BUG: Can't use iloc assign a list of None to a 1-d subset of a string dataframe.

[mvashishtha]

Pandas version checks

• I have checked that this issue has not already been reported.

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

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
s = pd.DataFrame([["A"], ["B"], ["C"]], dtype="string")
s.iloc[[0, 1], :] = [None, None]

Issue Description

On the latest pandas source code (version 1.5.0.dev0+122.gb4e578db85) I get a NotImplementedError:

Stack trace

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
Input In [9], in <module>
----> 1 s.iloc[[0, 1], :] = [None, None]

File ~/pandas/pandas/core/indexing.py:731, in _LocationIndexer.__setitem__(self, key, value)
    728 self._has_valid_setitem_indexer(key)
    730 iloc = self if self.name == "iloc" else self.obj.iloc
--> 731 iloc._setitem_with_indexer(indexer, value, self.name)

File ~/pandas/pandas/core/indexing.py:1704, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1702     self._setitem_with_indexer_split_path(indexer, value, name)
   1703 else:
-> 1704     self._setitem_single_block(indexer, value, name)

File ~/pandas/pandas/core/indexing.py:1952, in _iLocIndexer._setitem_single_block(self, indexer, value, name)
   1949 self.obj._check_is_chained_assignment_possible()
   1951 # actually do the set
-> 1952 self.obj._mgr = self.obj._mgr.setitem(indexer=indexer, value=value)
   1953 self.obj._maybe_update_cacher(clear=True, inplace=True)

File ~/pandas/pandas/core/internals/managers.py:337, in BaseBlockManager.setitem(self, indexer, value)
    331 def setitem(self: T, indexer, value) -> T:
    332     """
    333     Set values with indexer.
    334
    335     For SingleBlockManager, this backs s[indexer] = value
    336     """
--> 337     return self.apply("setitem", indexer=indexer, value=value)

File ~/pandas/pandas/core/internals/managers.py:304, in BaseBlockManager.apply(self, f, align_keys, ignore_failures, **kwargs)
    302         applied = b.apply(f, **kwargs)
    303     else:
--> 304         applied = getattr(b, f)(**kwargs)
    305 except (TypeError, NotImplementedError):
    306     if not ignore_failures:

File ~/pandas/pandas/core/internals/blocks.py:1394, in EABackedBlock.setitem(self, indexer, value)
   1391     nb = self.coerce_to_target_dtype(value)
   1392     return nb.setitem(indexer, value)
-> 1394 indexer = self._unwrap_setitem_indexer(indexer)
   1395 value = self._maybe_squeeze_arg(value)
   1397 values = self.values

File ~/pandas/pandas/core/internals/blocks.py:1652, in ExtensionBlock._unwrap_setitem_indexer(self, indexer)
   1650             indexer = indexer[0]
   1651         else:
-> 1652             raise NotImplementedError(
   1653                 "This should not be reached. Please report a bug at "
   1654                 "github.com/pandas-dev/pandas/"
   1655             )
   1656 return indexer

NotImplementedError: This should not be reached. Please report a bug at github.com/pandas-dev/pandas/

On the latest pandas release candidate (version 1.4.0rc0) I get a ValueError:

Stack trace

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [1], in <module>
      1 import pandas as pd
      3 s = pd.DataFrame([["A"], ["B"], ["C"]], dtype="string")
----> 4 s.iloc[[0, 1], :] = [None, None]

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:715, in _LocationIndexer.__setitem__(self, key, value)
    712 self._has_valid_setitem_indexer(key)
    714 iloc = self if self.name == "iloc" else self.obj.iloc
--> 715 iloc._setitem_with_indexer(indexer, value, self.name)

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:1689, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1687     self._setitem_with_indexer_split_path(indexer, value, name)
   1688 else:
-> 1689     self._setitem_single_block(indexer, value, name)

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:1937, in _iLocIndexer._setitem_single_block(self, indexer, value, name)
   1934 self.obj._check_is_chained_assignment_possible()
   1936 # actually do the set
-> 1937 self.obj._mgr = self.obj._mgr.setitem(indexer=indexer, value=value)
   1938 self.obj._maybe_update_cacher(clear=True, inplace=True)

