def preprocess_source(base_dir=os.curdir):
    """
    A special method for convert all source files to compatible with current
    python version during installation time.

    The source directory layout must like this :

    base_dir --+
               |
               +-- src (All sources are placed into this directory)
               |
               +-- preprocessed (Preprocessed sources are placed into this
               |   directory)
               |
               +-- setup.py
               |
               ...

    @return Preprocessed source directory
    """

    source_path = os.path.join(base_dir, SOURCE_DIR)
    destination_path = os.path.join(base_dir, PREPROCESSED_DIR)

    # The 'build' and 'dist' folder sometimes will not update! So we need to
    # remove them all !
    shutil.rmtree(os.path.join(base_dir, 'build'), ignore_errors=True)
    shutil.rmtree(os.path.join(base_dir, 'dist'), ignore_errors=True)

    # Remove all unused directories
    directories = []
    directory_patterns = ['__pycache__', '*.egg-info']
    for root, dirs, files in os.walk(destination_path):
        for adir in dirs:
            for pattern in directory_patterns:
                if fnmatch.fnmatch(adir, pattern):
                    directories.append(os.path.join(root, adir))
                    break

    for adir in directories:
        shutil.rmtree(adir, ignore_errors=True)

    if sys.version_info[0] >= 3:
        # We wrote program implicated by version 3, if python version
        # large or equal than 3, we need not change the sources.
        return source_path

    # Check and prepare 3to2 module.
    try:
        from lib3to2.main import main as lib3to2_main
    except ImportError:
        try:
            from pip import main as pipmain
        except:
            from pip._internal import main as pipmain

        pipmain(['install', '3to2'])

        from lib3to2.main import main as lib3to2_main

    # Remove old preprocessed sources.
    if not os.path.exists(destination_path):
        __copy_tree(source_path, destination_path)
        lib3to2_main("lib3to2.fixes",
                     ["-w", "-n", "--no-diffs"] + [destination_path])
    else:
        # Remove all files that only in right side
        # Copy all files that only in left side to right side, then
        # 3to2 on these files

        files = []
        dirs = []

        cmp_result = filecmp.dircmp(source_path, destination_path)
        dirs.append(cmp_result)

        while len(dirs) > 0:

            # Get the last one compare result
            cmp_result = dirs[-1]
            del dirs[-1]

            # Append all sub-dirs compare results, so that we could
            # continue our loop.
            dirs.extend(list(cmp_result.subdirs.values()))

            # Remove all files that only in right side
            for file_name in cmp_result.right_only:
                file_path = os.path.join(cmp_result.right, file_name)
                if os.path.isdir(file_path):
                    shutil.rmtree(file_path, ignore_errors=True)
                    continue

                # Only parse files.
                try:
                    os.remove(file_path)
                except:
                    pass

            # Copy all files that only in left side to right side or
            # different files, then 3to2 on these files
            for file_name in (cmp_result.left_only + cmp_result.diff_files):
                left_file_path = os.path.join(cmp_result.left, file_name)
                right_file_path = os.path.join(cmp_result.right, file_name)

                if os.path.isdir(left_file_path):
                    __copy_tree(left_file_path, right_file_path)
                    files.append(right_file_path)
                    continue

                if not fnmatch.fnmatch(file_name, "*.py"):
                    continue

                try:
                    os.remove(right_file_path)
                except:
                    pass

                shutil.copy2(left_file_path, right_file_path)
                files.append(right_file_path)

        if len(files) > 0:
            lib3to2_main("lib3to2.fixes", ["-w", "-n", "--no-diffs"] + files)

    return destination_path