Mixing iteration and read methods would lose data

I just upgraded to Python 2.7.12 and my code is now erroring with rather unhelpful Mixing iteration and read methods would lose data errors. Googling for this yields a number of StackOverflow posts where people are iterating over a file, and then within the loop, doing calls on the file. This sounds like a good error for their case, but in my case the fixes are somewhat more inane.

This errors:

    persistence.write_dict_uint32_to_list_of_uint32s(output_file, { k: [] for k in read_set_of_uint32s(input_file) })

This fixes that error:

    set_value = persistence.read_set_of_uint32s(input_file)
    persistence.write_dict_uint32_to_list_of_uint32s(output_file, { k: [] for k in set_value })

Another error at another location:

    persistence.write_set_of_uint32s(output_file, persistence.read_set_of_uint32s(input_file))

Some simple rearranging makes this error go away:

    set_value = persistence.read_set_of_uint32s(input_file)
    persistence.write_set_of_uint32s(output_file, set_value)

And finally, the next error at another location:

def read_string(f):
    s = ""
    while 1:
        v = f.read(1) # errors here
        if v == '\0':
            break
        s += v
    return s

And the fix for this:

def read_bytes(f, num_bytes):
    return f.read(num_bytes)

def read_string(f):
    s = ""
    while 1:
        v = read_bytes(f, 1)
        if v == '\0':
            break
        s += v
    return s

This error seems like a horribly broken idea, that is popping up in places it wasn't intended.