YTEP-0025: The ytdata Frontend


Created: August 31, 2015 Author: Britton Smith

This YTEP proposes to make data products created by yt into loadable datasets. Primarily, this will provide the following features:

  • exporting geometric data containers to datasets that can be reloaded for further geometric selection and analysis.
  • exporting plot-type data (projections, slices, profiles) so that they can be moved, reloaded, and manipulated to make new images.



Detailed Description

Currently, yt’s main data products (data containers, projections, slices, profiles) can only be used with their full functionality with the original dataset loaded. This is cumbersome when the datasets are so large that they can only be hosted at remote facilities. Creating publication-quality images from such data either requires a cycle of tweaking, transferring, viewing, and cursing or creating custom intermediate data products and plotting codes.

This YTEP proposes to create functionality that will allow for the above data products to be exported to a format that can be reloaded as a full-fledged dataset.

The proposed functionality consists of two main components: functionality to save objects to disk and a frontend responsible for reloading the saved objects.


A general function for saving array data associated with an open dataset will be responsible for writing data to disk. Data will be written to a single hdf5 file. Metadata associated with the dataset (i.e., current_time, current_redshift, cosmological parameters, domain dimensions) will be saved as attributes of the root file group. By default, data will be saved to a “grid” group with “units” attributes saved for each dataset. This function is implemented as save_as_dataset in yt/frontends/ytdata/ and imported in the main yt import.

The above function will be called by the user-facing functions, YTDataContainer.save_as_dataset, ProfileND.save_as_dataset, and FixedResolutionBuffer.save_as_dataset, which will optionally take a filename and a list of fields. If no field list is given, then the fields that have been queried and cached will be saved. This function will also make sure that fields necessary for geometric selection (grid position/cell size, particle position) are also saved. Mesh data will be saved to the “grid” group and particle data will be saved to groups named after the specific particle type.

ytdata Frontend

This frontend will be responsible for reloading all data saved with the above method. As this data is of multiple types, this will actually be multiple frontends living under the general “ytdata” heading. All dataset types will inherit from the YTDataset class. See ytdata Dataset Types for a description of each class. For each loaded dataset, ds, in the ytdata frontend, will provide direct access to the field data.

Geometrical Data Containers

Fields queried from data containers are returned as unordered, one-dimension arrays and, thus, most closely resemble particle datasets. All geometric data containers are reloaded as type YTDataContainerDataset, which is a particle dataset type. Mesh data is stored with the corresponding dx, dy, and dz fields such that derived fields like cell_volume can be created. All mesh data is aliased to the “gas” field type. The data attribute associated with the loaded dataset will be a data container configured identically to the original data container. In the case of ray data containers, this is not possible as a ray is defined by cells it intersects and not cells/particles enclosed within. In this case, data will be an instance of ds.all_data(). Field access through conventional data containers is also possible.

ds = yt.load("enzo_tiny_cosmology/DD0046/DD0046")

sphere = ds.sphere([0.5]*3, (10, "Mpc"))
sphere.save_as_dataset(fields=["density", "particle_mass"])

sds = yt.load("DD0046_sphere.h5")

# sphere with the same center and radius
print (
print (["grid", "density"])
print (["gas", "density"])
print (["all", "particle_mass"])
print (["all", "particle_position_x"])

# create a data container
ad = sds.all_data()
print (ad["grid", "density"])
print (ad["all", "particle_mass"])

Grid Data Containers

Covering grids, smoothed covering grids, and arbitrary grids return 3D arrays and so can be treated as uniform grid datasets. After being saved with save_as_dataset, these are reloaded as type YTGridDataset, which is a uniform grid that also supports particles. FixedResolutionBuffer objects saved with save_as_dataset will be reloaded as this type as well, only 2D. In this case, will give access to the multi-dimensional field arrays.

ds = yt.load("enzo_tiny_cosmology/DD0046/DD0046")

cg = ds.covering_grid(level=0, left_edge=[0.25]*3, dims=[16]*3)
cg.save_as_dataset("cg.h5", ["density", "particle_mass"])
cg_ds = yt.load("cg.h5")

# this has the dimensions of the original covering grid
print (["gas", "density"]).shape

# access via geometric selection
ad = cg_ds.all_data()
print (ad["gas", "density"])
print (ad["all", "particle_mass"])

ray = cg_ds.ray(cg_ds.domain_left_edge, cg_ds.domain_right_edge)
print (ray["gas", "density"])

