I want to select columns with a specific value (say 1) in a specific row (say first row) for Pandas Dataframe

Use iloc with boolean indexing, for performance is better filtering index not DataFrame and then select index (see performance):

df = pd.DataFrame({'A':list('abcdef'),'B':[4,5,4,5,5,4],'C':[7,8,9,4,2,3],'D':[1,3,5,7,1,0],'E':[5,3,6,9,2,4],'F':list('aaabbb')
})print (df)A  B  C  D  E  F
0  a  4  7  1  5  a
1  b  5  8  3  3  a
2  c  4  9  5  6  a
3  d  5  4  7  9  b
4  e  5  2  1  2  b
5  f  4  3  0  4  bs = df.iloc[0]        
a = s.index[s == 1]
print (a)
Index(['D'], dtype='object')a = s.index.values[(s == 1)]
print (a)
['D']