What’s New and Different in yt 4.0?

If you are new to yt, welcome! If you’re coming to yt 4.0 from an older version, however, there may be a few things in this version that are different than what you are used to. We have tried to build compatibility layers to minimize disruption to existing scripts, but necessarily things will be different in some ways.

Updating to yt 4.0 from Old Versions (and going back)

Converting Old Scripts to Work with yt 4.0

After installing yt-4.0, you’ll want to change your old scripts in a few key ways. After accounting for the changes described in the list below, try running your script. If it still fails, the Python tracebacks should be fairly descriptive and it may be possible to deduce what remaining changes are necessary. If you continue to have trouble, please don’t hesitate to request help.

The list below is arranged in order of most to least important changes.

  • Fields should be specified as tuples not as strings In the past, you could specify fields as strings like "density", but with the growth of yt and its many derived fields, there can be sometimes be overlapping field names (e.g., ("gas", "density") and ("PartType0", "density")), where yt doesn’t know which to use. To remove any ambiguity, it is now strongly recommended to explicitly specify the full tuple form of all fields. Just search for all field accesses in your scripts, and replace strings with tuples (e.g. replace "a" with ("gas", "a" )). There is a compatibility rule in yt-4.0 to allow strings to continue to work until yt-4.1, but you may get unexpected behavior. Any field specifications that are ambiguous will throw an error in future versions of yt. See our Fields in yt, and available field list documentation for more information.

  • Use Newer Versions of Python The yt-4.0 release will be the final release of yt to support Python 3.6. Starting with yt-4.1, python 3.6 will no longer be supported, so please start using 3.7+ as soon as possible.

  • Particle-based datasets no longer accept n_ref and over_refine_factor One of the major upgrades in yt-4 is native treatment of particle-based datasets. This is in contrast to previous yt behavior which loaded particle-based datasets as octrees, which could then be treated like grid-based datasets. In order to define the octrees, users were required to specify n_ref and over_refine_factor values at load time. Please remove any reference to n_ref and over_refine_factor in your scripts.

  • Neutral ion fields changing format In previous versions, neutral ion fields were specified as ELEMENT_number_density (e.g., H_number_density to represent H I number density). This led to a lot of confusion, because some people assumed these fields were the total hydrogen density, not neutral hydrogen density. In yt-4.0, we have resolved this issue by explicitly calling total hydrogen number density H_nuclei_density and neutral hydrogen density H_p0_number_density (where p0 refers to plus 0 charge). This syntax follows the rule for other ions: H II = H_p1 = ionized hydrogen. Change your scripts accordingly. See Species Fields for more information.

  • Change in energy and momentum field names Fields representing energy and momentum quantities are now given names which reflect their dimensionality. For example, the ("gas", "kinetic_energy") field was actually a field for kinetic energy density, and so it has been renamed to ("gas", "kinetic_energy_density"). The old name still exists as an alias as of yt v4.0.0, but it will be removed in yt v4.1.0. See next item below for more information. Other examples include "gas", "specific_thermal_energy" for thermal energy per unit mass, and ("gas", "momentum_density_x") for the x-axis component of momentum density. See Energy and Momemtum Fields for more information.

  • Deprecated field names Certain field names are deprecated within yt v4.0 and will be removed in yt v4.1. For example, ("gas", "kinetic_energy") has been renamed to ("gas", "kinetic_energy_density"), though the former name has been added as an alias. Other fields, such as ("gas", "cylindrical_tangential_velocity_absolute"), are being removed entirely. When the deprecated field names are used for the first time in a session, a warning will be logged, so it is advisable to set your logging level to WARNING (yt.set_log_level("error")) at a minimum to catch these. See How can I change yt’s log level? for more information on setting your log level and Field List to see all available fields.

  • cmocean colormaps need prefixing yt used to automatically load and register external colormaps from the cmocean package unprefixed (e.g., set_cmap(FIELD, "balance"). This became unsustainable with the 3.4 release of Matplotlib, in which colormaps with colliding names raise errors. The fix is to explicitly import the cmocean module and prefix cmocean colormaps (like balance) with cmo. (e.g., cmo.balance). Note that this solution works with any yt-supported version of Matplotlib, but is not backward compatible with earlier versions of yt.

  • Position and velocity fields now default to using linear scaling in profiles and phase plots, whereas previously behavior was determined by whether the dataset was particle- or grid-based. Efforts have been made to standardize the treatment of other fields in profile and phase plots for particle and grid datasets.

Important New Aliases

With the advent of supporting SPH data at the particle level instead of smoothing onto an octree (see below), a new alias for both gas particle masses and cell masses has been created: ("gas", "mass"), which aliases to ("gas", "cell_mass") for grid-based frontends and to the gas particle mass for SPH frontends. In a number of places in yt, code that used ("gas", "cell_mass") has been replaced by ("gas", "mass"). Since the latter is an alias for the former, old scripts which use ("gas", "cell_mass") should not break.