# FRBs
proj = ds.proj("density", "x", weight_field="density")
frb = proj.to_frb(1.0, (800, 800))
fds = yt.load("DD0046_proj_frb.h5")
print (["density"])

Projections and Slices

Projections and slices are like two-dimensional particle datasets where the x and y fields are “px” and “py”. They are reloaded as type YTProjectionDataset, which is a subclass of YTDataContainerDataset. Reloaded projection or slice data can be selected geometrically or fed into a ProjectionPlot or SlicePlot. In these cases, is an instance of ds.all_data().

ds = yt.load("enzo_tiny_cosmology/DD0046/DD0046")

proj = ds.proj("density", "x", weight_field="density")

gds = yt.load("proj.h5")
print (["gas", "density"])
p = yt.ProjectionPlot(gds, "x", "density", weight_field="density")

The above would enable someone to make projections or slices of large datasets remotely, then download the exported dataset, and perfect the final image on a local machine. On and off-axis slices are implemented. Off-axis projections are not implemented at this time as they use totally different machinery. In this case, the best strategy would be to create an FRB and call save_as_dataset on that.

General Array Data

Array data written with the base save_as_dataset function can be reloaded as a non-spatial dataset. Geometric selection is not possible, but the data can be accessed through the YTNonspatialGrid object, This object will only grab data from the hdf5 file and do further selection on it.

from yt.frontends.ytdata.api import save_as_dataset

ds = yt.load("enzo_tiny_cosmology/DD0046/DD0046")

region =[0.25]*3, [0.75]*3)
sphere = ds.sphere(ds.domain_center, (10, "Mpc"))

my_data = {}
my_data["region_density"] = region["density"]
my_data["sphere_density"] = sphere["density"]
save_as_dataset(ds, "test_data.h5", my_data)

ads = yt.load("test_data.h5")
print (["region_density"])
print (["sphere_density"])


1, 2, and 3D profiles are like 1, 2, and 3D uniform grid datasets where dx, dy, and dz are different and have different dimensions. YTProfileDataset objects inherit from the YTNonspatialDataset class. Similarly, the data can be accessed from The x and y bins will be saved as 1D fields and fields named after the x and y bin field names will be saved with the same shape as the actual profile data. This will allow for easy array slicing of the profile based on the bin fields.

ds = yt.load("enzo_tiny_cosmology/DD0046/DD0046")
profile = yt.create_profile(ds.all_data(), ["density", "temperature"],
                            "cell_mass", weight_field=None)

pds = yt.load("DD0046_profile.h5")
# print the profile data
# print the x and y bins
# bin data shaped like the profile

ytdata Dataset Types

Name Inheritance Purpose Dataset Type Geometric Selection
YTDataset Dataset common functionality for other dataset types n/a n/a
YTDataContainerDataset YTDataset geometric data containers (sphere, region, ray, disk) particle yes
YTSpatialPlotDataset YTDataContainerDataset projections, slices, cutting planes particle yes
YTGridDataset YTDataset covering grids, arbitrary grids, fixed resolution buffers grid w/particles yes
YTNonspatialDataset YTGridDataset general array data grid no
YTProfileDataset YTNonspatialDataset 1, 2, and 3D profiles grid no

Backwards Compatibility

Currently, the only API breakage is in the AbsorptionSpectrum. Previously, it accepted a generic hdf5 file created by the LightRay. As per the open PR, the LightRay now writes out a yt.loadable dataset that is loaded by the AbsorptionSpectrum.

Other than the above, this is all new functionality and so has no backward incompatibility. One general change made to the yt codebase is that places that refer to index fields (x, y, z, dx, etc.) now refer to (<fluid_type>, "dx") instead of ("index", "dx"). This is to allow fields like cell_volume to be created from the ("grid", "dx") field that, for the ytdata frontend, lives on disk instead of the version being generated by the geometry handler. For actual grid datasets, we simply create an alias from (<fluid_type>, "dx") to ("index", "dx") upon loading. This should be completely transparent to the user.


We could create custom binary files for every type of plot and data container. We could also revive the concept of saving pickled objects that was used somewhat in yt-2.