Contents

Using the ATLAS xAOD Typed Backend

Using the ATLAS xAOD Typed Backend

Come here to learn:

  • How to setup your environment to access xAOD data in ATLAS (R21)

  • Collections from the xAOD that are available

  • Access methods, attributes, and decorations on the xAOD Event Data Model (EDM).

This tutorial book takes for granted that you:

  • Have access to an ATLAS xAOD ServiceX backend

  • Know the basics about how to use ServiceX to filter, select data, etc.

  • Basic knowledge of the ATLAS xAOD data model (at the level of a standard ATLAS tutorial)

  • The names of the xAOD collections you are interested in, etc.

Typed?

Normally, one does not think of Python as a typed language, like C++ or many other languages. However, most Python code does not take advantage of this - a variable is always an int or a string or similar. Type Hints are the Python way of expressing this. They are a fundamental part of the modern Python language (see PEP 482, PEP 483, PEP 484, etc.). Typing offers several advantages:

  • Various tools like pylance and mypy can spot errors before the code is run

  • Editors, like vscode, can give you suggestions motivated by the type definitions

Further, func_adl libraries can use this type information to configure the backend C++ on the fly:

  • New collections can be made accessible.

  • Objects that are returned by various methods can be properly interpreted by the func_adl C++ backend.

  • Arbitrary C++ code can be downloaded and executed as part of the query to ServiceX.

For example, here is a Jupyter notebook opened in visual studio code, showing the possible collections in an event:

Example of using VSCode's built in type-checker to predict method names

And here the type checker pylance has flagged a bad method name, and a mouse-over shows more details on the error

Example of using VSCode's pylance type checker to flag an error

Setup and config

Everything in this book can be executed and, at the time when the book was published, ran. For this to work we need an up-to-date version of ServiceX running. We call this backend xaod_r21 for the C++ ATLAS xAOD backend based on the Release 21 of the ATLAS analysis software. As per normal for ServiceX, xaod_r21 should be defined in your servicex.yaml or .servicex file in your $HOME directory.

See the chapter on configuration for more information.

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Configuration