concat() on Sparse dataframe returns strange results

I open a stackoverflow question here :
http://stackoverflow.com/questions/35083277/pandas-concat-on-sparse-dataframes-a-mystery

And someone ask me to open an issue on Github.
To summarise, I don't really understand what's going on after a concat of 2 sparse data frame...
After such concat, df.density or df.memory_usage, for example, will throw an error.
Moreover, the basic structure of the sparse result seems strange...
I'm sorry that the bug is not defined better.

[jreback]

dupe of #10536

when making a report, ideally would have a simple copy-pastable example as well as pd.show_versions()

Hi,

I don't understand, the issue #10536 says that :

The [code] above does work correctly for SparseDataFrames.

More over, pd.show_versions() get me :

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.0.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-229.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.0
nose: 1.3.7
pip: 8.0.2
setuptools: 18.4
Cython: 0.23.4
numpy: 1.10.1
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.6
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.4.4
matplotlib: 1.4.3
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None

[jreback]

well, then pls show an example

This is a pretty long example, sorry about it :
Maybe it's not a bug, and I don't get something. (?)

import pandas as pd

df1 = pd.DataFrame({'A': [0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
              'B': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
              'C': [0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0],
              'D': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
              'E': [0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0]},
            index=['a','b','c','d','e','f','g','h','i','j','k','l']).to_sparse(fill_value=0)

df2 = pd.DataFrame({'F': [0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0],
              'G': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 0],
              'H': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
              'I': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
              'J': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 6]},
            index=['a','b','c','d','e','f','g','h','i','j','k','l']).to_sparse(fill_value=0)

print("df1 sparse size =", df1.memory_usage().sum(),"Bytes, density =", df1.density)
print(type(df1))
print('default_fill_value =', df1.default_fill_value)
print(df1.values)

print("df2 sparse size =", df2.memory_usage().sum(),"Bytes, density =", df2.density)
print(type(df2))
print('default_fill_value =', df2.default_fill_value)
print(df2.values)

result = pd.concat([df1,df2], axis=1)

print(type(result)) # Seems alright
print('default_fill_value =', result.default_fill_value) # The default fill value is not 0 ???
print(result.values) # What's that "nan" blocks ?
# result.density # Throw an error
# result.memory_usage # Throw an error

[jreback]

its the same issue as I pointed

[jreback]

added to the master issue. Sparse have not gotten a lot of TLC; need someone to step up and give them some :)

Yes #10627 is certainly root cause

[yonatanp]

In case this ever helps someone, here is my favorite workaround for fixing semi-sparse dataframes:

def fix_broken_semi_sparse_columns_inplace(df, fill_value=0):
        """
        Things happen in a DataFrame's life that gets some of its columns to be "semi-sparse".
        The tell-tale of this illness is when a column is of type `SparseSeries` but has no `sp_values` member.
        This function takes `df` and identifies its broken columns; and then cuts-fixes-reinserts them in same order.
        Hopefully this is more efficient than just calling df.to_sparse(fill_value=fill_value).
        """
        # TODO: what should we do if `df` is not a SparseDataFrame?
        #       right now we raise to make sure we notice if it happens.
        if not isinstance(df, pd.SparseDataFrame):
            raise Exception("df is not a SparseDataFrame (while not necessarily bad, it's unexpected and wasn't tested")
        columns = df.columns.tolist()
        for (index, column) in enumerate(columns):
            # we only want SparseSeries
            if not isinstance(df[column], pd.SparseSeries):
                continue
            # we only want broken ones
            if hasattr(df[column], "sp_values"):
                continue
            # cut-fix-insert
            df_of_col = df[[column]]
            df_of_col_fixed = df_of_col.to_sparse(fill_value=fill_value)
            col_fixed = df_of_col_fixed[column]
            del df[column]
            df.insert(index, column, col_fixed)

[jreback]

@yonatanp not sure what you are showing here. This issue was fixed in 0.18.1