CLN/API: implemented to_html in terms of .style

[jreback]

Implement to_html / notebook repr based on .style.

prob need to expand this to take a use argument (to select the style, needs to be 'classic' for a while, to replicate the current .to_html one).

[jorisvandenbossche]

Some discussion related to this was going on in #14975 (comment). Summarizing some elements here:

Barriers: some missing features are needed before such a replacement is possible (see also some elements in #11610)

• truncated display
• writing to a file (ENH: write Styler rendered output to file #13379)

Advantages:

• would eliminate a lot of code that gives similar functionality (HTMLFormatter, possibly other formatters) -> converging to one formatting system

Disadvantages:

• formally adding jinja2 as a dependency.
• performance?
  • plain html rendering on dataframe of 10 columns /10,000 rows of floats: df.style.render(): 19.6 s vs df.to_html() 2.7 s
  • for notebook reprs (which are typically truncated) this will probably not be a problem

cc @TomAugspurger For basic html output / notebook repr, it would maybe be useful to have a base class that has a simpler template and does not support all the different customization methods? For example, I can imagine that leaving out all the id=.. (which are not needed for basic display I think?) can improve perf / simplify things.

[TomAugspurger]

For basic html output / notebook repr, it would maybe be useful to have a base class that has a simpler template and does not support all the different customization methods?

100% agree with your comments here. This wouldn't really be implementing df.to_html using .style.
Instead we'd have a common Jinja2 template that would handle the logic of iterating over rows, inserting tags.
Then .to_html() and .style would extend that base template. .to_html probably wouldn't change much from the base really.

Also, Jinja depends on MarkupSafe, so that becomes another dependency.