Buggy .ix assignment
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```
1) With numeric values everywhere:

df['tt1'] = df['tt2'] = 0
df.ix[1, ['tt1', 'tt2']] = [1, 2]
print df.ix[0:2,['tt1', 'tt2']]

Output:

   tt1  tt2
0    0    0
1    1    2
2    0    0

2) With string values everywhere:

df['tt1'] = df['tt2'] = '0'
df.ix[1, ['tt1', 'tt2']] = ['1', '2']
print df.ix[0:2,['tt1', 'tt2']]

Output:

      tt1     tt2
0       0       0
1  [1, 2]  [1, 2]
2       0       0
```
