# Writing a Cyclus Input File¶

This section will provide an introduction to creating a valid input file for Cyclus by hand. Because Cyclus uses XML, JSON, or Python, input files have clearly defined sections, or blocks, that can be automatically validated for correctness.

Work is underway to provide a drag-and-drop graphical user interface to facilitate building Cyclus input files in the future.

## A Brief Introduction to XML¶

XML stands for EXtensible Markup Language, and was designed to provide structure to data in a generic way by grouping the data between starting and ending “tags”. The tags used to provide that structure are not defined universally, but each project can invent their own language for those tags. The data that lies between a pair starting and ending tags may be additional sections defined by other tags.

Here is an example of a XML code block.

<note>
<to>Matt</to>          <!-- message recipient -->
<from>Anthony</from>   <!-- message sender -->
<body>We have released Cyclus v1.0 for everyone to use.</body>
</note>


In this example, a section named note includes four other sections named to, from, heading, and body, respectively.

The set of tags for any given project, and their relationship to each other in the hierarchy, is referred to as a grammar. In addition to defining which tags exist, the grammar defines how many times each tag must or may exist in a given section. A tag may be completely optional, required one or more times, or allowed to appear as many times as the user desires.

Although XML grammars are designed to be self-documenting, comments can be inserted into XML files anywhere using the <!-- comment here --> syntax, as shown above.

## A Brief Introduction to JSON¶

JSON stands for JavaScript Object Notation, and is similar in many ways to XML. JSON was created to replace XML with a more user friendly language. It was designed to be a simpler and easier to use markup language. It does this by cutting out unnecessary parts of the XML language, such as closing tags. One area where JSON excels is that it is a data-oriented language, compared to XML, which is document-oriented.

Here is the same code block using JSON.

{
"note": {
"to": "Matt",
"from": "Anthony",
"body": "We have released Cyclus v1.0 for everyone to use."
}
}


## A Brief Introduction to Python¶

Python_ is a fully featured dynamic programming language. It hosts a wide variety of primitives built in to the language as well as powerful 3rd part libraries. Python input files allow for the computation of mathematical expressions in the input file itself. Static, proecomputed values are not required (as with XML and JSON).

Here is the same code block using Python.

{"note": {
"to": "Matt",       # message recipient
"from": "Anthony",  # message sender
"body": "We have released Cyclus v1.0 for everyone to use.",
}
}


In this example, a dictionary with a note key is has a dictionary vary that includes four other keys named to, from, heading, and body respectively.

## The Cyclus Input File¶

Every Cyclus input file must have exactly one simulation section that contains all data for a simulation.

XML:

<simulation>
... simulation data will go here ...
</simulation>


JSON:

{
"simulation": {
"… simulation data will go here …"
}


Python:

# Python input files will look for a variable named
# simulation, Simulation, or SIMULATION. It's value should be a
# dictionary with a single "simulation" key.
simulation = {"simulation": {...}}

# Alternitavely, the simualtion variable can also be a Python function or
# callable that returns a simulation dictionary. This function should not
# take any arguments.
def simulation():
return {"simulation": {...}}


Although not all sections are required, the following sections may appear in any order in the input file:

## Including XML Files¶

One feature of XML is that you are able to include external XML files inside of your current document. This lets you reuse common parts of your input file that do not change across multiple simulations. To enable XML include semantics the attribute xmlns:xi="http://www.w3.org/2001/XInclude" must be added to the <simulation> tag. Then to include an external file use the <xi:include href="path/to/file" /> tag where you want the included file to go.

A common inclusion pattern is to have a recipe book of materials in one file and then include this in your simulation.

input.xml:

<simulation xmlns:xi="http://www.w3.org/2001/XInclude">
...
<xi:include href="recipebook.xml" />
...
</simulation>


recipebook.xml:

<recipe>
<name>proton_recipe</name>
<basis>mass</basis>
<nuclide>
<id>010010000</id>
<comp>1</comp>
</nuclide>
</recipe>

<recipe>
<name>natU_recipe</name>
<basis>atom</basis>
<nuclide>
<id>922350000</id><comp>0.007</comp>
</nuclide>
<nuclide>
<id>922380000</id><comp>0.993</comp>
</nuclide>
</recipe>