title: pandas cannot delete columns
body: <p>I used pd.merge() to combine to two dataframes. So far it worked quite fine, but this time pd creates extra columns and I don't know why.
  Not big deal so far, but I can not delete unwanted columns and I don't know why</p>
  <pre><code>values_columns = ['LST_Day_1km', 'LST_Night_1km']
  table = month_mean.pivot(index=['population', 'country'], columns='month', values=values_columns)
  table = table.reset_index()
  
  results.dropna(inplace=True)
  table = pd.merge(table, results, on=['population', 'country'])
  table.reset_index()
  </code></pre>
  <pre><code>results = pd.Dataframe({'country': {0: 'uzbk', 1: 'uzbk', 2: 'uzbk', 3: 'uzbk', 6: 'uzbk'}, 'population': {0: 1, 1: 2, 2: 13, 3: 14, 6: 26}, 'n_plants': {0: 29.0, 1: 41.0, 2: 25.0, 3: 24.0, 6: 30.0}, 'n_dm3': {0: 0.0, 1: 0.0, 2: 0.0, 3: 0.0, 6: 7.0}, 'perc_dm3': {0: 0.0, 1: 0.0, 2: 0.0, 3: 0.0, 6: 23.33333333}})
  </code></pre>
  <pre><code>pd.DataFrame({'population': {0: 1, 1: 2, 2: 13, 3: 14, 4: 26},
   'country': {0: 'uzbk', 1: 'uzbk', 2: 'uzbk', 3: 'uzbk', 4: 'uzbk'},
   ('population', ''): {0: 1, 1: 2, 2: 13, 3: 14, 4: 26},
   ('country', ''): {0: 'uzbk', 1: 'uzbk', 2: 'uzbk', 3: 'uzbk', 4: 'uzbk'},
   ('LST_Day_1km', 'April'): {0: 21.52598039215687,
    1: 27.82574879227056,
    2: 27.23307692307695,
    3: 28.99916256157639,
    4: 26.263700000000036},
   ('LST_Day_1km', 'August'): {0: 36.79163021868789,
    1: 39.95906666666667,
    2: 38.78652777777779,
    3: 46.96329983249583,
    4: 39.641850594227506},
   ('LST_Day_1km', 'December'): {0: 0.6611801242236314,
    1: 6.980344827586234,
    2: 10.035423728813587,
    3: 11.079760000000029,
    4: 6.721406250000029},
   ('LST_Day_1km', 'February'): {0: -0.5136666666666424,
    1: 9.338888888888919,
    2: 11.175762711864435,
    3: 10.999791666666695,
    4: 7.987187500000024},
   ('LST_Day_1km', 'January'): {0: -2.583491124260326,
    1: 5.6667441860465395,
    2: 6.85056603773588,
    3: 7.350000000000033,
    4: 2.717241379310376},
   ('LST_Day_1km', 'July'): {0: 36.44175337186901,
    1: 42.49046332046341,
    2: 41.024377104377116,
    3: 48.574181523500776,
    4: 40.77033167495858},
   ('LST_Day_1km', 'June'): {0: 31.96304123711347,
    1: 41.723317191283336,
    2: 39.96400000000003,
    3: 45.81320000000002,
    4: 38.27521384928721},
   ('LST_Day_1km', 'March'): {0: 12.131560283687973,
    1: 19.09487179487184,
    2: 20.453040000000026,
    3: 21.504645669291357,
    4: 19.016351351351386},
   ('LST_Day_1km', 'May'): {0: 25.559118773946373,
    1: 36.469663299663324,
    2: 33.800755287009075,
    3: 37.53063400576372,
    4: 31.62000000000006},
   ('LST_Day_1km', 'November'): {0: 10.985531914893652,
    1: 14.045539568345351,
    2: 17.69792270531405,
    3: 19.28898989898993,
    4: 17.179842931937205},
   ('LST_Day_1km', 'October'): {0: 22.78096551724142,
    1: 26.49048109965638,
    2: 27.541752021563386,
    3: 32.219545454545475,
    4: 26.592708333333377},
   ('LST_Day_1km', 'September'): {0: 31.7962770562771,
    1: 34.359823874755435,
    2: 34.65355805243449,
    3: 41.5586535008977,
    4: 34.987818181818206},
   ('LST_Night_1km', 'April'): {0: 7.428039215686304,
    1: 11.624009661835778,
    2: 10.993653846153878,
    3: 12.09098522167491,
    4: 9.67710000000003},
   ('LST_Night_1km', 'August'): {0: 17.047176938369812,
    1: 19.450419047619054,
    2: 19.302986111111167,
    3: 21.32366834170856,
    4: 18.71767402376913},
   ('LST_Night_1km', 'December'): {0: -5.529627329192519,
    1: -1.8968965517241128,
    2: -1.5199999999999723,
    3: -1.1770399999999708,
    4: -3.743749999999973},
   ('LST_Night_1km', 'February'): {0: -8.199999999999973,
    1: -0.4308080808080539,
    2: -0.6925423728813282,
    3: -0.3518749999999719,
    4: -3.641718749999971},
   ('LST_Night_1km', 'January'): {0: -8.32088757396447,
    1: -2.084186046511599,
    2: -3.6739622641509166,
    3: -2.578965517241353,
    4: -6.3384482758620395},
   ('LST_Night_1km', 'July'): {0: 18.013121387283277,
    1: 21.493127413127457,
    2: 21.020235690235747,
    3: 23.224975688816883,
    4: 20.515671641791084},
   ('LST_Night_1km', 'June'): {0: 15.94376288659797,
    1: 20.185447941888633,
    2: 19.214458333333337,
    3: 21.213320000000014,
    4: 18.589877800407365},
   ('LST_Night_1km', 'March'): {0: -0.0034751773049363833,
    1: 5.004102564102594,
    2: 4.989680000000029,
    3: 5.707322834645698,
    4: 2.8841891891892186},
   ('LST_Night_1km', 'May'): {0: 12.303180076628376,
    1: 16.889124579124594,
    2: 15.533564954682795,
    3: 17.093919308357382,
    4: 14.693125000000032},
   ('LST_Night_1km', 'November'): {0: -0.1641489361701835,
    1: 0.9734532374101026,
    2: 2.970289855072489,
    3: 3.347373737373767,
    4: 1.9593193717277761},
   ('LST_Night_1km', 'October'): {0: 6.293034482758647,
    1: 7.753298969072191,
    2: 8.515768194070107,
    3: 9.684141414141447,
    4: 7.5757812500000234},
   ('LST_Night_1km', 'September'): {0: 12.172943722943756,
    1: 13.638454011741713,
    2: 14.51179775280902,
    3: 16.17215439856377,
    4: 13.910836363636404},
   'n_plants': {0: 29.0, 1: 41.0, 2: 25.0, 3: 24.0, 4: 30.0},
   'n_dm3': {0: 0.0, 1: 0.0, 2: 0.0, 3: 0.0, 4: 7.0},
   'perc_dm3': {0: 0.0, 1: 0.0, 2: 0.0, 3: 0.0, 4: 23.33333333}})
  </code></pre>
  <p>as you can see for population and country a new column was created, is pd.pivot the reason for that?</p>
  <p>anyway when I try to delete the columns:</p>
  <pre><code>table.drop(&quot;('population', '')&quot;, axis=1, inplace=True)
  # same results for table.drop(&quot;(population, )&quot;, axis=1, inplace=True)
  
  C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\reshape\merge.py:648: UserWarning: merging between different levels can give an unintended result (2 levels on the left,1 on the right)
    warnings.warn(msg, UserWarning)
  Traceback (most recent call last):
    File &quot;C:\ProgramData\Anaconda3\lib\site-packages\IPython\core\interactiveshell.py&quot;, line 3437, in run_code
      exec(code_obj, self.user_global_ns, self.user_ns)
    File &quot;&lt;ipython-input-6-02927c003eef&gt;&quot;, line 37, in &lt;module&gt;
      table.drop(&quot;('population', '')&quot;, axis=1, inplace=True)
    File &quot;C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\frame.py&quot;, line 4308, in drop
      return super().drop(
    File &quot;C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\generic.py&quot;, line 4153, in drop
      obj = obj._drop_axis(labels, axis, level=level, errors=errors)
    File &quot;C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\generic.py&quot;, line 4188, in _drop_axis
      new_axis = axis.drop(labels, errors=errors)
    File &quot;C:\ProgramData\Anaconda3\lib\site-packages\pandas\core\indexes\base.py&quot;, line 5591, in drop
      raise KeyError(f&quot;{labels[mask]} not found in axis&quot;)
  KeyError: '[&quot;(\'population\', \'\')&quot;] not found in axis'
  </code></pre>
  <p>Anyone has an Idea why
  (i) the columns are created in the first place
  (ii) I cannot drop the columns</p>
  <p>Cheers</p>