"columns must have matching element counts" while performing explode in pandas

Question

I'm performing explode on 3 columns in a dataframe, all the 3 columns have the same number of elements. But while performing the explode operation, the code runs in a error

columns must have matching element counts

Below is the code snippet I'm trying to perform.

 list1 = primary_secondary[['Column_x', 'Column_y']].replace('', np.nan).stack().groupby(level=0).apply(list)

list2 = primary_secondary[['Column_a', 'Column_b']].replace('', np.nan).stack().groupby(level=0).apply(list)

list3 = primary_secondary[['Column_c', 'Column_d']].replace('', np.nan).stack().groupby(level=0).apply(list)

df = df.assign(Combination_1=list1.fillna(''), Combination_2=list2.fillna(''),Combination_3=list3.fillna('') ).explode(['Combination_1', 'Combination_2', 'Combination_3'])

The operation I intend to perform is, combine columns column_x, column_y into a new column Combination_1, and that should be performed in the following manner.

I want to merge 2 columns of the same dataframe, and add a duplicate row using the same values as it has in the other columns

consider the following dataframe:

Column A	Column B	Column C
ABC	''	1
GHI	XYZ	2
''	PQR	3
''	''	4

The conditions are:

1. If the Column A has a alphanumeric value and the Column B has a Nan value or a '' (empty string) -> the Result column should only consider the value from Number-first
2. If the Column A has a Nan or '' (empty string) value and the Column B has a alphanumeric value -> the Result column should only consider the value from Number-second
3. If the values from both the columns are alphanumeric the result column should duplicate itself where the first value should be Column A and the second value should be Column B
4. If both the Columns have Nan or empty string values, the result should consist of a '' (empty string) value

Following would be the output for the above dataframe:

Column A	Column B	Column C	Result
ABC	''	1	ABC
GHI	XYZ	2	GHI
GHI	XYZ	2	XYZ
''	PQR	3	PQR
''	''	4	''

I have been unsuccessful in making it work

Thanks a lot in advance for your help !