Return label of the column which has the maximum value if it is higher than a certain number

Question

I have a pandas dataframe with 18 columns, the columns from 2:17 hold numerical values with labels 2-17. I want to add a new column ('category') to the dataframe to include the label of the column which holds the maximum value if that value is higher than a certain number (0.5). I used the below code after utilizing a related question on #Pandas (python): max in columns define new value in new column

  test_df['Category'] = test_df.values[:,2:18].argmax(1) if (test_df.values[:,2:18].max(1) >=0.5) else 'none'

when I run the above code, I get the error message "The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()", any advice is appreciated.