Python script that change md5 of binary file randomly

I want to write a python script that will change the md5 of a binary file in every execution of this script. I want that after execution of this script, the binary file will be still executable with different md5 every time. I tried this script:

with open('C:\file.exe', 'wb') as fout:
                fout.write(os.urandom(1024))

After execution of this script, file.exe wasn't executable anymore. Is there any other way to do it ?