BUG: concat of MultiIndex with names passed

[seth-a]

Code Sample, a copy-pastable example if possible

    import numpy as np
    import pandas as pd

    res = []
    for _ in range(2):
        res1 = []
        # Only occurs when dataframe is used with measure
        data = np.zeros((30, 21))
        idx = np.random.randint(0, 5, 30)
        df = pd.DataFrame(data, index=idx).loc[3]
        #df = pd.DataFrame(data[::5, :])  # Uncomment for example of correct behavior

        res1.append(pd.DataFrame(sum(data.dot(df.T))))
        tmp = pd.concat(res1, keys=[1], names=['level1'])

        res.append(tmp)
    final = pd.concat(res, keys=[i for i in range(2)], names=['level2'])
    print(final)

Problem description

In python, datatypes generally don't matter. A dataframe is a dataframe, but as shown in the example code concat'ing dataframes with an index does not have the same behavior as dataframes without an index. The label for a level of the index is dropped. This is a small bug. Run it several times (10-12 seems to do it) and you will see a much more worrisome issue: on occasion, the label is not dropped. Yes, the output of concat is random.

Expected Output

level2 level1       
0      1      0  0.0
              1  0.0
              2  0.0
              3  0.0
              4  0.0
              5  0.0
1      1      0  0.0
              1  0.0
              2  0.0
              3  0.0
              4  0.0
              5  0.0

Output of pd.show_versions()

level2         
0      1 0  0.0
         1  0.0
         2  0.0
         3  0.0
         4  0.0
         5  0.0
1      1 0  0.0
         1  0.0
         2  0.0
         3  0.0

[jreback]

In python, datatypes generally don't matter. A dataframe is a dataframe, but as shown in the example

not sure what you mean

[jreback]

can you update your example to show exactly what is the problem. you seems to be doing lots of stuff, but its not clear what the issue is (if any).

and show a constructed frame that matches your expectations. IOW if this is a bug, then ultimately we need an example that we can do something like

from pandas.util import testing as tm
result = .....
expected = DataFrame(....)
tm.assert_frame_equal(result, expected)

[seth-a]

The label of the second column of the index is being changed to None. Run the example, uncomment the line that says "Uncomment for example of correct behavior" to see what is expected. Yes, the example is doing lots of "stuff", but that's what it takes to reproduce the bug.

[jreback]

@seth-a can you post an expected result and then run your example and post. the more information you show and the more obvious the quicker this will get looked at.

[seth-a]

I've updated the formatting in the expected results section, are you looking for something beyond what is there?

[jreback]

slightly modified.

In [1]: import numpy as np
   ...: import pandas as pd
   ...: 
   ...: res = []
   ...: for _ in range(2):
   ...:     res1 = []
   ...:     # Only occurs when dataframe is used with measure
   ...:     data = np.zeros((30, 21))
   ...:     idx = np.random.randint(0, 5, 30)
   ...:     df = pd.DataFrame(data, index=idx).loc[3]
   ...:     #df = pd.DataFrame(data[::5, :])  # Uncomment for example of correct behavior
   ...: 
   ...:     df2 = pd.DataFrame(sum(data.dot(df.T)))
   ...:     df2.index.name = 'level2'
   ...:     res1.append(df2)
   ...:     tmp = pd.concat(res1, keys=[1], names=['level1'])
   ...: 
   ...:     res.append(tmp)
   ...: final = pd.concat(res, keys=[i for i in range(2)])
   ...: 

In [2]: final
Out[2]: 
                   0
  level1 level2     
0 1      0       0.0
         1       0.0
         2       0.0
         3       0.0
         4       0.0
1 1      0       0.0
         1       0.0
         2       0.0

The the issue is that you are concatting 2 MultiIndexes with names of ['level1', None]
your original res is this:

In [2]: res
Out[2]: 
[            0
 level1       
 1      0  0.0
        1  0.0
        2  0.0
        3  0.0,             0
 level1       
 1      0  0.0
        1  0.0
        2  0.0
        3  0.0
        4  0.0
        5  0.0
        6  0.0
        7  0.0]

so you are expecting the passed in named to 'automatically' figure out that the new names are the existing first level name plus the passed in names?

how does that make sense?

I suppose that its actually an error to pass in any names if you have MultiIndex that you are concatting in the first place. So would take a PR for that.

[jreback]

FYI here is the code for creating the concatted index. It is slightly non-trivial!

[seth-a]

Hmm, clearly you've missed the point. In fact pandas does automatically figure out that the new names should be the existing index plus the passed in names, as can be shown by un-commenting the line in my example saying "Uncomment for example of correct behavior". Here's an example that may make it a little easier to see,

    def concat_multiple(bug=False):
        """
        Make a mess with DataFrames, if bug is True change behavior so that bug shows
        """
        res = []
        for _ in range(2):
            res1 = []
            # Only occurs when dataframe is used with measure
            data = np.zeros((30, 21))
            idx = np.random.randint(0, 5, 30)

            df = pd.DataFrame(data, index=idx)
            # Bug is right here.  Same dataframe, just different method of indexing
            # This is the only thing that changes
            if bug:
                df = df.loc[3]

            res1.append(pd.DataFrame(sum(data.dot(df.T))))
            tmp = pd.concat(res1, keys=[1], names=['level1'])

            res.append(tmp)
        return pd.concat(res, keys=[i for i in range(2)], names=['level2'])

    res_bad = concat_multiple(bug=True)   # Results when bug is present
    res_good = concat_multiple(bug=False) # Results when bug is not present

    print("These indexes should be the same")
    print (res_bad.index.names)
    print (res_good.index.names)
    if (res_bad.index.names != res_good.index.names):
        print("But they aren't")

    # It gets worse, let's run it several times and see how often the bug is present
    good = [concat_multiple(bug=True).index.names ==res_good.index.names for _ in range(20)]
    print("pandas is broken  {}% of the time".format((1-sum(good)/len(good))*100 ))

And here's the output:

These indexes should be the same
['level2', None, None]
['level2', 'level1', None]
But they aren't
pandas is broken  95.0% of the time

And when a bug like this exists you don't need to link to the source code to convince me it's a mess.

[jreback]

@seth-a

this is really really odd thing to do: of course this doesn't work
idx = np.random.randint(0, 5, 30)