Diff the files in a directory using Python

gistfile1.py

#used as part of a unit test
def _compare_directories(self, expected_dir, actual_dir, header=None):
    """Compares two directories and diffs any different files.
    
    The file list of both directories must match and the files must match.
    Sub directories are not considered as I don't need to in my case.
    
    The diff will not show that a file was deleted from one side. It shows
    that all the lines were removed.
    
    :raises: AssertionError if there are differences. Error will 
    contain the diff as it's message.  
    
    :param expected_dir: directory of expected files
    
    :param actual_dir: directory of actual files
    
    :param header: Optional string header to prepend to the diff.
    """
    
    if os.path.exists(expected_dir) and os.path.exists(actual_dir):
        dir_diff = filecmp.dircmp(expected_dir, actual_dir)
        diff_files = list(itertools.chain(dir_diff.diff_files, 
            dir_diff.left_only, dir_diff.right_only))
    else:
        try:
            diff_files = os.listdir(actual_dir)
        except (OSError):
            diff_files = os.listdir(expected_dir)

    if not diff_files:
        return
        
    sb = []
    if header:
        sb.append(header)
        
    def safe_read_lines(path):
        if not os.path.exists(path):
            return []
        f = open(path)
        try:
            return f.readlines()
        finally:
            f.close()
            
    for diff_file in diff_files:
        expected_file = os.path.join(expected_dir, diff_file)
        actual_file = os.path.join(actual_dir, diff_file)
        
        if os.path.isdir(expected_file) or os.path.isdir(actual_file):
            raise RuntimeError("Unexpected sub dir")
            
        diff = difflib.context_diff(
            safe_read_lines(expected_file),
            safe_read_lines(actual_file),
            fromfile=expected_file, 
            tofile=actual_file)
        sb.extend(diff)

    self.fail(msg="".join(sb))