Data Objects

What are Data Objects in yt?

Data objects (also called Data Containers) are used in yt as convenience structures for grouping data in logical ways that make sense in the context of the dataset as a whole. Some of the data objects are geometrical groupings of data (e.g. sphere, box, cylinder, etc.). Others represent data products derived from your dataset (e.g. slices, streamlines, surfaces). Still other data objects group multiple objects together or filter them (e.g. data collection, cut region).

To generate standard plots, objects rarely need to be directly constructed. However, for detailed data inspection as well as hand-crafted derived data, objects can be exceptionally useful and even necessary.

How to Create and Use an Object

To create an object, you usually only need a loaded dataset, the name of the object type, and the relevant parameters for your object. Here is a common example for creating a Region object that covers all of your data volume.

import yt
ds = yt.load("RedshiftOutput0005")
ad = ds.all_data()

Alternatively, we could create a sphere object of radius 1 kpc on location [0.5, 0.5, 0.5]:

import yt
ds = yt.load("RedshiftOutput0005")
sp = ds.sphere([0.5, 0.5, 0.5], (1, 'kpc'))

After an object has been created, it can be used as a data_source to certain tasks like ProjectionPlot (see ProjectionPlot), one can compute the bulk quantities associated with that object (see Processing Objects: Derived Quantities), or the data can be examined directly. For example, if you want to figure out the temperature at all indexed locations in the central sphere of your dataset you could:

import yt
ds = yt.load("RedshiftOutput0005")
sp = ds.sphere([0.5, 0.5, 0.5], (1, 'kpc'))

# Show all temperature values

# Print things in a more human-friendly manner: one temperature at a time
print("(x,  y,  z) Temperature")
for i in range(sp["temperature"].size):
    print("(%f,  %f,  %f)    %f" %
          (sp["x"][i], sp["y"][i], sp["z"][i], sp["temperature"][i]))

Slicing Syntax for Selecting Data

yt provides a mechanism for easily selecting data while doing interactive work on the command line. This allows for region selection based on the full domain of the object. Selecting in this manner is exposed through a slice-like syntax. All of these attributes are exposed through the RegionExpression object, which is an attribute of a DataSet object, called r.

Getting All The Data

The .r attribute serves as a persistent means of accessing the full data from a dataset. You can access this shorthand operation by querying any field on the .r object, like so:

ds = yt.load("RedshiftOutput0005")
rho = ds.r["density"]

This will return a flattened array of data. The region expression object (r) doesn’t have any derived quantities on it. This is completely equivalent to this set of statements:

ds = yt.load("RedshiftOutput0005")
dd = ds.all_data()
rho = dd["density"]


One thing to keep in mind with accessing data in this way is that it is persistent. It is loaded into memory, and then retained until the dataset is deleted or garbage collected.

Selecting Multiresolution Regions

To select rectilinear regions, where the data is selected the same way that it is selected in a 3D Objects, you can utilize slice-like syntax, supplying start and stop, but not supplying a step argument. This requires that three components of the slice must be specified. These take a start and a stop, and are for the three axes in simulation order (if your data is ordered z, y, x for instance, this would be in z, y, x order).

The slices can have both position and, optionally, unit values. These define the value with respect to the domain_left_edge of the dataset. So for instance, you could specify it like so::

ds.r[(100, 'kpc'):(200,'kpc'),:,:]

This would return a region that included everything between 100 kpc from the left edge of the dataset to 200 kpc from the left edge of the dataset in the first dimension, and which spans the entire dataset in the second and third dimensions. By default, if the units are unspecified, they are in the “native” code units of the dataset.

