Constructing DataFrame from values in variables yields "ValueError: If using all scalar values, you must pass an index"

I have two variables as follows.
a = 2
b = 3


I want to construct a DataFrame from this:
df2 = pd.DataFrame({'A':a, 'B':b})


This generates an error:
ValueError: If using all scalar values, you must pass an index


I tried this also:
df2 = (pd.DataFrame({'a':a, 'b':b})).reset_index()


This gives the same error message. How do I do what I want?

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, 3])
>>> df
   A  B
0  2  3
3  2  3

Stack Overflow Q#17839973, score: 1258