[python] How do I copy an entire directory of files into an existing directory using Python?

Here is my pass at the problem. I modified the source code for copytree to keep the original functionality, but now no error occurs when the directory already exists. I also changed it so it doesn't overwrite existing files but rather keeps both copies, one with a modified name, since this was important for my application.

import shutil
import os


def _copytree(src, dst, symlinks=False, ignore=None):
    """
    This is an improved version of shutil.copytree which allows writing to
    existing folders and does not overwrite existing files but instead appends
    a ~1 to the file name and adds it to the destination path.
    """

    names = os.listdir(src)
    if ignore is not None:
        ignored_names = ignore(src, names)
    else:
        ignored_names = set()

    if not os.path.exists(dst):
        os.makedirs(dst)
        shutil.copystat(src, dst)
    errors = []
    for name in names:
        if name in ignored_names:
            continue
        srcname = os.path.join(src, name)
        dstname = os.path.join(dst, name)
        i = 1
        while os.path.exists(dstname) and not os.path.isdir(dstname):
            parts = name.split('.')
            file_name = ''
            file_extension = parts[-1]
            # make a new file name inserting ~1 between name and extension
            for j in range(len(parts)-1):
                file_name += parts[j]
                if j < len(parts)-2:
                    file_name += '.'
            suffix = file_name + '~' + str(i) + '.' + file_extension
            dstname = os.path.join(dst, suffix)
            i+=1
        try:
            if symlinks and os.path.islink(srcname):
                linkto = os.readlink(srcname)
                os.symlink(linkto, dstname)
            elif os.path.isdir(srcname):
                _copytree(srcname, dstname, symlinks, ignore)
            else:
                shutil.copy2(srcname, dstname)
        except (IOError, os.error) as why:
            errors.append((srcname, dstname, str(why)))
        # catch the Error from the recursive copytree so that we can
        # continue with other files
        except BaseException as err:
            errors.extend(err.args[0])
    try:
        shutil.copystat(src, dst)
    except WindowsError:
        # can't copy file access times on Windows
        pass
    except OSError as why:
        errors.extend((src, dst, str(why)))
    if errors:
        raise BaseException(errors)