reading an f-string from a file

[Skaperen]

i am reading a string from a file. how can i have it be interpreted as an f-string?

   num = 123456789
   with open('thefile') as openf: # this file has: number is {num}
       bar = do_f_string(openf.readline().rstrip())
   foo = f'number is {num}'
   print(foo)
   print(bar)

the above code should output the same line twice when do_f_string() is the function doing what i seek.