# Codelens¶

The codelens directive creates an interactive environment for you to step through small python code examples.

## Synopsis¶

The general format of the codelens directive is:

.. codelens:: unique_id
:options:

+ --- Content area ---+
|
| one or more lines of python code
|
+ --------------------+


Codelens displays the values of variables and shows the contents and links between your objects. Unlike a normal code debugger intended for solving bugs, codelens lets you step forward and backward through the code.

In addition to stepping through the code, you as an author can embed a single question into the codelens example. You may ask the student to predict what the value of a variable will be after a line executes, what the value of an element on the heap is at the point you pause the code (if the term heap is unfamiliar to you, you need note only that you should be asking questions about values of variables, not e.g. an element of a Python list), or you may ask the student to predict which line of code will be executed next. This is an excellent way to help students develop a good mental model of how python works.

It’s worth noting that you can also make use of codelens in a live environment where you can edit code and run new examples. To use codelens interactively go to http://www.pythontutor.com/.

Although codelens in Runestone currently supports python only, the core visualization available within codelens is also available for any language supported by pythontutor, which currently includes C, C++, Java, Ruby and others.

## Required Arguments¶

unique id
A unique identifier after a space and the :: in the codelens directive. Valid identifiers must not contain spaces. You should also avoid the characters   , ,, :, and *.
content area

The content of a codelens directive is the similar to an :docactivecode directive block: lines of code. There are a few differences:

• The question text or additional instructions section separated from code content is not supported. The book will compile, but the codelens will not render correctly.
• The hidden code section is not supported.

Note that if your code has any errors, it will definitely cause a problem when tracing through the codelens example, so make sure to test your code before deploying your book!

## Optional Arguments¶

caption
String. Define a caption for the bottom of the title frame.
showoutput
Boolean. Sometimes it is desirable to ignore any output from print statements, in which case you would include this argument. Or sometimes you just want to save space and not show console output, in which case you should not use this argument.
question

String. This is the question text that will be shown to the student.

Only one question per codelens for now.

correct
String. This is the correct answer to the question. This should be specified as a value from the trace data (see above). For example in the first example above, where you want to know the value of variable b, the correct answer parameter is globals.b. Note also that if you are asking a question about what line will be next executed, you can use the variable line (see example above), which refers to the line number that will be next executed (which may be a complex question if the code includes a loop or a conditional statement).
feedback
String. If the student gives the wrong answer you can give them a few sentences of feedback; the parameter to this argument is any string. The feedback will be the same for every wrong answer, so it’s a good idea to make the feedback generic reminders or hints.
breakline
Integer. This is the line number that you want the program to stop at and ask the question. Note that the lines in the code start at 1, and the breakpoint at which the code will stop and ask you the question breaks BEFORE executing the line you specify in the breakline.
tracedata

JSON Object. Normally this value is filled in automatically with a JavaScript object of the stack trace, but you can provide your own tracedata if you wish. The tracedata is the object from which you access the value of the :correct: answer (see below) if you are including a question in the codelens directive.

Developer notes

You can see an example of the tracedata of a codelens directive by writing the codelens directive with content, building your book, and then looking in the html document that was built from your .rst file, which you can find in the build folder, in the corresponding directory to the directory in _sources where you saved your current .rst file (e.g. if your current rst file is in _sources/Functions/introduction.rst, you can see the tracedata for a codelens example in build/Functions/introduction.html.

You can index into that tracedata object with dot notation, but index into any list within it with [], as usual in Python.

Note that globals are the variables in the global scope. locals is populated only if the codelens question refers to an inner, local scope within the program, and elements within lists, for example, are stored on the heap.

How Tracedata works

The way codelens works is that when a Runestone book is built, codelens takes the code and runs it through the python debugger where a series of stack frames are collected. I will refer to this list of stack frames as the trace data. The trace data is then embedded into the page, so when a student is reading the book and wants to step through a codelens example the trace data is visualized for the student.

## Languages supported¶

Python only at this time.

It is however to embed the core code visualization feature directly from python tutor. This workaround lacks the question and answer features of codelens, and is styled differently.

## Sphinx configuration options¶

No directive specific configuration options exist.

tbd

## Known limitations¶

Codelens does not process code containing preambles or hidden code correctly.

Codelens support is for core python only - libraries such as turtle or unittest are not supported.

You can’t place a codelens directive inside a Tab Groups directive.

## Examples¶

A codelens example with zero, or 1 line, while legal, does not make for a compelling demonstration.

(cl_ex_null)

.. codelens:: cl_ex_null


A simple example to help visualize a concept: insertion sort.

(cl_ex1)

This example comes from http://pythontutor.com itself.

.. codelens:: cl_ex1

# from: http://www.ece.uci.edu/~chou/py02/python.html
def InsertionSort(A):
for j in range(1, len(A)):
key = A[j]
i = j - 1
while (i >= 0) and (A[i] > key):
A[i+1] = A[i]
i = i - 1
A[i+1] = key

input = [8, 3, 9, 15, 29, 7, 10]
InsertionSort(input)
print(input)


This example asks a student to predict the value of a variable:

(question_example)

.. codelens:: question_example
:question: What is the value in b after line 4 executes?
:feedback: When d is set to a copy of the value of b it doesn't change the value of b.
:breakline: 4
:correct: globals.b

a = 1
b = 12.3
c = "Fred"
d = b


This is the default trace generated by runestone.

