python: pandas np.where vs. df.loc with multiple conditions

Question

Np.where has been giving me a lot of errors, so I am looking for a solution with df.loc instead.

This is the np.where error I have been getting:

C:\Users\xxx\AppData\Local\Continuum\Anaconda2\lib\site-packages\ipykernel\__main__.py:1: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  if __name__ == '__main__':

I am working with the following dataframe df:

df = pd.DataFrame({'Column_A': ['AAA','AAA','ABC','CDE'],'checked': ['0','0','1','0'],'duplicate': ['True','True','False','False']})

    Column_A    checked   duplicate
0   AAA             0      True
1   AAA             0      True
2   ABC             1      False
3   CDE             0      False

I want to create an additional flag, if checked is 0 and duplicate is True.

I tried this and it didn't work:

df['flag'] = (np.where((df['checked'] == 'Y') &(df['duplicate'] == 'True'), 'Y', '0'))

TypeError: invalid type comparison

I tried it with df.loc:

df['flag'] = (df.loc[df['checked'] == 'Y']& df.loc[df['duplicate'] == 'True'], 'Y','0')

TypeError: invalid type comparison

and I get the same error!

Answer 1

I think your boolean are not strings, so need remove ':

df = pd.DataFrame({'Column_A': ['AAA','AAA','ABC','CDE'],
                  'checked': ['0','0','1','0'],
                  'duplicate': [True, True, False, False]})

df['flag'] = np.where((df['checked'] == 'Y') &(df['duplicate'] == True), 'Y', '0')
print (df)
  Column_A checked  duplicate flag
0      AAA       0       True    0
1      AAA       0       True    0
2      ABC       1      False    0
3      CDE       0      False    0

Or if compare with boolean column, == True can be omited:

df['flag'] = np.where((df['checked'] == 'Y') &(df['duplicate']), 'Y', '0')
print (df)
  Column_A checked  duplicate flag
0      AAA       0       True    0
1      AAA       0       True    0
2      ABC       1      False    0
3      CDE       0      False    0

Also if need check checked need ' because strings:

df['flag'] = np.where((df['checked'] == '0') &(df['duplicate'] == True), 'Y', '0')
print (df)
  Column_A checked  duplicate flag
0      AAA       0       True    Y
1      AAA       0       True    Y
2      ABC       1      False    0
3      CDE       0      False    0

EDIT:

Solution with loc:

df['flag'] = '0'
mask = (df['checked'] == '0') &(df['duplicate'])
df.loc[mask, 'flag'] = 'Y'
print (df)
  Column_A checked  duplicate flag
0      AAA       0       True    Y
1      AAA       0       True    Y
2      ABC       1      False    0
3      CDE       0      False    0