_metadata items of subclassed pd.Series are not propagated into corresponding SubclassedDataFrame

Code Sample

import pandas as pd

# Define a subclass for pd.Series with metadata myprop
class SubclassedSeries(pd.Series):

    _metadata = ['myprop']

    @property
    def _constructor(self):
        return SubclassedSeries

    @property
    def _constructor_expanddim(self):
        return SubclassedDataFrame

# Define a subclass for pd.DataFrame that slices to SubclassedSeries
class SubclassedDataFrame(pd.DataFrame):
    @property
    def _constructor(self):
        return SubclassedDataFrame

    @property
    def _constructor_sliced(self):
        return SubclassedSeries

# make an instance of SubclassedSeries and set myprop
sr = SubclassedSeries([1,2,3])
sr.myprop = 'foo'
print("myprob is", sr.myprop) # Works

# put the SubclassedSeries object into a SubclassedDataFrame and try to get myprop
df = SubclassedDataFrame({'a': sr})
print(type(df.a)) # Works (prints <class '__main__.SubclassedSeries'>)
print("myprob is", df.a.myprop) # does not work (AttributeError: 'SubclassedSeries' object has no attribute 'myprop')

Problem description

_metadata items of pd.Series subclasses are not propagated when the SubclassedSeries object is put into a SubclassedDataFrame. I would expect myprop to be available in the new SubclassedDataFrame.

Expected Output

myprob is foo
<class '__main__.SubclassedSeries'>
myprob is foo

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : None python : 3.8.1.final.0 python-bits : 64 OS : Linux OS-release : 5.3.0-42-lowlatency machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : de_DE.UTF-8 LOCALE : de_DE.UTF-8

pandas : 1.0.2 numpy : 1.18.1 pytz : 2019.3 dateutil : 2.8.1 pip : 20.0.2 setuptools : 46.0.0.post20200309 Cython : 0.29.15 pytest : 5.4.1 hypothesis : None sphinx : None blosc : None feather : None xlsxwriter : None lxml.etree : None html5lib : None pymysql : None psycopg2 : None jinja2 : 2.11.1 IPython : 7.13.0 pandas_datareader: None bs4 : None bottleneck : None fastparquet : None gcsfs : None lxml.etree : None matplotlib : 3.1.3 numexpr : 2.7.1 odfpy : None openpyxl : None pandas_gbq : None pyarrow : None pytables : None pytest : 5.4.1 pyxlsb : None s3fs : None scipy : 1.4.1 sqlalchemy : None tables : 3.6.1 tabulate : None xarray : 0.15.0 xlrd : 1.2.0 xlwt : None xlsxwriter : None numba : None

[samuelduchesne]

@Flix6x Somehow, returning a function instead of a class fails at line 396: https://github.com/pandas-dev/pandas/blob/8f6ec1e83a5de6d30cd187b832853797fd569d31/pandas/core/reshape/concat.py#L394-L398 with error AttributeError: 'function' object has no attribute '_get_axis_number'

Any workarounds?

[jorisvandenbossche]

I opened a PR to fix this _get_axis_number case: https://github.com/pandas-dev/pandas/pull/46018