This works in all types of datasets, as well. For instance, if you have a geographic dataset (which is usually ordered latitude, longitude, altitude) you can easily select, for instance, one hemisphere with a region selection::


If you specify a single slice, it will be repeated along all three dimensions. For instance, this will give all data::


And this will select a box running from 0.4 to 0.6 along all three dimensions::


Selecting Fixed Resolution Regions

yt also provides functionality for selecting regions that have been turned into voxels. This returns an Arbitrary Grids Objects object. It can be created by specifying a complex slice “step”, where the start and stop follow the same rules as above. This is similar to how the numpy mgrid operation works. For instance, this code block will generate a grid covering the full domain, but converted to being 21x35x100 dimensions::

region = ds.r[::21j, ::35j, ::100j]

The left and right edges, as above, can be specified to provide bounds as well. For instance, to select a 10 meter cube, with 24 cells in each dimension, we could supply::

region = ds.r[(20,'m'):(30,'m'):24j, (30,'m'):(40,'m'):24j,

This can select both particles and mesh fields. Mesh fields will be 3D arrays, and generated through volume-weighted overlap calculations.

Selecting Slices

If one dimension is specified as a single value, that will be the dimension along which a slice is made. This provides a simple means of generating a slice from a subset of the data. For instance, to create a slice of a dataset, you can very simply specify the full domain along two axes::

sl = ds.r[:,:,0.25]

This can also be very easily plotted::

sl = ds.r[:,:,0.25]

This accepts arguments the same way::

sl = ds.r[(20.1, 'km'):(31.0, 'km'), (504.143,'m'):(1000.0,'m'),
          (900.1, 'm')]

Available Objects

As noted above, there are numerous types of objects. Here we group them into:

  • Geometric Objects Data is selected based on spatial shapes in the dataset
  • Filtering Objects Data is selected based on other field criteria
  • Collection Objects Multiple objects grouped together
  • Construction Objects Objects represent some sort of data product constructed by additional analysis

If you want to create your own custom data object type, see Creating Data Objects.

Geometric Objects

For 0D, 1D, and 2D geometric objects, if the extent of the object intersects a grid cell, then the cell is included in the object; however, for 3D objects the center of the cell must be within the object in order for the grid cell to be incorporated.

0D Objects

Class YTPoint
Usage: point(coord, ds=None, field_parameters=None, data_source=None)
A point defined by a single cell at specified coordinates.

1D Objects

Ray (Axis-Aligned)
Usage: ortho_ray(axis, coord, ds=None, field_parameters=None, data_source=None)
A line (of data cells) stretching through the full domain aligned with one of the x,y,z axes. Defined by an axis and a point to be intersected. Please see this note about ray data value ordering.
Ray (Arbitrarily-Aligned)
Class YTRay
Usage: ray(start_coord, end_coord, ds=None, field_parameters=None, data_source=None)
A line (of data cells) defined by arbitrary start and end coordinates. Please see this note about ray data value ordering.

2D Objects

Slice (Axis-Aligned)
Class YTSlice
Usage: slice(axis, coord, center=None, ds=None, field_parameters=None, data_source=None)
A plane normal to one of the axes and intersecting a particular coordinate.
Slice (Arbitrarily-Aligned)
Usage: cutting(normal, coord, north_vector=None, ds=None, field_parameters=None, data_source=None)
A plane normal to a specified vector and intersecting a particular coordinate.

3D Objects

All Data
Function all_data()
Usage: all_data(find_max=False)
all_data() is a wrapper on the Box Region class which defaults to creating a Region covering the entire dataset domain. It is effectively ds.region(ds.domain_center, ds.domain_left_edge, ds.domain_right_edge).
Box Region
Class YTRegion
Usage: region(center, left_edge, right_edge, fields=None, ds=None, field_parameters=None, data_source=None)
Alternatively: box(left_edge, right_edge, fields=None, ds=None, field_parameters=None, data_source=None)
A box-like region aligned with the grid axis orientation. It is defined by a left_edge, a right_edge, and a center. The left_edge and right_edge are the minimum and maximum bounds in the three axes respectively. The center is arbitrary and must only be contained within the left_edge and right_edge. By using the box wrapper, the center is assumed to be the midpoint between the left and right edges.
Class: YTDisk
Usage: disk(center, normal, radius, height, fields=None, ds=None, field_parameters=None, data_source=None)
A cylinder defined by a point at the center of one of the circular bases, a normal vector to it defining the orientation of the length of the cylinder, and radius and height values for the cylinder’s dimensions.
Usage: ellipsoid(center, semi_major_axis_length, semi_medium_axis_length, semi_minor_axis_length, semi_major_vector, tilt, fields=None, ds=None, field_parameters=None, data_source=None)
An ellipsoid with axis magnitudes set by semi_major_axis_length, semi_medium_axis_length, and semi_minor_axis_length. semi_major_vector sets the direction of the semi_major_axis. tilt defines the orientation of the semi-medium and semi_minor axes.
Class YTSphere
Usage: sphere(center, radius, ds=None, field_parameters=None, data_source=None)
A sphere defined by a central coordinate and a radius.

