import itertools
  [(c, len(list(it))) for (c, it) in itertools.groupby("aaaabbbcca")]

It's not the first time I've wanted

  def ilen(it):
    return sum(1 for _ in it)

as part of itertools. Which would make the second line be:

  [(c, itertools.ilen(it)) for (c, it) in itertools.groupby("aaaabbbcca")]

Rejected many times, eg, from 2010: https://bugs.python.org/issue9547 .