[BUG] Consistency in column selection

[samukweku]

• Did you find a bug in datatable, or maybe the bug found you? f symbol accepts mixed-selector types in the j section, which is not allowed when not selecting with the f symbol.

• How to reproduce the bug?

from datatable import dt, f

DF = dt.Frame(
    {
        "V1": [1, 2, 1, 2, 1, 2, 1, 2, 1],
        "V2": [1, 2, 3, 4, 5, 6, 7, 8, 9],
        "V3": [0.5, 1.0, 1.5] * 3,
        "V4": ["A", "B", "C"] * 3,
    }
)

DF

   |    V1     V2       V3  V4   
   | int32  int32  float64  str32
-- + -----  -----  -------  -----
 0 |     1      1      0.5  A    
 1 |     2      2      1    B    
 2 |     1      3      1.5  C    
 3 |     2      4      0.5  A    
 4 |     1      5      1    B    
 5 |     2      6      1.5  C    
 6 |     1      7      0.5  A    
 7 |     2      8      1    B    
 8 |     1      9      1.5  C    
[9 rows x 4 columns]


In [161]: DF[:, ['V1', 0]]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-161-ada00c210ff6> in <module>
----> 1 DF[:, ['V1', 0]]

TypeError: Mixed selector types are not allowed. Element 1 is of type integer, whereas the previous element(s) were of type string


In [162]: DF[:, f['V1', 0]]
Out[162]: 
   |    V1     V2
   | int32  int32
-- + -----  -----
 0 |     1      1
 1 |     2      2
 2 |     1      1
 3 |     2      2
 4 |     1      1
 5 |     2      2
 6 |     1      1
 7 |     2      2
 8 |     1      1
[9 rows x 2 columns]

• What was the expected behavior? I think f should return a TypeError if the selector types are mixed

• Your environment? python: '3.9.2 | packaged by conda-forge | (default, Feb 21 2021, 05:02:46) \n[GCC 9.3.0]' datatable: '1.0.0a0+build.1615412817.sam' os: linux