BUG: StringArray __array_ufunc__ validation of result should not require strings

[johnflavin]

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• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

Note: This code sample requires pygeos, a python interface to the GEOS library. I don't know how to reproduce in a more minimal way without pygeos because the sample requires the numpy ufunc mechanism, which I can't easily exercise with sample code. Maybe someone with more C experience could make a smaller reproduction.

>>> import pandas as pd
>>> import pygeos
>>> pd.array(["POINT (0 0)"])
<StringArray>
['POINT (0 0)']
Length: 1, dtype: string
>>> pygeos.from_wkt(pd.array(["POINT (0 0)"]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Caskroom/miniconda/base/envs/geopandas-dev/lib/python3.9/site-packages/pygeos/io.py", line 181, in from_wkt
    return lib.from_wkt(geometry, **kwargs)
  File "/usr/local/Caskroom/miniconda/base/envs/geopandas-dev/lib/python3.9/site-packages/pandas/core/arrays/numpy_.py", line 254, in __array_ufunc__
    result = type(self)(result)
  File "/usr/local/Caskroom/miniconda/base/envs/geopandas-dev/lib/python3.9/site-packages/pandas/core/arrays/string_.py", line 195, in __init__
    self._validate()
  File "/usr/local/Caskroom/miniconda/base/envs/geopandas-dev/lib/python3.9/site-packages/pandas/core/arrays/string_.py", line 200, in _validate
    raise ValueError("StringArray requires a sequence of strings or pandas.NA")
ValueError: StringArray requires a sequence of strings or pandas.NA

Problem description

The pygeos.from_wkt function takes an array of strings and returns an array of geometry objects. But when this input array is a pandas StringArray we get an error. It seems that the PandasArray.__array_ufunc__ implementation assumes the results of the ufunc will go into the same type of array as the input, in this case a StringArray, which produces an error when the results are not a string. (Credit to @jorisvandenbossche in a comment on pygeos/pygeos#338.)

Expected Output

If we pass a numpy array into pygeos.from_wkt it works fine and produces an output array of geometries.

>>> import numpy as np
>>> np.array(["POINT (0 0)"])
array(['POINT (0 0)'], dtype='<U11')
>>> pygeos.from_wkt(np.array(["POINT (0 0)"]))
array([<pygeos.Geometry POINT (0 0)>], dtype=object)

I would expect the same output using a StringArray.

>>> pygeos.from_wkt(pd.array(["POINT (0 0)"]))
array([<pygeos.Geometry POINT (0 0)>], dtype=object)

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f2c8480
python : 3.9.2.final.0
python-bits : 64
OS : Darwin
OS-release : 20.3.0
Version : Darwin Kernel Version 20.3.0: Thu Jan 21 00:07:06 PST 2021; root:xnu-7195.81.3~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.2.3
numpy : 1.20.2
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 49.6.0.post20210108
Cython : None
pytest : 6.2.3
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : 7.22.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[jorisvandenbossche]

@johnflavin thanks for moving the issue here.

To be able to reproduce it without pygeos, we need a ufunc that works on string data. I am not sure there is a built-in one in numpy, but we can create one from a python function using np.frompyfunc:

In [10]: str_len_ufunc = np.frompyfunc(lambda x: len(x), 1, 1)

In [11]: arr = pd.array(["a", "bb"], dtype="string")

In [12]: str_len_ufunc(arr)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-12-3738fc027ef2> in <module>
----> 1 str_len_ufunc(arr)

~/scipy/pandas/pandas/core/arrays/numpy_.py in __array_ufunc__(self, ufunc, method, *inputs, **kwargs)
    180             if not lib.is_scalar(result):
    181                 # re-box array-like results, but not scalar reductions
--> 182                 result = type(self)(result)
    183             return result
    184 

~/scipy/pandas/pandas/core/arrays/string_.py in __init__(self, values, copy)
    209         self._dtype = StringDtype()  # type: ignore[assignment]
    210         if not isinstance(values, type(self)):
--> 211             self._validate()
    212 
    213     def _validate(self):

~/scipy/pandas/pandas/core/arrays/string_.py in _validate(self)
    214         """Validate that we only store NA or strings."""
    215         if len(self._ndarray) and not lib.is_string_array(self._ndarray, skipna=True):
--> 216             raise ValueError("StringArray requires a sequence of strings or pandas.NA")
    217         if self._ndarray.dtype != "object":
    218             raise ValueError(

ValueError: StringArray requires a sequence of strings or pandas.NA

[johnflavin]

@jorisvandenbossche Perfect! Thank you for the simpler reproduction.