Constructing pandas DataFrame from values in variables gives “ValueError: If using all scalar values, you must pass an index”

The error message says that if you’re passing scalar values, you have to pass an index. So you can either not use scalar values for the columns — e.g. use a list:

>>> df = pd.DataFrame({'A': [a], 'B': [b]})
>>> df
   A  B
0  2  3

or use scalar values and pass an index:

>>> df = pd.DataFrame({'A': a, 'B': b}, index=[0])
>>> df
   A  B
0  2  3