Fix and test scalar extension dtype op corner case

[jbrockmendel]

Fixes the following behavior in master:

ser = pd.Series(['a', 'b', 'c'])

>>> ser + "category"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/ops.py", line 1233, in wrapper
    return dispatch_to_extension_op(op, left, right)
  File "pandas/core/ops.py", line 1163, in dispatch_to_extension_op
    res_values = op(new_left, new_right)
TypeError: can only concatenate list (not "str") to list


>>> ser + "Int64"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/ops.py", line 1233, in wrapper
    return dispatch_to_extension_op(op, left, right)
  File "pandas/core/ops.py", line 1163, in dispatch_to_extension_op
    res_values = op(new_left, new_right)
TypeError: can only concatenate list (not "str") to list

and the same for the reversed ops.

[jorisvandenbossche]

Good catch :)

[jorisvandenbossche]

Is checking that it is a string a bit more logical?

[jbrockmendel]

I guess that would work. Elsewhere the same check is done with not is_scalar

[jorisvandenbossche]

then ignore my comment and go for consistency

[codecov]

Codecov Report

Merging #22378 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22378   +/-   ##
=======================================
  Coverage   92.05%   92.05%           
=======================================
  Files         169      169           
  Lines       50709    50709           
=======================================
  Hits        46679    46679           
  Misses       4030     4030

Flag	Coverage Δ
#multiple	90.46% <ø> (ø)	⬆️
#single	42.25% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/ops.py	96.71% <ø> (ø)	⬆️

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[jbrockmendel]

@jorisvandenbossche think this is mergeable? I'm looking through the queue for stuff sufficiently small/easy to take off @jreback's plate.

[jorisvandenbossche]

Yes, merged, thanks!