index into list of pandas series

Question

I have a list of pandas Series objects obj and a list of indices idx . What I want is a new Series out that for each row in idx contains the value of obj[idx] if idx is not 255 and -1 otherwise.

The following code does what I want to achieve, but I'd like to know if there's a better way of doing this, especially without the overhead of first creating a Python list and then converting that into a pandas series.

>>> import pandas as pd
>>> obj = [pd.Series([1, 2, 3]), pd.Series([4, 5, 6]), pd.Series([7, 8, 9])]
>>> idx = pd.Series([0, 255, 2])
>>> out = pd.Series([obj[idx[row]][row] if idx[row] != 255 else -1 for row in range(len(idx))])
>>> out
0    1
1   -1
2    9
dtype: int64
>>>

Thanks in advance.

Answer 1

Using reindex + lookup

pd.Series(pd.concat(obj,1).reindex(idx).lookup(idx,idx.index)).fillna(-1)
Out[822]: 
0    1.0
1   -1.0
2    9.0
dtype: float64