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YALPT is a tool to assist in literate programming. Specifically, it is designed for using literate programing to write interactive tutorials.

What Is Literate Programming?

The term literate programing was coined by Donald Knuth (see [Wikipedia][1]) to describe a style of programming whereby documentation and information about the code was written using normal language, generally in some markup language, and then interspersed with actual code. The resulting files were then supposed to be compilable into normal programs. In the case of Python, this means that there should be a way to run them like normal Python programs.

What is YALPT?

YALPT is a tool for running properly formatted literate Python files. A YALPT-compatible file consists of normal text interspersed with interactive Python sessions. These sessions should be detectable by doctest (in fact, you can run YALPT files through doctest if you want). Other than that, the formatting is up to you. YALPT comes with a formatter that takes Markdown-formatted text and displays it using ANSI escape codes to make bold text show up as bold, etc. However, this is optional.

Since YALPT is based on the doctest and code modules from the Python standard library, it behaves very similarly to the tools you already know. In fact, YALPT will check the results of each chunk of code and warn you if it differs from what you have written in your file.

Why YALPT?

At this point, you may be asking yourself why you should use YALPT instead of some other literate Python tool, like [IPython notebooks](http://ipython.org/notebook.html) or [python-literate](https://github.com/stdbrouw/python-literate).

YALPT differs from tools like python-literate because it’s designed to run the code right in an interactive session, instead of running the code and the generating the output later.

YALPT differs from IPython notebooks in that it has a very loose format. IPython notebooks are stored in a JSON-based format that’s not particularly easy for a human to read or write quickly. One of the goals of YALPT is to have files that are stand-alone, and are perfectly useful without YALPT. This means that they should be readable and writable by humans. Additionally it is easy to write YALPT files that are also valid files in your markup language of choice. For instance, Markdown-flavored YALPT files can be passed to a Markdown parser, so you can generate static pages as well without having to rely on YALPT knowning how to read and output your markup and output formats of choice.

How do I run YALPT?

The basic invocation of YALPT is as such:

$ run-lit.py my-file.txt

If your file is in Markdown, you can use the Markdown formatter:

$ run-lit.py -f markdown my-file.md

By default, YALPT will drop into an interactive session after each code block, so that you can experiment around with the results of that block before continuing on. To continue, simply enter two blank lines at the interactive session (press enter twice). If you wish to disable this, and only show an interactive session at the end, you may pass the –no-interactive flag:

$ run-lit.py my-file.txt –no-interactive

This will simply pause after each code block. You can press enter to continue. If you do not even want the pause, you can run YALPT with the –no-pause flag:

$ run-lit.py my-file.txt –no-pause

By default, YALPT will enable readline support for completion and history. Completion is triggered with Control-space, and history is stored in your home directory in the file .literate-python-history. You can disable this functionality with the –no-readline flag:

$ run-lit.py my-file.txt –no-readline

Finally, by default YALPT will use ANSI escape codes to color the code a different color and format error messages. You can disable this using the –no-ansi flag:

$ run-lit.py my-file.txt –no-ansi

That’s all there is to it!

I didn’t read that description above, so how do I write YALPT files?

Any file that looks like doctest-compatible interactive python sessions interspersed between chunks of other text-based “stuff” is a YALPT-compatible files. As long as doctest can find the code chunks, so can YALPT, since YALPT makes use of doctest. You are otherwise free to use your markup language of choice for the text parts, or even no markup at all.

However, as mentioned above, YALPT does know how to format some Markdown, but using Markdown is by no means necessary. If you want to write a formatter for some other language, you can use the formatter for Markdown as a guideline, and submit a pull request on [GitHub](https://github.com/directxman12/yalpt).

Can I use this on files that don’t have doctest code blocks?

YALPT now technically has support for running any code in Markdown code blocks. Any indented code blocks, as well as both untyped fenced code blocks and fenced code blocks marked as Python, will be interpreted as Python code blocks and executed.

However, this functionality has not been tested as extensively as the the doctest parser, and is not as “self-documenting”, since it does not include the output of each line like the doctest format does.

If you wish to use a different parser, you can call YALPT with the -p flag:

$ run-lit.py -p markdown my-file.md

My YALPT file needs a lot a setup that distracts from the main contents!

While it’s generally preferable to provide basic setup as part of the literate Python file, it can be cumbersome and distracting when too much boilerplate is required. In this case, and “environment driver” may be specified. Environment drivers perform some setup and teardown, and may also provide objects that the literate Python files can use.

To run YALPT with an environment driver, use the -e flag:

$ run-lit.py -e gssapi basic-tutorial.md

One example of a package which provides an environment driver is [GSSAPI Console](https://pypi.python.org/pypi/gssapi_console). This environment driver sets up a self-contained MIT Krb5 environment for use when writing literate Python files that involve GSSAPI.

Reusing/Extending YALPT

The core functionality of YALPT exists in the class yalpt.core.LiterateInterpreter. This class may be subclassed to modify functionality.

It functions much like the standard library’s code.InteractiveConsole, from which it inherits.

Custom formatters should implement a format method which takes a string and returns a formatted string.

Additionally, YALPT now has several setuptools entry points for extension:

  • yalpt.formatters can be used to introduce custom formatters for pretty-printing the non-code portions of the files. See yalpt.formatters for more information.
  • yalpt.parsers can be used to introduce custom parsers to separate code from non-code. See yalpt.parsers for more information.
  • yalpt.env_drivers can be used to introduce environment drivers to provide boilerplate setup for literate Python files. Environment driver classes should implement a setup() method (which returns a dict of locals to include in the environment), a teardown() method, and have DRIVER_NAME and banner properties/field.

[1]: http://en.wikipedia.org/wiki/Literate_programming

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