CODE EXAMPLE FOR PYTHON

dynamically create python dictionary

dict= {} # create an empty dictionary
list= [['a', 1], ['b', 2], ['a', 3], ['c', 4]]
#list is our input where 'a','b','c', are keys and 1,2,3,4 are values
for i in range(len(list)):
     if list[i][0] in dic.keys():# if key is present in the list, just append the value
         dic[list[i][0]].append(list[i][1])
     else:
         dic[list[i][0]]= [] # else create a empty list as value for the key
         dic[list[i][0]].append(list[i][1]) # now append the value for that key