Dictionary with tuple keys and two lists as values, where a value is added to just one of the lists

Question

I have a set of integers in the form of a tuple forming my keys (a,b). I need to construct a dictionary where, for each key, a value (float) is put in one of the two lists that acts as pair-value:

(a,b) : [x_list,y_list]

I am constructing the dictionary from a txt file where each line has the tuple (a,b) and ONE of either x or y-type value that should be added to the list. However, I fail to understand how this can be done.

To be more precise:

if the txt file contains:

15,17,x_type,-1.1
15,17,y_type,44
1,2,y_type,-0.38
15,17,y_type,5

the dictionary should produce

d: {(1,2): [[], [-0.38]] , (15,17): [[-1.1], [5,44]]

What I am trying:

example = ['15,17,x_type,-1.1','15,17,y_type,44','1,2,y_type,-0.38','15,17,y_type,5']

for _ in example:
    [a,b,val_type,val] = _.split(',')
    if val_type == 'x_type':
       d[(a,b)] = [val,]
    if val_type == 'y_type':
       d[(a,b)] = [,val]

The syntax allows for [x,], but does not for [,y]. Why?