[Python] Python list of tuples to nested dict

I have a data structure as follows:

[(a,x1,1),(a,x2,5),(b,x1,1) ...]


and i want to turn it into a nested dictionary of the form

{a:{x1:1, x2:5}, b:{x1:1}...}


I tried

dictdata = {}
for row in rows:
ean = row[1].encode('ascii','ignore')
period = str(row[0])
value = row[2]
dictdata[ean]={} # init sub dictionary
dictdata[ean][period] = value


but every time I do dictdata[ean]={}, the contents get erased, so this will not work. If I do not initialise the sub-dictionary, I also cannot get it to work.

Any help appreciated

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