BUG: Set type check is too strict when creating Series from dict keys

[krassowski]

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

• 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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Code Sample, a copy-pastable example

from pandas import Series
Series({'a': 1, 'b': 2}.keys())

Problem description

Since Python 3.7 dictionaries are ordered, and therefore their keys are also ordered. In older pandas versions (prior to 1.1?) it was possible to create a Series from keys of a dictionary. A fix to #32582 intended to prevent an issue with sets seems to be breaking the creation of Series from keys of a dictionary.

Currently the following is raised:

/pandas/core/series.py in __init__(self, data, index, dtype, name, copy, fastpath)
    325                     data = data.copy()
    326             else:
--> 327                 data = sanitize_array(data, index, dtype, copy, raise_cast_failure=True)
    328 
    329                 data = SingleBlockManager.from_array(data, index)

/pandas/core/construction.py in sanitize_array(data, index, dtype, copy, raise_cast_failure)
    450         subarr = _try_cast(arr, dtype, copy, raise_cast_failure)
    451     elif isinstance(data, abc.Set):
--> 452         raise TypeError("Set type is unordered")
    453     elif lib.is_scalar(data) and index is not None and dtype is not None:
    454         data = maybe_cast_to_datetime(data, dtype)

TypeError: Set type is unordered

Expected Output

Same as of Series(list({'a': 1, 'b': 2}.keys())), i.e.:

0    a
1    b
dtype: object

Output of pd.show_versions()

INSTALLED VERSIONS

commit : f2ca0a2
python : 3.7.5.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-42-generic
Version : #46-Ubuntu SMP Fri Jul 10 00:24:02 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.1.1
numpy : 1.18.1
pytz : 2019.3
dateutil : 2.8.1
pip : 19.2.3
setuptools : 41.2.0
Cython : None
pytest : 5.3.5
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.12.0
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
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[krassowski]

The current check:

from collections import abc

isinstance(data, abc.Set):
    raise TypeError("Set type is unordered")

will also fall on OrderedSet which is a recipe linked from the collections.abc documentation (see "See also" notes at the very bottom of the page).

It will also fail on dict().items(); the failures on keys and items are because abc.KeysView inherits from abc.MappingView and abc.Set while abc.ItemsView inherits from abc.MappingView and abc.Set. The inheritance from abc.Set is a statement about uniqueness not about being unordered.

[simonjayhawkins]

cc @dsaxton