Filtering and Collection Objects

See also the section on Filtering your Dataset.

Intersecting Regions
Most Region objects provide a data_source parameter, which allows you to subselect
one region from another (in the coordinate system of the DataSet). Note, this can
easily lead to empty data for non-intersecting regions.
Usage: slice(axis, coord, ds, data_source=sph)
Boolean Regions
Note: not yet implemented in yt 3.0
Usage: boolean()
Usage: cut_region(base_object, conditionals, ds=None, field_parameters=None)
A cut_region is a filter which can be applied to any other data object. The filter is defined by the conditionals present, which apply cuts to the data in the object. A cut_region will work for either particle fields or mesh fields, but not on both simultaneously. For more detailed information and examples, see Cut Regions.
Collection of Data Objects
Usage: data_collection(center, obj_list, ds=None, field_parameters=None)
A data_collection is a list of data objects that can be sampled and processed as a whole in a single data object.

Construction Objects

Fixed-Resolution Region
Usage: covering_grid(level, left_edge, dimensions, fields=None, ds=None, num_ghost_zones=0, use_pbar=True, field_parameters=None)
A 3D region with all data extracted to a single, specified resolution. See Examining Grid Data in a Fixed Resolution Array.
Fixed-Resolution Region with Smoothing
Usage: smoothed_covering_grid(level, left_edge, dimensions, fields=None, ds=None, num_ghost_zones=0, use_pbar=True, field_parameters=None)
A 3D region with all data extracted and interpolated to a single, specified resolution. Identical to covering_grid, except that it interpolates as necessary from coarse regions to fine. See Examining Grid Data in a Fixed Resolution Array.
Fixed-Resolution Region
Usage: arbitrary_grid(left_edge, right_edge, dimensions, ds=None, field_parameters=None)
When particles are deposited on to mesh fields, they use the existing mesh structure, but this may have too much or too little resolution relative to the particle locations (or it may not exist at all!). An arbitrary_grid provides a means for generating a new independent mesh structure for particle deposition and simple mesh field interpolation. See Arbitrary Grids Objects for more information.
Usage: proj(field, axis, weight_field=None, center=None, ds=None, data_source=None, method="integrate", field_parameters=None)
A 2D projection of a 3D volume along one of the axis directions. By default, this is a line integral through the entire simulation volume (although it can be a subset of that volume specified by a data object with the data_source keyword). Alternatively, one can specify a weight_field and different method values to change the nature of the projection outcome. See Types of Projections for more information.
Usage: streamline(coord_list, length, fields=None, ds=None, field_parameters=None)
A streamline can be traced out by identifying a starting coordinate (or list of coordinates) and allowing it to trace a vector field, like gas velocity. See Streamlines: Tracking the Trajectories of Tracers in your Data for more information.
Class YTSurface
Usage: surface(data_source, field, field_value)
The surface defined by all an isocontour in any mesh field. An existing data object must be provided as the source, as well as a mesh field and the value of the field which you desire the isocontour. See 3D Surfaces and Sketchfab.

Processing Objects: Derived Quantities

Derived quantities are a way of calculating some bulk quantities associated with all of the grid cells contained in a data object. Derived quantities can be accessed via the quantities interface. Here is an example of how to get the angular momentum vector calculated from all the cells contained in a sphere at the center of our dataset.

ds = load("my_data")
sp = ds.sphere('c', (10, 'kpc'))

Quickly Processing Data

Most data objects now have multiple numpy-like methods that allow you to quickly process data. More of these methods will be added over time and added to this list. Most, if not all, of these map to other yt operations and are designed as syntactic sugar to slightly simplify otherwise somewhat obtuse pipelines.