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/managers.py:337, in BaseBlockManager.setitem(self, indexer, value)
    331 def setitem(self: T, indexer, value) -> T:
    332     """
    333     Set values with indexer.
    334
    335     For SingleBlockManager, this backs s[indexer] = value
    336     """
--> 337     return self.apply("setitem", indexer=indexer, value=value)

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/managers.py:304, in BaseBlockManager.apply(self, f, align_keys, ignore_failures, **kwargs)
    302         applied = b.apply(f, **kwargs)
    303     else:
--> 304         applied = getattr(b, f)(**kwargs)
    305 except (TypeError, NotImplementedError):
    306     if not ignore_failures:

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/blocks.py:1595, in ExtensionBlock.setitem(self, indexer, value)
   1592     value = value._ixs(0, axis=1)._values
   1594 check_setitem_lengths(indexer, value, self.values)
-> 1595 self.values[indexer] = value
   1596 return self

File /usr/local/lib/python3.9/site-packages/pandas/core/arrays/string_.py:412, in StringArray.__setitem__(self, key, value)
    410         value = np.asarray(value, dtype=object)
    411     if len(value) and not lib.is_string_array(value, skipna=True):
--> 412         raise ValueError("Must provide strings.")
    414 super().__setitem__(key, value)

ValueError: Must provide strings.

On the version of pandas that I was using when I found this bug (version 1.3.5) I get a ValueError:

Stack trace

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [1], in <module>
      1 import pandas as pd
      3 s = pd.DataFrame([["A"], ["B"], ["C"]], dtype="string")
----> 4 s.iloc[[0, 1], :] = [None, None]

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:723, in _LocationIndexer.__setitem__(self, key, value)
    720 self._has_valid_setitem_indexer(key)
    722 iloc = self if self.name == "iloc" else self.obj.iloc
--> 723 iloc._setitem_with_indexer(indexer, value, self.name)

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:1732, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1730     self._setitem_with_indexer_split_path(indexer, value, name)
   1731 else:
-> 1732     self._setitem_single_block(indexer, value, name)

File /usr/local/lib/python3.9/site-packages/pandas/core/indexing.py:1968, in _iLocIndexer._setitem_single_block(self, indexer, value, name)
   1965 self.obj._check_is_chained_assignment_possible()
   1967 # actually do the set
-> 1968 self.obj._mgr = self.obj._mgr.setitem(indexer=indexer, value=value)
   1969 self.obj._maybe_update_cacher(clear=True)

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/managers.py:355, in BaseBlockManager.setitem(self, indexer, value)
    354 def setitem(self: T, indexer, value) -> T:
--> 355     return self.apply("setitem", indexer=indexer, value=value)

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/managers.py:327, in BaseBlockManager.apply(self, f, align_keys, ignore_failures, **kwargs)
    325         applied = b.apply(f, **kwargs)
    326     else:
--> 327         applied = getattr(b, f)(**kwargs)
    328 except (TypeError, NotImplementedError):
    329     if not ignore_failures:

File /usr/local/lib/python3.9/site-packages/pandas/core/internals/blocks.py:1491, in ExtensionBlock.setitem(self, indexer, value)
   1488     indexer = indexer[0]
   1490 check_setitem_lengths(indexer, value, self.values)
-> 1491 self.values[indexer] = value
   1492 return self

File /usr/local/lib/python3.9/site-packages/pandas/core/arrays/string_.py:413, in StringArray.__setitem__(self, key, value)
    411         value = np.asarray(value, dtype=object)
    412     if len(value) and not lib.is_string_array(value, skipna=True):
--> 413         raise ValueError("Must provide strings.")
    415 super().__setitem__(key, value)

ValueError: Must provide strings.

