon configuration will be loaded, then the setup file is executed. To load only specific objects, the YAML configuration file of the session must be adapted.

The setup file associated to a session is a Python script that is executed after session objects are created. It can load user scripts and execute user-defined functions.

Launching a BLISS session

A session can be started in the BLISS shell command line with -s option:

% bliss -s eh1
                       __         __   __
                      |__) |   | /__` /__`
                      |__) |__ | .__/ .__/

Welcome to BLISS version 0.02 running on pcsht (in bliss Conda environment)
Copyright (c) 2015-2023 Beamline Control Unit, ESRF
Connected to Beacon server on lid421 (port 3412)
eh1: Executing setup...
Initializing 'pzth'
Initializing 'simul_mca'
Initializing 'pzth_enc'
Hello eh1 session !!

EH1 [1]:


Two identical sessions cannot be started in a BLISS shell. This is ensured by usage of Tmux and by a locking mechanim. Tmux will display the previously started session instead of re-starting a new one. If Tmux is not used, an error message from the locking mechanism should appear at the second start of demo session:

demo is already running on host:pcsht,pid:8825 cmd: **bliss -s demo**

The error message gives all info needed to find where is running the previously started session and to deal with it (to kill it or to keep it).

Files organization

The files used to configure a session are located in ~/local/beamline_configuration/sessions/ directory.

   ├── ...
   ├── sessions/
   .   ├── eh1.yml           <---- session configuration file
   .   ├──      <---- session setup file
       ├── __init__.yml
       └── scripts/
           ├──  <---- eh1 session related scripts


__init__.yml file contains plugin: session ; then, all YAML files in the directory are loaded using the Session plugin.

YAML Session configuration file

The file ~/local/beamline_configuration/sessions/<session_name>.yml is used to define for a session:

  • the name of the session
  • a list of objects to include or not in this session
  • a list of measurment groups to include or not in this session
- class: Session
  name: eh1
  setup-file: ./

The file eh1.yml defines a session called eh1, with a setup file.

!! note The file path is relative to the session YAML file location if it starts with ./, otherwise it is absolute from the root of the Beacon file database.

By default, all objects defined in the configuration will be loaded in the session. It is possible to specify which objects must be included or not by using the config-objects keyword with the list of object names:

    - class: Session
      name: eh1
      setup-file: ./
      config-objects: [pzth, simul_mca]

The list of objects can also be written as:

    - class: Session
      name: eh1
      setup-file: ./
        - pzth
        - simul_mca

Conversely, exclude-objects can be used to not load unused objects.


Sessions can be nested using include-sessions. This allows to have a session for a specific equipment, for example, and to be able to include the whole equipment and its setup in another session.

Setup file

The setup file associated to a session (usualy ~/local/beamline_configuration/sessions/<session_name> is a Python script that is executed after session objects are created.

All objects from the session are available in the setup script. The globals defined in the setup script, and all session objects, are automatically added to the bliss.setup_globals namespace, to be used in user scripts.

import os
from bliss.common.standard import * # import all default functions, scans, etc.

SCAN_SAVING.base_path = os.path.join(os.environ["HOME"], "scans")
SCAN_SAVING.template = "{session}/{date}"
print "Setting scanfile to", SCAN_SAVING.get_path()

All objects can be made available in the session with:

from bliss.setup_globals import *

User scripts

Python files defined under the special scripts/ directory can be loaded in the setup file of a bliss session using the load_script(<file_name>) function.

All commands in the file are then executed.

In case of error, the load_script() function catches and displays exceptions, but do not prevent the rest of the setup from executing. Each call to load_script re-execute the commands again.

load_script is the equivalent of the execfile Python function, but for session scripts.


User script located in session directory should be session-specific.

Beamline specific should be placed in another directory to ease their maintenance. see:


The file is: ~/local/beamline_configuration/sessions/scripts/

import time
# 'time' module is now usable in this file, but also in the session
# 'demo' after loading of the script.

print("[loading script]: align_spectrometer()")

def align_spectrometer(energy=5.0):
    print(f"Aligning spectrometer for energy {energy}...")
    print(f"Spectrometer is aligned :)")

Example of script loading and usage:

DEMO [1]: load_script('demo')
[loading script]: align_spectrometer()
DEMO [2]:
DEMO [2]: align_spectrometer(energy=7.5)
Aligning spectrometer for energy 7.5  ...
Spectrometer is aligned :)
DEMO [3]:

Automatic session creation

A skeleton of BLISS session can be created automatically with the -c option:

example for a session named demo_session

(bliss) pcsht:~ % bliss -c demo_session
Creating 'demo_session' BLISS session on BEACON_HOST=epilobe:25000
Creating sessions/demo_session.yml
Creating sessions/
Creating sessions/scripts/

At start-up, the new session will display a message to help customization:

% bliss -c demo_session
Welcome to your new 'demo_session' BLISS session !!
You can now customize your 'demo_session' session by changing files:
   * .../
   * .../demo_session.yml
   * .../scripts/

If a session already exists, it will not be erased or reset.

% bliss -c demo
Session 'demo' cannot be created: it already exists.

A session can be deleted with the -d option:

% bliss -d demo_session
Removing 'demo_session' session.
removing .../sessions/
removing .../sessions/demo_session.yml
removing .../sessions/scripts/