PERF: Excel Styler treatment of CSS side expansion is slow

[tehunter]

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Reproducible Example

The styler approach to resolving CSS string expansion for conversion to Excel is slow. In particular:

• atomize contains an inefficient string addition and replace.
• No caching is conducted by CSSResolver or CSSToExcelConverter

from pathlib import Path

import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.randint(0, 100, (10, 1000)))
style = df.style
style.applymap(lambda x: "border-color: black; border-style: solid; border-width: thin")
style.to_excel(Path(__file__).parent / "test.xlsx")

On my machine, this example took 12.2s to run. Of that time, 6.57s was cumulatively spent in css.py:CSSResolver.__call__, including 2.37s in parse and 1.5s in atomize.

Installed Versions

Replace this line with the output of pd.show_versions()

Prior Performance

No response

[attack68]

The CSS resolving has always been an additional hack tagged on to the excel conversion.

If changed, there were some particular orders of calculation that might make some of these things a bit tricky to implement. IIRC there are some tests that might catch these items I am referring to, hopefully. Otherwise.. contributions welcome :)

[tehunter]

Put together a PR to address some of the issues. Had to modify some of the argument types, but I think I eliminated a lot of the unnecessary string joining and parsing. Also added Python's functools.lru_cache wrapper to the CSSToExcelConverter.