# Element developer guide

See elements/ for example elements.

Element code uses the libraries in freeformPythonLib/.

### Anatomy of an element

The system-wide elements available in the current build of the PrairieLearn server live in [PrairieLearn directory]/elements inside a folder corresponding to the element name. You can also have course-specific elements in a directory inside the root of your course repository, such as [course directory]/elements. See exampleCourse/elements for a real example of this.

By convention, all element files are named the same as the element they belong to. That directory should contain an info.json file that contains metadata about the element, including which file is the element controller and any dependencies of the element. See the section on dependencies for more information

Each element should have a .py controller that contains the functions listed in the next section. This controller is responsible for rendering the element, parsing the student's submission, and optionally grading the submission.

As a simple example, element pl-my-element would have the following file structure:

pl-my-element
+-- info.json
|-- pl-my-element.py
|-- pl-my-element.mustache
|-- pl-my-element.js
-- pl-my-element.css


And an info.json with the following contents:

{
"controller": "pl-my-element.py",
"dependencies": {
"elementScripts": [
"pl-my-element.js"
],
"elementStyles": [
"pl-my-element.css"
]
}
}


### Element functions

All element functions have the signature:

def fcn(element_html, data)


The arguments are:

Argument Type Description
element_html string The template HTML for the element.
data dict Mutable data for the question, which can be modified and returned.

The data dictionary has the following possible keys (not all keys will be present in all element functions):

Key Type Description
data["params"] dict Parameters that describe the question variant.
data["correct_answers"] dict The true answer (if any) for the variant.
data["submitted_answers"] dict The answer submitted by the student (after parsing).
data["format_errors"] dict Any errors encountered while parsign the student input.
data["partial_scores"] dict Partial scores for individual variables in the question.
data["score"] float The total final score for the question.
data["feedback"] dict Any feedback to the student on their submitted answer.
data["variant_seed"] integer The random seed for this question variant.
data["options"] dict Any options associated with the question.
data["raw_submitted_answers"] dict The answer submitted by the student before parsing.
data["editable"] boolean Whether the question is currently in an editable state.
data["panel"] string Which panel is being rendered (question, submission, or answer).
data["extensions"] dict A list of extensions that are available to be loaded by this element. For more information see the element extensions page.

So that multiple elements can exist together in one question, the convention is that each element instance is associated with one or more variables. These variables are keys in the dictionaries for the data elements. For example, if there are variables x and y then we might have:

data["correct_answers"]["x"] = 4


This structure, where dictionaries have variables as keys, is used for all dictionaries in data.

The element functions are:

Function Return object modifiable data keys unmodifiable data keys Description
generate() correct_answers, params options, variant_seed, extensions Generate the parameter and true answers for a new random question variant. Set data["params"][name] and data["correct_answers"][name] for any variables as needed. Return the modified data dictionary.
prepare() correct_answers, params options, variant_seed, extensions Final question preparation after element code has run. Can modify data as necessary. Return the modified data dictionary.
render() html (string) correct_answers, editable, feedback, format_errors, options, panel, params, partial_scores, raw_submitted_answers, score, submitted_answers, variant_seed, extensions Render the HTML for one panel and return it as a string.
parse() format_errors, submitted_answers, correct_answers options, params, raw_submitted_answers, variant_seed, extensions Parse the data["submitted_answers"][var] data entered by the student, modifying this variable. Return the modified data dictionary.
grade() correct_answers, feedback, format_errors, partial_scores, score, submitted_answers params, options, raw_submitted_answers, variant_seed, extensions Grade data["submitted_answers"][var] to determine a score. Store the score and any feedback in data["partial_scores"][var]["score"] and data["partial_scores"][var]["feedback"]. Return the modified data dictionary.
test() format_errors, partial_scores, score, raw_submitted_answers gradable, test_type, extensions Creates a test submission for this element, used when running tests from the "Settings" panel. Should set a value in data['raw_submitted_answers'][var] and expected score in data['partial_scores'][var] (or data['format_errors'][var] if invalid). The type of input to test is given in data['test_type'] and can be one of correct, incorrect, or invalid.

The above function descriptions describe the typical variables that will be read and modified by each function. However, any function that returns data (i.e., not parse()) is allowed to change any of the modifiable values in data (see above table) and these changes will be persisted to the database. No function is allowed to add or delete keys in data.

### Element dependencies

It's likely that your element will depend on certain client-side assets, such as scripts or stylesheets. To keep clean separation of HTML, CSS, and JS, you can place those dependencies in other files. If you depend on libraries like lodash or d3, you can also express that need. PrairieLearn will compile a list of all dependencies needed by all elements on a page, dedup the dependencies, and ensure they are loaded on the page.

Dependencies are listed in your element's info.json. You can configure them for your element as follows:

{
"controller": "pl-my-element.py",
"dependencies": {
"coreScripts": [
"d3.min.js"
],
"nodeModulesScripts": [
"three/build/three.min.js"
],
"elementScripts": [
"pl-my-element.js"
],
"elementStyles": [
"pl-my-element.css"
],
"clientFilesCourseStyles": [
"courseStylesheet1.css",
"courseStylesheet2.css"
]
}
}


The different types of dependency properties currently available are summarized in this table:

Property Description
coreStyles The styles required by this element, relative to [PrairieLearn directory]/public/stylesheets.
coreScripts The scripts required by this element, relative to [PrairieLearn directory]/public/javascripts.
nodeModulesStyles The styles required by this element, relative to [PrairieLearn directory]/node_modules.
nodeModulesScripts The scripts required by this element, relative to [PrairieLearn directory]/node_modules.
elementStyles The styles required by this element relative to the element's directory, which is either [PrairieLearn directory]/elements/this-element-name or [course directory]/elements/this-element-name.
elementScripts The scripts required by this element relative to the element's directory, which is either [PrairieLearn directory]/elements/this-element-name or [course directory]/elements/this-element-name.
clientFilesCourseStyles The styles required by this element relative to [course directory]/clientFilesCourse. (Note: This property is only available for elements hosted in a specific course's directory, not system-wide PrairieLearn elements.)
clientFilesCourseScripts The scripts required by this element relative to [course directory]/clientFilesCourse`. (Note: This property is only available for elements hosted in a specific course's directory, not system-wide PrairieLearn elements.)

You can also find the types of dependencies defined in these schema files: