title: pandas DataFrame dropped column appearing again
body: <p>Sorry if I am doing something stupid, but I am very puzzled by this issue: I pass a DataFrame to a function, and inside that function I add a column and drop it. Nothing strange until here, but after the function has finished the DataFrame of the global namescope is showing the added&amp;dropped column. If I declare the DF as global, this is not happening...</p>
  
  <p>This test code is showing the issue in the four cases resulting from the combination of Python 3.3.3/2.7.6 and pandas 0.13.0/0.12.0:</p>
  
  <pre><code>#!/usr/bin/python
  import pandas as pd
  
  # FUNCTION DFcorr
  def DFcorr(df):
      # Calculate column of accumulated elements
      df['SUM']=df.sum(axis=1)
      print('DFcorr: DataFrame after add column:')
      print(df)
      # Drop column of accumulated elements
      df=df.drop('SUM',axis=1)
      print('DFcorr: DataFrame after drop column:')
      print(df)  
  
  # FUNCTION globalDFcorr
  def globalDFcorr():
      global C
      # Calculate column of accumulated elements
      C['SUM']=C.sum(axis=1)
      print('globalDFcorr: DataFrame after add column:')
      print(C)
      # Drop column of accumulated elements
      print('globalDFcorr: DataFrame after drop column:')
      C=C.drop('SUM',axis=1)
      print(C)  
  
  ######################### MAIN #############################
  C = pd.DataFrame.from_items([('A', [1, 2]), ('B', [3 ,4])], orient='index', columns['one', 'two'])
  print('\nMAIN: Initial DataFrame:')
  print(C)
  DFcorr(C)
  print('MAIN: DataFrame after call to DFcorr')
  print(C)
  
  C = pd.DataFrame.from_items([('A', [1, 2]), ('B', [3 ,4])], orient='index', columns=['one', 'two'])
  print('\nMAIN: Initial DataFrame:')
  print(C)
  globalDFcorr()
  print('MAIN: DataFrame after call to globalDFcorr')
  print(C)
  </code></pre>
  
  <p>And here you are the output:</p>
  
  <pre><code>MAIN: Initial DataFrame:
     one  two
  A    1    2
  B    3    4
  
  [2 rows x 2 columns]
  DFcorr: DataFrame after add column:
     one  two  SUM
  A    1    2    3
  B    3    4    7
  
  [2 rows x 3 columns]
  DFcorr: DataFrame after drop column:
     one  two
  A    1    2
  B    3    4
  
  [2 rows x 2 columns]
  MAIN: DataFrame after call to DFcorr
     one  two  SUM
  A    1    2    3
  B    3    4    7
  
  [2 rows x 3 columns]
  
  MAIN: Initial DataFrame:
     one  two
  A    1    2
  B    3    4
  
  [2 rows x 2 columns]
  globalDFcorr: DataFrame after add column:
     one  two  SUM
  A    1    2    3
  B    3    4    7
  
  [2 rows x 3 columns]
  globalDFcorr: DataFrame after drop column:
     one  two
  A    1    2
  B    3    4
  
  [2 rows x 2 columns]
  MAIN: DataFrame after call to globalDFcorr
     one  two
  A    1    2
  B    3    4
  
  [2 rows x 2 columns]
  </code></pre>
  
  <p>What am I missing? Many thanks!</p>