# Transitioning From yt-2 to yt-3¶

If you’re used to halo analysis in yt-2.x, here’s a guide to how to update your analysis pipeline to take advantage of the new halo catalog infrastructure. If you’re starting from scratch, see Halo Finding and Analysis.

## Finding Halos¶

Previously, halos were found using calls to HaloFinder, FOFHaloFinder and RockstarHaloFinder. Now it is encouraged that you find the halos upon creation of the halo catalog by supplying a value to the finder_method keyword when calling HaloCatalog. Currently, only halos found using rockstar or a previous instance of a halo catalog are able to be loaded using the halos_ds keyword.

To pass additional arguments to the halo finders themselves, supply a dictionary to finder_kwargs where each key in the dictionary is a keyword of the halo finder and the corresponding value is the value to be passed for that keyword.

## Getting Halo Information¶

All quantities that used to be present in a halo_list are still able to be found but are not necessarily included by default. Every halo will by default have the following properties:

• particle_position_i (where i can be x,y,z)
• particle_mass
• particle_identifier

If other quantities are desired, they can be included by adding the corresponding quantity before the catalog is created. See the full halo catalog documentation for further information about how to add these quantities and what quantities are available.

You no longer have to iterate over halos in the halo_list. Now a halo dataset can be treated as a regular dataset and all quantities are available by accessing all_data. Specifically, all quantities can be accessed as shown:

import yt
from yt.analysis_modules.halo_analysis.api import HaloCatalog
hc = HaloCatalog(data_ds=data_ds, finder_method='hop')
hc.create()


## Prefiltering Halos¶

Prefiltering halos before analysis takes place is now done by adding a filter before the call to create. An example is shown below

import yt
from yt.analysis_modules.halo_analysis.api import HaloCatalog
hc = HaloCatalog(data_ds=data_ds, finder_method='hop')
hc.create()


## Profiling Halos¶

The halo profiler available in yt-2.x has been removed, and profiling functionality is now completely contained within the halo catalog. A complete example of how to profile halos by radius using the new infrastructure is given in Worked Example of Halo Analysis.

## Plotting Halos¶

Annotating halo locations onto a slice or projection works in the same way as in yt-2.x, but now a halo catalog must be passed to the annotate halo call rather than a halo list.

import yt
from yt.analysis_modules.halo_analysis.api import HaloCatalog


Data is now written out in the form of h5 files rather than text files. The directory they are written out to is controlled by the keyword output_dir. Each quantity is a field in the file.