These operations are parallelized.

You can compute the extrema of a field by using the max or min functions. This will cache the extrema in between, so calling min right after max will be considerably faster. Here is an example::

ds = yt.load("IsolatedGalaxy/galaxy0030/galaxy0030")
reg = ds.r[0.3:0.6, 0.2:0.4, 0.9:0.95]
min_rho = reg.min("density")
max_rho = reg.max("density")

This is equivalent to::

min_rho, max_rho = reg.quantities.extrema("density")

The max operation can also compute the maximum intensity projection::

proj = reg.max("density", axis="x")

This is equivalent to::

proj = ds.proj("density", "x", data_source=reg, method="mip")

The min operator does not do this, however, as a minimum intensity projection is not currently implemented.

You can also compute the mean value, which accepts a field, axis and wight function. If the axis is not specified, it will return the average value of the specified field, weighted by the weight argument. The weight argument defaults to ones, which performs an arithmetic average. For instance::

mean_rho = reg.mean("density")
rho_by_vol = reg.mean("density", weight="cell_volume")

This is equivalent to::

mean_rho = reg.quantities.weighted_average("density", weight_field="ones")
rho_by_vol = reg.quantities.weighted_average("density",

If an axis is provided, it will project along that axis and return it to you::

rho_proj = reg.mean("temperature", axis="y", weight="density")

The sum function will add all the values in the data object. It accepts a field and, optionally, an axis. If the axis is left unspecified, it will sum the values in the object::

vol = reg.sum("cell_volume")

If the axis is specified, it will compute a projection using the method sum (which does not take into account varying path length!) and return that to you.:

cell_count = reg.sum("ones", axis="z")

To compute a projection where the path length is taken into account, you can use the integrate function::

proj = reg.integrate("density", "x")

All of these projections supply the data object as their base input.

Often, it can be useful to sample a field at the minimum and maximum of a different field. You can use the argmax and argmin operations to do this.:

reg.argmin("density", axis="temperature")

This will return the temperature at the minimum density.

If you don’t specify an axis, it will return the spatial position of the maximum value of the queried field. Here is an example::

x, y, z = reg.argmin("density")

Available Derived Quantities

Angular Momentum Vector
Usage: angular_momentum_vector(use_gas=True, use_particles=True)
The mass-weighted average angular momentum vector of the particles, gas, or both.
Bulk Velocity
Usage: bulk_velocity(use_gas=True, use_particles=True)
The mass-weighted average velocity of the particles, gas, or both.
Center of Mass
Usage: center_of_mass(use_cells=True, use_particles=False)
The location of the center of mass. By default, it computes of the non-particle data in the object, but it can be used on particles, gas, or both.
Class Extrema
Usage: extrema(fields, non_zero=False)
The extrema of a field or list of fields.
Maximum Location Sampling
Class SampleAtMaxFieldValues
Usage: sample_at_max_field_values(fields, sample_fields)
The value of sample_fields at the maximum value in fields.
Minimum Location Sampling
Class SampleAtMinFieldValues
Usage: sample_at_min_field_values(fields, sample_fields)
The value of sample_fields at the minimum value in fields.
Minimum Location
Usage: min_location(fields)
The minimum of a field or list of fields as well as the x,y,z location of that minimum.
Maximum Location
Usage: max_location(fields)
The maximum of a field or list of fields as well as the x,y,z location of that maximum.
Spin Parameter
Usage: spin_parameter(use_gas=True, use_particles=True)
The spin parameter for the baryons using the particles, gas, or both.
Total Mass
Class TotalMass
Usage: total_mass()
The total mass of the object as a tuple of (total gas, total particle) mass.
Total of a Field
Usage: total_quantity(fields)
The sum of a given field (or list of fields) over the entire object.
Weighted Average of a Field
Usage: weighted_average_quantity(fields, weight)
The weighted average of a field (or list of fields) over an entire data object. If you want an unweighted average, then set your weight to be the field: ones.
Weighted Variance of a Field
Usage: weighted_variance(fields, weight)
The weighted variance of a field (or list of fields) over an entire data object and the weighted mean. If you want an unweighted variance, then set your weight to be the field: ones.

