Tuesday, January 29, 2019

Things that start badly

Today's Example of Starting Badly: Building HTML.

The code has a super-simple email message with f"<html><body><p>stuff {data}</p></body></html>". It was jammed into an email object along with the text version. All very nice.

For a moment, I considered suggesting that f-string substitution wasn't a good long-term solution, since it doesn't cover anything more than the most trivial case.

Two things stopped me from complaining:
  • The case really was trivial.
  • It's administrative code: it sends naggy reminder emails periodically. Why over-engineer it?
What an idiot I was.

Today, the {data} has been replaced with a complex table instead of a summary. (Why? The user story evolved. And we needed to replace the summary with details.)

The engineer was pretty sure they could use htmlify(data) or data.htmlify() to transform the data into an HTML structure without seriously breaking the f-string nature of the app.

I should have commented "Don't build HTML that way, it's a bad way to start" on the previous release.

The f-string solution turns rapidly into complexities layered on complexities dusted over the top with sprinkles of NOPE.

This is a job for Jinja2 or Mako or something similar. 

There's a step function change in the app's perceived "complexity". Instead of a simple f-string, we now have to populate a template. It goes from one line of code to more than one (three seems typical.) And. The file-system loader for templates seems more appropriate rather than hard-coding the template in the body of the code. So there are now more files in the app with the HTML templates in them.

However. The Jinja {{variable|round(2)}} was an immediate victory. The use of {%for%} to build the HTML table was the goal, and that simplification was worth the price of entry. Now we're arguing over CSS nuances to get the columns to look "right."

Lessons learned.

Don't let the currently superficial trivial case slide past without at least a warning. Make the suggestion that functions like "get template" and "populate template" will be necessary even for trivial f-string or string.Template processing.

HTML isn't a first-class part of anything. It's external serialization.  Yes, it's for people, but it's only serialization. Serialization has to be separated from the other aspects of the data gathering, map-reduce summarization, and email distribution. There's a pipeline of steps there and the final app should reflect the complete separation of these concerns. Even if it is admin overhead.

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