Fix StringArray.astype for category dtype

[siboehm]

• closes BUG: Casting string series to category raises TypeError in nightlies #40351
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

This was failing due to elif np.issubdtype(dtype, np.floating) here, which fails for the pandas dtypes.
StringArrays are now being cast to Categorical just as they were before #38530, when the casting still happened inside Block.

[jorisvandenbossche]

Looks good to me, thanks!

[siboehm]

This uncovered a bit of a larger issue, basically #37626, but for datetimes & period.

Example:

import pandas as pd
t = pd.Series(["1/1/2021", "2/1/2021", None], dtype="period[M]")
s = pd.Series(["1/1/2021", "2/1/2021", None], dtype="string").astype("period[M]")

t work, s raises because the None is converted to NA which isn't recognized as datetimelike by period.

Example

import pandas as pd
t = pd.Series(["1/1/2021", "2/1/2021"], dtype="object").astype("datetime64[ns]")
s = pd.Series(["1/1/2021", "2/1/2021"], dtype="string").astype("datetime64[ns]")

t work, s raises.

However if it's ok I'd open a new issue for it, since it'll be a larger revamp which will take me a few more days.
These "new" issues also aren't regressions like this Categorical issue, they're all already present in 1.2.3.

[jorisvandenbossche]

Yes, that's a more general issue, and can be left out of scope for this PR.

[jreback]

needs a whatsnew note for 1.2.4

[jreback]

can u add an example for Timedelta, Datetime w/time zone and Interval (all the EA types)

[siboehm]

I added test for all ExtensionArray dtypes. BooleanArray and IntervalArray I had to exclude since there's no way to to parse them back from a list of strings. TimedeltaArray xfails due to #40478. For PeriodArrayand DatetimeArray the NaT get converted to NA strings. But converting the NA strings back to NaT fails. I added XFails, unless expecting EA ⇒ StringArray ⇒ EA to roundtrip successfully is still up for debate.

[jorisvandenbossche]

needs a whatsnew note for 1.2.4

This doesn't need a whatsnew, it's only a regression on master

[jreback]

needs a whatsnew note for 1.2.4

This doesn't need a whatsnew, it's only a regression on master

ok great

[jreback]

is there an issue for this?

[siboehm]

I created one here: #40566 to track this

[jreback]

BooleanArray can be parsed from string (see _from_sequence_of_strings, the general method)

[siboehm]

I had an implementation that used _from_sequence_of_strings instead of _from_sequence to in StringArray.astype(). That required bigger code changes. I'd like to merge this Regression PR and then implement to implement _from_sequence_of_strings as part of #40566

[siboehm]

Why does Codecov say 0% coverage for the diff, when the added if-branch has multiple tests running over it?

[jreback]

looks fine, can you add a whatsnew note (bug fixes on 1.3 / conversion or extension types section). ping on greenish (also merge master)

[siboehm]

@jreback added a Whatsnew, should be good for merge.

[jreback]

thanks @siboehm