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An extension of yt for working with merger tree data.

Project description


CircleCI codecov Documentation Status PyPI version Anaconda-Server Badge DOI yt-project

This is ytree, a yt extension for working with merger tree data.

Structure formation in cosmology proceeds in a hierarchical fashion, where dark matter halos grow via mergers with other halos. This type of evolution can be conceptualized as a tree, with small branches connecting to successively larger ones, and finally to the trunk. A merger tree describes the growth of halos in a cosmological simulation by linking a halo appearing in a given snapshot to its direct ancestors in a previous snapshot and its descendent in the next snapshot.

Merger trees are computationally expensive to generate and a great number of codes exist for computing them. However, each of these codes saves the resulting data to a different format. ytree is Python package for reading and working with merger tree data from multiple formats. If you are already familiar with using yt to analyze snapshots from cosmological simulations, then think of ytree as the yt of merger trees.

To load a merger tree data set with ytree and print the masses of all the halos in a single tree, one could do:

>>> import ytree
>>> a = ytree.load('tree_0_0_0.dat')
>>> my_tree = a[0]
>>> print(my_tree['tree', 'mass'].to('Msun'))
[6.57410072e+14 6.57410072e+14 6.53956835e+14 6.50071942e+14 ...
 2.60575540e+12 2.17122302e+12 2.17122302e+12] Msun

A list of all currently supported formats can be found in the online documentation. If you would like to see support added for another format, we would be happy to work with you to make it happen. In principle, any type of tree-like data where an object has one or more ancestors and a single descendent can be supported.


ytree can be installed with pip or conda:

pip installation

pip install ytree

conda installation

conda install -c conda-forge ytree

source installation

To get the development version, clone this repository and install like this:

git clone
cd ytree
pip install -e .

Getting Started

The ytree documentation will walk you through installation, get you started analyzing merger trees, and help you become a contributor to the project. Have a look!

Sample Data

Sample data for all merger tree formats supported by ytree is available on the yt Hub in the ytree data collection.


ytree would be much better with your contribution! As an extension of the yt Project, we follow the yt guidelines for contributing.

Citing ytree

If you use ytree in your work, please cite the following:

Smith et al., (2019). ytree: A Python package for analyzing merger trees.
Journal of Open Source Software, 4(44), 1881,

For BibTeX users:

    doi = {10.21105/joss.01881},
    url = {},
    year  = {2019},
    month = {dec},
    publisher = {The Open Journal},
    volume = {4},
    number = {44},
    pages = {1881},
    author = {Britton D. Smith and Meagan Lang},
    title = {ytree: A Python package for analyzing merger trees},
    journal = {Journal of Open Source Software}

If you would like to also cite the specific version of ytree used in your work, include the following reference:

  author       = {Britton Smith and
                  Meagan Lang and
                  Juanjo Bazán},
  title        = {ytree-project/ytree: ytree 3.2.0 Release},
  month        = nov,
  year         = 2022,
  publisher    = {Zenodo},
  version      = {ytree-3.2.0},
  doi          = {10.5281/zenodo.7335683},
  url          = {}


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