Cool New Things

Changes for Working with SPH Data

In yt-3.0 most user-facing operations on SPH data are produced by interpolating SPH data onto a volume-filling octree mesh. Historically this was easier to implement When support for SPH data was added to yt as it allowed re-using a lot of the existing infrastructure. This had some downsides because the octree was a single, global object, the memory and CPU overhead of smoothing SPH data onto the octree can be prohibitive on particle datasets produced by large simulations. Constructing the octree during the initial indexing phase also required each particle (albeit, in a 64-bit integer) to be present in memory simultaneously for a sorting operation, which was memory prohibitive. Visualizations of slices and projections produced by yt using the default settings are somewhat blocky since by default we use a relatively coarse octree to preserve memory.

In yt-4.0 this has all changed! Over the past two years, Nathan Goldbaum, Meagan Lang and Matt Turk implemented a new approach for handling I/O of particle data, based on storing compressed bitmaps containing Morton indices instead of an in-memory octree. This new capability means that the global octree index is now no longer necessary to enable I/O chunking and spatial indexing of particle data in yt.

The new I/O method has opened up a new way of dealing with the particle data and in particular, SPH data.

Scatter and Gather approach for SPH data

As mentioned, previously operations such as slice, projection and arbitrary grids would smooth the particle data onto the global octree. As this is no longer used, a different approach was required to visualize the SPH data. Using SPLASH as inspiration, SPH smoothing pixelization operations were created using smooting operations via “scatter” and “gather” approaches. We estimate the contributions of a particle to a single pixel by considering the point at the centre of the pixel and using the standard SPH smoothing formula. The heavy lifting in these functions is undertaken by cython functions.

It is now possible to generate slice plots, projection plots, covering grids and arbitrary grids of smoothed quanitities using these operations. The following code demonstrates how this could be achieved. The following would use the scatter method:

import yt

ds = yt.load("snapshot_033/snap_033.0.hdf5")

plot = yt.SlicePlot(ds, 2, ("gas", "density"))

plot = yt.ProjectionPlot(ds, 2, ("gas", "density"))

arbitrary_grid = ds.arbitrary_grid([0.0, 0.0, 0.0], [25, 25, 25], dims=[16, 16, 16])
ag_density = arbitrary_grid[("gas", "density")]

covering_grid = ds.covering_grid(4, 0, 16)
cg_density = covering_grid[("gas", "density")]

In the above example the covering_grid and the arbitrary_grid will return the same data. In fact, these containers are very similar but provide a slighlty different API.

The above code can be modified to use the gather approach by changing a global setting for the dataset. This can be achieved with ds.sph_smoothing_style = "gather", so far, the gather approach is not supported for projections.

The default behaviour for SPH interpolation is that the values are normalized inline with Eq. 9 in SPLASH, Price (2009). This can be disabled with ds.use_sph_normalization = False. This will disable the normalization for all future interpolations.

The gather approach requires finding nearest neighbors using the KDTree. The first call will generate a KDTree for the entire dataset which will be stored in a sidecar file. This will be loaded whenever neccesary.

Off-Axis Projection for SPH Data

The current OffAxisProjectionPlot class will now support SPH projection plots.

The following is a code example:

import yt

ds = yt.load("Data/GadgetDiskGalaxy/snapshot_200.hdf5")

smoothing_field = ("gas", "density")

_, center = ds.find_max(smoothing_field)

sp = ds.sphere(center, (10, "kpc"))

normal_vector = sp.quantities.angular_momentum_vector()

prj = yt.OffAxisProjectionPlot(ds, normal_vector, smoothing_field, center, (20, "kpc"))


Smoothing Data onto an Octree

Whilst the move away from the global octree is a promising one in terms of perfomance and dealing with SPH data in a more intuitive manner, it does remove a useful feature. We are aware that many users will have older scripts which take advantage of the global octree.

As such, we have added support to smooth SPH data onto an octree when desired by the users. The new octree is designed to give results consistent with those of the previous octree, but the new octree takes advantage of the scatter and gather machinery also added.

import numpy as np

import yt

ds = yt.load("GadgetDiskGalaxy/snapshot_200.hdf5")
left = np.array([0, 0, 0], dtype="float64")
right = np.array([64000, 64000, 64000], dtype="float64")

# generate an octree
octree = ds.octree(left, right, n_ref=64)

# Scatter deposition is the default now, and thus this will print scatter

# the density will be calculated using SPH scatter
density = octree[("PartType0", "density")]

# this will return the x positions of the octs
x = octree[("index", "x")]

The above code can be modified to use the gather approach by using ds.sph_smoothing_style = 'gather' before any field access. The octree just uses the smoothing style and number of neighbors defined by the dataset.

