Often, technical documentations include lists or other snippets of text that are strongly related to some of the project’s code. vpype’s documentation is no exception to this.

For instance, the Built-in symbols section lists the units available to expressions:

partial screenshot of vpype's documentation showing a list of units related to code

These units are related to the following piece of code:

# vpype/utils.py

    "px": 1.0,
    "in": 96.0,
    "inch": 96.0,
    "ft": 12.0 * 96.0,
    "yd": 36.0 * 96.0,
    "mi": 1760.0 * 36.0 * 96.0,
    "mm": 96.0 / 25.4,
    "cm": 96.0 / 2.54,
    "m": 100.0 * 96.0 / 2.54,
    "km": 100_000.0 * 96.0 / 2.54,
    "pc": 16.0,
    "pt": 96.0 / 72.0,

I recently added support for more units and, of course, the documentation was at risk of running out of sync. Obviously, generating the list of units based on the code would be a better solution. After some Googling, here is how I did it.

The basic idea is to use substitutions. A substitution consists of assigning a text snippet to a keyword, and subsequently use said keyword (with the |keyword| syntax) in the documentation’s body. The second insight is to use the rst_prolog variable (within the conf.py file) for the definition. This being regular Python, the definition can easily be auto-generated based on the original code.

Here is how it looks for the case above:

# docs/conf.py

import vpype as vp

# [...]

UNIT_STRINGS = ", ".join(f"``{s}``" for s in sorted(vp.UNITS.keys()) if s != "in")

rst_prolog = f"""
.. |units| replace:: {UNIT_STRINGS}

(Note that in is explicitly excluded from the list because it is a reserved Python keyword and cannot be used in the context of vpype expressions.)

And this is how the substitution is used in the actual documentation file:


* Units constants (|units|).

  These variables may be used to convert values to CSS pixels unit, which *vpype* uses internally. For example, the expression ``%(3+4)*cm%`` evaluates to the pixel equivalent of 7 centimeters (e.g. ~264.6 pixels). (Note that expressions may overwrite these variables, e.g. to use the ``m`` variable for another purpose.)

Et voilà! Nice and easy. I certainly expect to use this technique often in the future.