Pandas long format DataFrame from multiple lists of different length

Question

Consider I have multiple lists

A = [1, 2, 3]
B = [1, 4]

and I want to generate a Pandas DataFrame in long format as follows:

type | value
------------
A    | 1
A    | 2
A    | 3
B    | 1
B    | 4

What is the easiest way to achieve this? The way over the wide format and melt is not possible(?) because the lists may have different lengths.

Answer 1

Create dictionary for types and create list of tuples by list comprehension:

A = [1, 2, 3]
B = [1, 4]

d = {'A':A,'B':B}

print ([(k, y) for k, v in d.items() for y in v])
[('A', 1), ('A', 2), ('A', 3), ('B', 1), ('B', 4)]

df = pd.DataFrame([(k, y) for k, v in d.items() for y in v], columns=['type','value'])
print (df)
  type  value
0    A      1
1    A      2
2    A      3
3    B      1
4    B      4

Another solution, if input is list of lists and types should be integers:

L = [A,B]
df = pd.DataFrame([(k, y) for k, v in enumerate(L) for y in v], columns=['type','value'])
print (df)
   type  value
0     0      1
1     0      2
2     0      3
3     1      1
4     1      4