The octree implementation is very simple. It uses a recursive algorithm to build a depth-first which is consistent with the results from yt-3. Depth-first search (DFS) means that tree starts refining at the root node (this is the largest node which contains every particles) and refines as far as possible along each branch before backtracking.

yt.units Is Now a Wrapper for unyt

We have extracted yt.units into unyt, its own library that you can install separately from yt from pypi and conda-forge. You can find out more about using unyt in its documentation and in a paper in the Journal of Open Source Software.

From the perspective of a user of yt, very little should change. While things in unyt have different names – for example YTArray is now called unyt_array – we have provided wrappers in yt.units so imports in your old scripts should continue to work without issue. If you have any old scripts that don’t work due to issues with how yt is using unyt or units issues in general please let us know by filing an issue on GitHub.

Moving unyt into its own library has made it much easier to add some cool new features, which we detail below.


Each dataset now has a set of unit symbols and physical constants associated with it, allowing easier customization and smoother interaction, especially in workflows that need to use code units or cosmological units. The ds.units object has a large number of attributes corresponding to the names of units and physical constants. All units known to the dataset will be available, including custom units. In situations where you might have used ds.arr or ds.quan before, you can now safely use ds.units:

>>> ds = yt.load('IsolatedGalaxy/galaxy0030/galaxy0030')
>>> u = ds.units
>>> ad = ds.all_data()
>>> data = ad['Enzo', 'Density']
>>> data + 12*u.code_mass/u.code_length**3
unyt_array([1.21784693e+01, 1.21789148e+01, 1.21788494e+01, ...,
            4.08936836e+04, 5.78006836e+04, 3.97766906e+05], 'code_mass/code_length**3')
>>> data + .0001*u.mh/u.cm**3
unyt_array([6.07964513e+01, 6.07968968e+01, 6.07968314e+01, ...,
            4.09423016e+04, 5.78493016e+04, 3.97815524e+05], 'code_mass/code_length**3')

Automatic Unit Simplification

Often the results of an operation will result in a unit expression that can be simplified by cancelling pairs of factors. Before yt 4.0, these pairs of factors were only cancelled if the same unit appeared in both the numerator and denominator of an expression. Now, all pairs of factors have have inverse dimensions are cancelled, and the appropriate scaling factor is incorporated into the result. For example, Hz and s will now appropriately be recognized as inverses:

>>> from yt.units import Hz, s
>>> frequency = 60*Hz
>>> time = 60*s
>>> frequency*time
unyt_quantity(3600, '(dimensionless)')

Similar simplifications will happen even if units aren’t reciprocals of each other, for example here hour and minute automatically cancel each other:

>>> from yt.units import erg, minute, hour
>>> power = [20, 40, 80] * erg / minute
>>> elapsed_time = 3*hour
>>> print(power*elapsed_time)
[ 3600.  7200. 14400.] erg

Alternate Unit Name Resolution

It’s now possible to use a number of common alternate spellings for unit names and if unyt knows about the alternate spelling it will automatically resolve alternate spellings to a canonical name. For example, it’s now possible to do things like this:

>>> import yt.units as u
>>> d = 20*u.mile
>>> d.to('km')
unyt_quantity(32.18688, 'km')
>>> d.to('kilometer')
unyt_quantity(32.18688, 'km')
>>> d.to('kilometre')
unyt_quantity(32.18688, 'km')

You can also use alternate unit names in more complex algebraic unit expressions:

>>> v = d / (20*u.minute)
>>> v.to('kilometre/hour')
unyt_quantity(96.56064, 'km/hr')

In this example the common british spelling "kilometre" is resolved to "km" and "hour" is resolved to "hr".

Field-Specific Configuration

You can now set configuration values on a per-field basis. For instance, this means that if you always want a particular colormap associated with a particular field, you can do so!

This is documented under Available per-field Configuration Options, and was added in PR 1931.

New Method for Accessing Sample Datasets

There is now a function entitled load_sample() that allows the user to automatically load sample data from the yt hub in a local yt session. Previously, users would have to explicitly download these data directly from https://yt-project.org/data, unpackage them, and load them into a yt session, but now this occurs from within a python session. For more information see: Loading Sample Data

Some Widgets

In yt, we now have some simple display wrappers for objects if you are running in a Jupyter environment with the ipywidgets package installed. For instance, the ds.fields object will now display field information in an interactive widget, and three-element unyt arrays (such as ds.domain_left_edge) will be displayed interactively as well.

The package widgyts provides interactive, yt-specific visualization of slices, projections, and additional dataset display information.

New External Packages

As noted above (yt.units Is Now a Wrapper for unyt), unyt has been extracted from yt, and we now use it as an external library. In addition, other parts of yt such as Interactive Data Visualization have been extracted, and we are working toward a more modular approach for things such as Jupyter widgets and other “value-added” integrations.