python generator endless stream without using yield

Question

i'm trying to generate an endless stream of results given a function f and an initial value x so first call should give the initial value, second call should give f(x), third call is f(x2) while x2 is the previous result of f(x) and so on..

what i have come up with:

def generate(f, x): 
   return itertools.repeat(lambda x: f(x))

which does not seem to work. any ideas? (i cant use yield in my code). also i cant use more than 1 line of code for this problem. any help would be appreciated.

also note that in a previous ex. i was asked to use the yield. with no problems:

while True:
    yield x
    x = f(x)

this works fine. but now.. no clue how to do it without

Answer 1

In Python 3.3, you can use itertools.accumulate:

import itertools

def generate(f, x):
  return itertools.accumulate(itertools.repeat(x), lambda v,_:f(v))

for i, val in enumerate(generate(lambda x: 2*x, 3)):
  print(val)
  if i == 10:
    break