Don't listen to anyone. There is only one obvious way to write this in
one line:

count = +(lambda *a: a[0](*a))((lambda q, r, s, i: r(r, q, s, i, next(i,
s))), (lambda r, q, s, i, x: x is not s and (x in seek) + q(q, r, s,
i)), [], iter(chars))

It is not even use such non-functional paradigm as a for loop.