# Example 3: An HOD-style model with a feature of your own creation¶

In this section of the Tutorial on building an HOD-style model, we’ll build a composite model that includes a component model that is not part of Halotools, but that you yourself have written.

There is also an IPython Notebook in the following location that can be used as a companion to the material in this section of the tutorial:

halotools/docs/notebooks/hod_modeling/hod_modeling_tutorial3.ipynb

By following this tutorial together with this notebook, you can play around with your own variations of the models we’ll build as you learn the basic syntax. The notebook also covers supplementary material that you may find clarifying, so we recommend that you read the notebook side by side with this documentation.

## Overview of the new model¶

The model we’ll build will be based on the zheng07 HOD, and we will use the baseline_model_instance feature described in the Model-building syntax candy: the baseline_model_instance mechanism section of the documentation. In addition to the basic zheng07 features, we’ll add a component model that governs galaxy size. Our model for size will have no physical motivation whatsoever. That part is up to you. This tutorial just teaches you the mechanics of incorporating a new feature into the factory.

## Building the new component model¶

All component models are instances of python classes, so we will define a new class for our new feature. You can always brush up on python classes by reading the Python documentation on classes. But for our purposes there is really only one basic thing to know. If you define a __init__ method inside your class, then this method is what gets called whenever your class is instantiated. This means that any data that gets bound to self inside __init__ will be bound to any instance of your class.

The example source code below shows the basic pattern you need to match when writing your own component model. We’ll unpack each line of this code in the discussion that follows. However, for now, while reading this code take note of the big picture.

1. You need to provide a few pieces of boilerplate data in the __init__ method so that the Halotools factory knows how to interface with your model.
2. You need to write the “physics function” that is responsible for the behavior of the model (assign_size, in this case).
class Size(object):

def __init__(self, gal_type):

self.gal_type = gal_type
self._mock_generation_calling_sequence = ['assign_size']
self._galprop_dtypes_to_allocate = np.dtype([('galsize', 'f4')])
self.list_of_haloprops_needed = ['halo_spin']

def assign_size(self, **kwargs):
table = kwargs['table']
table['galsize'][:] = table['halo_spin']/5.


Now we’ll build an instance of the Size component model for centrals and satellites and incorporate this feature into a composite model:

cen_size = Size('centrals')
sat_size = Size('satellites')
from halotools.empirical_models import PrebuiltHodModelFactory, HodModelFactory
zheng_model = PrebuiltHodModelFactory('zheng07')
new_model = HodModelFactory(baseline_model_instance = zheng_model, centrals_size = cen_size, satellites_size = sat_size)

# Your new model can generate a mock in the same way as always
from halotools.sim_manager import CachedHaloCatalog
halocat = CachedHaloCatalog(simname = 'bolshoi')
new_model.populate_mock(halocat)


## The __init__ method of your component model¶

There are four lines of code here, and each of them binds some new data to the class instance. Thus the component_model_instance above will have four attributes: gal_type, _mock_generation_calling_sequence, _galprop_dtypes_to_allocate and list_of_haloprops_needed. Each of these attributes plays an important role in structuring the interface between your model and the HodModelFactory, so we’ll now discuss them one by one.

### The role of the _mock_generation_calling_sequence¶

During the generation of a mock catalog, the HodMockFactory calls upon the component models one-by-one to assign their properties to the galaxy_table. When each component is called upon, every method whose name appears in the component model’s _mock_generation_calling_sequence gets passed the galaxy_table. These methods are called in the order they appear in the _mock_generation_calling_sequence list. So the purpose of the _mock_generation_calling_sequence list is to inform the HodModelFactory what to do when it comes time for the component model to play its role in creating the mock galaxy distribution.

See the The mock_generation_calling_sequence mechanism section of the Composite Model Bookkeeping Mechanisms page for further discussion.

### The role of the _galprop_dtypes_to_allocate¶

One of the tasks handled by the HodMockFactory is the allocation of the appropriate memory that will be stored in your galaxy_table. For every galaxy property in a composite model, there needs to be a corresponding column of the galaxy_table of the appropriate data type. The _galprop_dtypes_to_allocate ensures that this is the case.

The way this works is that every component model must declare a numpy.dtype object and bind it to the _galprop_dtypes_to_allocate attribute of the composite model instance. You can read more about Numpy dtype objects in the Numpy documentation, but the basic syntax is illustrated in the source code above: our new column will be named galsize, and each row stores a float.

You can see how to alter the syntax for the case of a component model that assigns more than one galaxy property in the next example of this tutorial. See the The galprop_dtypes_to_allocate mechanism section of the Composite Model Bookkeeping Mechanisms documentation page for further discussion.

### The role of the list_of_haloprops_needed¶

This attribute provides a list of all the keys of the halo_table that the functions appearing _mock_generation_calling_sequence will need to access during mock population. For example, the assign_size method requires access to the halo_spin column, and so the halo_spin string appears in list_of_haloprops_needed. This is fairly self-explanatory, but you can read more about the under-the-hood details in the The list_of_haloprops_needed mechanism section of the Composite Model Bookkeeping Mechanisms documentation page.

### The role of the HOD gal_type¶

Each of the component model methods appearing in _mock_generation_calling_sequence will not only be called by the composite model during mock generation, but these methods will also be passed on as methods that the composite model itself can use for other applications. The only difference will be that your choice for the gal_type string will be appended to these method names. For example, in the source code above, the composite model instance will have two methods: assign_size_centrals and assign_size_satellites. Besides mock population, you may find it useful to call upon these methods to make plots or study the behavior of your model.

## The “physics function” of your component model¶

In the example above, there is just one function responsible for the physics underlying this model: assign_size. The behavior of this simple function is pretty trivial: whatever the spin of the halo is, divide it by five and call the result the size of the galaxy. Obviously this is physically silly, but the calling signature illustrates how to write your more physically realistic model.

Any method in your _mock_generation_calling_sequence must accept a table keyword argument. That is because when the MockFactory calls your component model methods, it will pass the galaxy_table that is being built to each of your physics functions via a table keyword argument. The simplest way to handle this in a way that will be compatible with future Halotools updates is to have your physics functions use the Python built-in kwargs mechanism, as demonstrated in the source code above.

Note

As of v0.4, any physics functions must use the Python kwargs mechanism. This is so that the Halotools mock factories can pass arbitrary supplementary data to the component model functions, such as a random number seed. Rather than enumerating the list of necessary arguments that need to be caught, if you simply tack on the **kwargs argument at the end of the signature of your physics function, then the factories can pass in any additional data that your function can trivially ignore.

In order for your model’s underlying physics to propagate into the properties of the mock galaxy population, your physics function(s) must write values to the appropriate column of the table. In this case, we wrote to the galsize column. The [:] syntax is not strictly necessary, but it is good practice to use it because it ensures that an exception will be raised if you attempt to write to column that does not exist in the galaxy_table; you can omit the [:] if you want to eschew this safety mechanism, but there is no difference in performance and so this is syntax is recommended as a sanity check on all the bookkeeping.

This tutorial continues with Example 4: A more complex HOD component model.