Random string generation I/O python

I'm creating a python script which should create a new file every time it generates a new string (everytime the script is executed it prints out the string to file), and also compares this random string with a string which is a text like: THIS IS A STRING, which is located on another already created file.

For example, the string THIS IS A STRING is located on file called file.dat, and the random generated string should be written to a file, in this case I call it newfile.txt.

However, I have this code:

import string
import random

file = open('file.dat', 'r')
file=file.read()
print file

def id_generator(size=28, chars=string.ascii_uppercase + string.digits):
    return ''.join(random.choice(chars) for _ in range(size))
file = open("newfile.txt", "w")
file.write(id_generator())
file.close()

This code, simply reads the file.dat archive, prints it to console, then generates a random string and stores it on a file called newfile.txt, but it doesn't compares anything, so, to accomplish that I've modified the code like this:

import string
import random

file = open('bullshit.dat', 'r')
file=file.read()
print file

def id_generator(size=28, chars=string.ascii_uppercase + string.digits):
    return ''.join(random.choice(chars) for _ in range(size))
with open("newfile.txt", "r") as f: stored = f.readline() 
if stored == id_generator(): 
    print('success!')

Now, my problem is, that this code just reads an already created file, which is newfile.txt, I need to create a new one like the code I had before, but comparing the strings.

I tried modifying the last three lines like this:

with open("newfile.txt", "w") as f: stored = f.readline() 
if stored == id_generator(): 
    print('success!')

But it throws me:

Traceback (most recent call last):
File "string2.py", line 20, in <module>
with open("newfile.txt", "w") as f: stored = f.readline() 
IOError: File not open for reading

How can I accomplish something like the first version, but comparing the strings as the second one?