Can python dictionary comprehension be used to create a dictionary of substrings and their locations?

The problem is that v[0] depends on the length or v[1], which means that either the operation to generate v[1] would have to operate twice, or that the dictionary would have to be iterated over in order to fill in v[0] to replace the dummy value included the first time.

Another problem is that dict comprehensions expect the entire key and value to be available immediately, which means that you would have to run a list comprehension to get all the indexes of the character, which means that the entire operation becomes O(n²).

The only optimization I would make would be to replace the creation of d so that you don't need to check for key containment.

d = collections.defaultdict(lambda: [0, []])

It is scary, but (I added just offsets, number of occurrences you may get from list of offsets). Yes, it may be done

In [83]: my_str = 'abcdabcxdabc'

In [84]: n=3

In [85]: {substr: [my_str.replace(substr, ' '*n, c).index(substr) 
                   for c in xrange(my_str.count(substr))]
   ....: for substr in set(my_str[idx:idx+n] for idx in xrange(len(my_str)-n))}
Out[85]: 
{'abc': [0, 4, 9],
 'bcd': [1],
 'bcx': [5],
 'cda': [2],
 'cxd': [6],
 'dab': [3, 8],
 'xda': [7]}