Convert a dictionary of list to a dataframe in a specific format - python-3.x

Question

I have a dictionary with each key holding a list of values. Now, I want to convert that to an dataframe in a specific format.
Example -
dct = {key1:["North", "South", "East"], key2:["East"], key3:["East", "West", "North", "South"]}
The table should look like
key1 North
South
East
key2 East
key3 East
West
North
South

Answer 1

First create DataFrame in list comprehension for tuples:
df = pd.DataFrame([(k, x) for k, v in dct.items() for x in v], columns=['a','b'])
print (df)
a b
0 key1 North
1 key1 South
2 key1 East
3 key2 East
4 key3 East
5 key3 West
6 key3 North
7 key3 South
In pandas for need replace non existing values by NaN or some another value, like here '' for empty string:
#replace values by NaN
#df['a'] = df['a'].mask(df['a'].duplicated())
df['a'] = df['a'].mask(df['a'].duplicated(), '')
print (df)
a b
0 key1 North
1 South
2 East
3 key2 East
4 key3 East
5 West
6 North
7 South
If need convert a column to index (but stil index values are key1, key2, key3 and ''):
s = df.set_index('a')['b']
print (s)
a
key1 North
South
East
key2 East
key3 East
West
North
South
Name: b, dtype: object