Deprecate xlwt

[WillAyd]

Similar to the conversation in #26487 xlwt isn't maintained. If we deprecate this could open up the path to simplify our Excel writers / extensions and supported file types going forward.

I would think this is also much easier than deprecating xlrd since we have other writers already implemented.

[jorisvandenbossche]

That would also mean deprecating the xls format? (or can the other engines write such files as well?)

I don't think I necessarily mind (for writing, as long as we can still read old formats), but just to make it clear.

[WillAyd]

That's correct - xlwt and xlrd are the only engines that support the .xls format

[jbrockmendel]

Is there any value in getting this (and the other excel-related deprecations) done for 1.0? If we're not enforcing the deprecations until 2.0, im tempted to just ignore all DEPR issues until [2 releases before 2.0]

[WillAyd]

Probably worth getting #29375 in before 1.0

[jreback]

cc @pandas-dev/pandas-core if anyone wants to do this for 1.2

[rhshadrach]

@jreback Is this something we could potentially do during the rc phase for 1.2? I can take this up, but not for another 2 days.

[jreback]

yeah i think this would be fine during rc

[rhshadrach]

Based on the decisions made in #35029, I'm not sure there is anything to do here. For reading, when a user specifies engine="xlrd" or the extension is .xls, the xlrd engine is used and no warning is emitted. I think we want to parallel that behavior for writing, and as far as I can tell, that is already the case.

Namely, we get the config f"io.excel.{ext}.writer" where ext is the file extension (defaults to "xlsx" if path is not a string). This config defaults to "auto" which uses the following logic to determine the engine based on the extension.

    _default_writers = {
        "xlsx": "openpyxl",
        "xlsm": "openpyxl",
        "xls": "xlwt",
        "ods": "odf",
    }
    xlsxwriter = import_optional_dependency(
        "xlsxwriter", raise_on_missing=False, on_version="warn"
    )
    if xlsxwriter:
        _default_writers["xlsx"] = "xlsxwriter"

So a user will only be using xlwt if they specify engine="xlwt", have overridden the default config, or if the path is a string with the extension xls. I think in each of these cases, we do not want to emit a warning.

[rhshadrach]

Ahh, I see. I think we want to deprecate writing xls but not reading xls, so a DeprecationWarning should be emitted anytime xlwt is used regardless of how we got there.