{"code": "a = 1\nb = 12.3\nc = \"Fred\"\nd = b", "trace": [{"ordered_globals": [], "stdout": "", "func_name": "<module>", "stack_to_render": [], "globals": {}, "heap": {}, "line": 1, "event": "step_line"}, {"ordered_globals": ["a"], "stdout": "", "func_name": "<module>", "stack_to_render": [], "globals": {"a": 1}, "heap": {}, "line": 2, "event": "step_line"}, {"ordered_globals": ["a", "b"], "stdout": "", "func_name": "<module>", "stack_to_render": [], "globals": {"a": 1, "b": 12.3}, "heap": {}, "line": 3, "event": "step_line"}, {"ordered_globals": ["a", "b", "c"], "question": {"text": "What is the value in b after line 4 executes?", "div": "question_example_modal", "feedback": "When d is set to a copy of the value of b it doesn't change the value of b.", "correct": "globals.b"}, "stdout": "", "func_name": "<module>", "stack_to_render": [], "globals": {"a": 1, "c": "Fred", "b": 12.3}, "heap": {}, "line": 4, "event": "step_line"}, {"ordered_globals": ["a", "b", "c", "d"], "question": {"text": "What is the value in b after line 4 executes?", "div": "question_example_modal", "feedback": "When d is set to a copy of the value of b it doesn't change the value of b.", "correct": "globals.b"}, "stdout": "", "func_name": "<module>", "stack_to_render": [], "globals": {"a": 1, "c": "Fred", "b": 12.3, "d": 12.3}, "heap": {}, "line": 4, "event": "return"}]}


This is the raw trace, but formatted. The globals object refernced in the directive is at the end of this object.

 {
"code":"a = 1\nb = 12.3\nc = \"Fred\"\nd = b",
"trace":[
{
"ordered_globals":[

],
"stdout":"",
"func_name":"<module>",
"stack_to_render":[

],
"globals":{

},
"heap":{

},
"line":1,
"event":"step_line"
},
{
"ordered_globals":[
"a"
],
"stdout":"",
"func_name":"<module>",
"stack_to_render":[

],
"globals":{
"a":1
},
"heap":{

},
"line":2,
"event":"step_line"
},
{
"ordered_globals":[
"a",
"b"
],
"stdout":"",
"func_name":"<module>",
"stack_to_render":[

],
"globals":{
"a":1,
"b":12.3
},
"heap":{

},
"line":3,
"event":"step_line"
},
{
"ordered_globals":[
"a",
"b",
"c"
],
"question":{
"text":"What is the value in b after line 4 executes?",
"div":"question_example_modal",
"feedback":"When d is set to a copy of the value of b it doesn't change the value of b.",
"correct":"globals.b"
},
"stdout":"",
"func_name":"<module>",
"stack_to_render":[

],
"globals":{
"a":1,
"c":"Fred",
"b":12.3
},
"heap":{

},
"line":4,
"event":"step_line"
},
{
"ordered_globals":[
"a",
"b",
"c",
"d"
],
"question":{
"text":"What is the value in b after line 4 executes?",
"div":"question_example_modal",
"feedback":"When d is set to a copy of the value of b it doesn't change the value of b.",
"correct":"globals.b"
},
"stdout":"",
"func_name":"<module>",
"stack_to_render":[

],
"globals":{
"a":1,
"c":"Fred",
"b":12.3,
"d":12.3
},
"heap":{

},
"line":4,
"event":"return"
}
]
}


The answer to a question need not relate to a value. In this example, we ask not for a value from a variable, but rather which line executes next.

(Ketchup_Speed)

.. codelens:: Ketchup_Speed
:question: What line will be executed after the current line executes?
:feedback: This code is executed one line at a time from top to bottom. (No loops.)
:breakline: 3
:correct: line

dripMPH = .028
FPM = 5280.0
dripFPH = dripMPH * FPM
MPH = 60
dripFPM = dripFPH / MPH
print("Ketchup speed in feet per minute:")
print(dripFPM)
print("Ketchup speed to move 4 feet in a minute:")
print(4 / dripFPM)


## Workarounds for languages other than Python¶

Although codelens is supported only for Python, that does not mean there is not a code visualization option available.

You can link directly to pythontutor.com. The site include a facility to generate either a link or an emdeddable iframe:

.. raw:: html

<iframe width="800" height="500" frameborder="0" src="http://pythontutor.com/iframe-embed.html#code=%23include%20%3Ciostream%3E%0A%23include%20%3Cstring%3E%0A%0Aint%20main%20%28%29%20%7B%0A%20%20int%20test%5B%5D%20%3D%20%7B%201,%202,%203,%205,%208%20%7D%3B%20%20//%20can%20we%20use%20an%20initializer%20list%3F%0A%20%20for%20%28auto%20i%3A%20test%29%20%7B%20%20%20%20%20%20%20%20%20%20%20%20%20//%20can%20we%20use%20a%20range-for%20loop%3F%0A%20%20%20std%3A%3Acout%20%3C%3C%20%22i%20is%20%22%20%3C%3C%20i%20%3C%3C%20'%5Cn'%3B%0A%20%20%7D%0A%0A%20%20return%200%3B%0A%7D%0A&codeDivHeight=400&codeDivWidth=350&py=cpp"> </iframe>


It’s not perfect as the output and visualizations of variables does not flow below the code. Also, note that this technique relies on the raw directive, which does make your book less portable and flexible.

It is a best practice to wrap markup specific to a single output generator with an only directive:

.. only:: html

.. raw:: html

<iframe ... />


One minor advantage of the embedded iframe over codelens` is the ability to click on a line of code and set breakpoints.

The back and forward buttons jump to the next breakpoint, if set.

## Logs & Grading¶

Clicks are logged. Answers to questions are also logged, but are currently not plugged into the grading interface and are used solely as a tool for checking understanding.

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