Expected Behavior

After the code example above, s should be missing entries for the first two rows, so it should look like:

      0
0  <NA>
1  <NA>
2     C

Installed Versions

INSTALLED VERSIONS

commit : 66e3805
python : 3.9.9.final.0
python-bits : 64
OS : Darwin
OS-release : 20.6.0
Version : Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:31 PDT 2021; root:xnu-7195.141.2~5/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.5
numpy : 1.22.1
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3.1
setuptools : 59.0.1
Cython : None
pytest : 6.2.5
hypothesis : None
sphinx : 4.4.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.3
IPython : 8.0.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : 2022.01.0
fastparquet : None
gcsfs : None
matplotlib : 3.5.1
numexpr : 2.8.1
odfpy : None
openpyxl : 3.0.9
pandas_gbq : 0.16.0
pyarrow : 6.0.1
pyxlsb : None
s3fs : 2022.01.0
scipy : 1.7.3
sqlalchemy : 1.4.29
tables : 3.7.0
tabulate : None
xarray : 0.20.2
xlrd : 2.0.1
xlwt : None
numba : None

Note that as I mentioned above, I get different errors even with newer versions of pandas.

[levibaguley]

Using pd.NA gives the desired results

import pandas as pd
s = pd.Series(["A", "B", "C"], dtype="string")
s[[0, 1]] = [pd.NA, pd.NA]
print(s)

# 0    <NA>
# 1    <NA>
# 2       C
# dtype: string

[mvashishtha]

@levibaguley I'm sorry, but the code snippet I posted was incorrect. The bug is for dataframes, not series. I updated the snippet. Still, if I substitute pd.NA for None, the code works as expected in versions 1.4.0rc0 and 1.3.5. However, in version 1.5.0.dev0+122.gb4e578db85, I get the same NotImplementedError:

Stack trace

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
Input In [4], in <module>
      1 import pandas as pd
      3 s = pd.DataFrame([["A"], ["B"], ["C"]], dtype="string")
----> 4 s.iloc[[0, 1], :] = [pd.NA, pd.NA]

File ~/pandas/pandas/core/indexing.py:731, in _LocationIndexer.__setitem__(self, key, value)
    728 self._has_valid_setitem_indexer(key)
    730 iloc = self if self.name == "iloc" else self.obj.iloc
--> 731 iloc._setitem_with_indexer(indexer, value, self.name)

File ~/pandas/pandas/core/indexing.py:1704, in _iLocIndexer._setitem_with_indexer(self, indexer, value, name)
   1702     self._setitem_with_indexer_split_path(indexer, value, name)
   1703 else:
-> 1704     self._setitem_single_block(indexer, value, name)

File ~/pandas/pandas/core/indexing.py:1952, in _iLocIndexer._setitem_single_block(self, indexer, value, name)
   1949 self.obj._check_is_chained_assignment_possible()
   1951 # actually do the set
-> 1952 self.obj._mgr = self.obj._mgr.setitem(indexer=indexer, value=value)
   1953 self.obj._maybe_update_cacher(clear=True, inplace=True)

File ~/pandas/pandas/core/internals/managers.py:337, in BaseBlockManager.setitem(self, indexer, value)
    331 def setitem(self: T, indexer, value) -> T:
    332     """
    333     Set values with indexer.
    334
    335     For SingleBlockManager, this backs s[indexer] = value
    336     """
--> 337     return self.apply("setitem", indexer=indexer, value=value)

File ~/pandas/pandas/core/internals/managers.py:304, in BaseBlockManager.apply(self, f, align_keys, ignore_failures, **kwargs)
    302         applied = b.apply(f, **kwargs)
    303     else:
--> 304         applied = getattr(b, f)(**kwargs)
    305 except (TypeError, NotImplementedError):
    306     if not ignore_failures:

File ~/pandas/pandas/core/internals/blocks.py:1394, in EABackedBlock.setitem(self, indexer, value)
   1391     nb = self.coerce_to_target_dtype(value)
   1392     return nb.setitem(indexer, value)
-> 1394 indexer = self._unwrap_setitem_indexer(indexer)
   1395 value = self._maybe_squeeze_arg(value)
   1397 values = self.values

File ~/pandas/pandas/core/internals/blocks.py:1652, in ExtensionBlock._unwrap_setitem_indexer(self, indexer)
   1650             indexer = indexer[0]
   1651         else:
-> 1652             raise NotImplementedError(
   1653                 "This should not be reached. Please report a bug at "
   1654                 "github.com/pandas-dev/pandas/"
   1655             )
   1656 return indexer

NotImplementedError: This should not be reached. Please report a bug at github.com/pandas-dev/p