I'm curious what you mean by "iterating variables have to be predeclared". In my experience mypy's type inference works with bog standard python constructs like a for loop that appends into a list that gets declared before the loop, aka:

  output = []
  for x in range(1):
      output.append(x)
      
  reveal_type(output)
  
  # note: Revealed type is 'builtins.list[builtins.int*]'