Arbitrary Grids Objects

The covering grid and smoothed covering grid objects mandate that they be exactly aligned with the mesh. This is a holdover from the time when yt was used exclusively for data that came in regularly structured grid patches, and does not necessarily work as well for data that is composed of discrete objects like particles. To augment this, the YTArbitraryGrid object was created, which enables construction of meshes (onto which particles can be deposited or smoothed) in arbitrary regions. This eliminates any assumptions on yt’s part about how the data is organized, and will allow for more fine-grained control over visualizations.

An example of creating an arbitrary grid would be to construct one, then query the deposited particle density, like so:

import yt
ds = yt.load("snapshot_010.hdf5")

obj = ds.arbitrary_grid([0.0, 0.0, 0.0], [0.99, 0.99, 0.99],
                       dims=[128, 128, 128])
print(obj["deposit", "all_density"])

While these cannot yet be used as input to projections or slices, slices and projections can be taken of the data in them and visualized by hand.

These objects, as of yt 3.3, are now also able to “voxelize” mesh fields. This means that you can query the “density” field and it will return the density field as deposited, identically to how it would be deposited in a fixed resolution buffer. Note that this means that contributions from misaligned or partially-overlapping cells are added in a volume-weighted way, which makes it inappropriate for some types of analysis.

Combining Objects: Boolean Data Objects


Boolean Data Objects have not yet been ported to yt 3.0 from yt 2.x. If you are interested in aiding in this port, please contact the yt-dev mailing list. Until it is ported, this functionality below will not work.

A special type of data object is the boolean data object. It works only on three-dimensional objects. It is built by relating already existing data objects with boolean operators. The boolean logic may be nested using parentheses, and it supports the standard “AND”, “OR”, and “NOT” operators:

  • “AND” When two data objects are related with an “AND”, the combined data object is the volume of the simulation covered by both objects, and not by just a single object.
  • “OR” When two data objects are related with an “OR”, the combined data object is the volume(s) of the simulation covered by either of the objects. For example, this may be used to combine disjoint objects into one.
  • “NOT” When two data objects are related with a “NOT”, the combined data object is the volume of the first object that the second does not cover. For example, this may be used to cut out part(s) of the first data object utilizing the second data object.
  • “(” or ”)” Nested logic is surrounded by parentheses. The order of operations is such that the boolean logic is evaluated inside the inner-most parentheses, first, then goes upwards. The logic is read left-to-right at all levels (crucial for the “NOT” operator).

Please see the The Cookbook for some examples of how to use the boolean data object.

Connected Sets and Clump Finding

The underlying machinery used in Clump Finding is accessible from any data object. This includes the ability to obtain and examine topologically connected sets. These sets are identified by examining cells between two threshold values and connecting them. What is returned to the user is a list of the intervals of values found, and extracted regions that contain only those cells that are connected.

To use this, call extract_connected_sets() on any 3D data object. This requests a field, the number of levels of levels sets to extract, the min and the max value between which sets will be identified, and whether or not to conduct it in log space.

sp = ds.sphere("max", (1.0, 'pc'))
contour_values, connected_sets = sp.extract_connected_sets(
     "density", 3, 1e-30, 1e-20)

The first item, contour_values, will be an array of the min value for each set of level sets. The second (connected_sets) will be a dict of dicts. The key for the first (outer) dict is the level of the contour, corresponding to contour_values. The inner dict returned is keyed by the contour ID. It contains YTCutRegion objects. These can be queried just as any other data object. The clump finder (Clump Finding) differs from the above method in that the contour identification is performed recursively within each individual structure, and structures can be kept or remerged later based on additional criteria, such as gravitational boundedness.

Storing and Loading Objects

Often, when operating interactively or via the scripting interface, it is convenient to save an object to disk and then restart the calculation later or transfer the data from a container to another filesystem. This can be particularly useful when working with extremely large datasets. Field data can be saved to disk in a format that allows for it to be reloaded just like a regular dataset. For information on how to do this, see Geometric Data Containers.