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

Run the following code from a directory that contains a directory named bar (containing one or more files) and a directory named baz (also containing one or more files). Make sure there is not a directory named foo.

import shutil
shutil.copytree('bar', 'foo')
shutil.copytree('baz', 'foo')

It will fail with:

$ python copytree_test.py 
Traceback (most recent call last):
  File "copytree_test.py", line 5, in <module>
    shutil.copytree('baz', 'foo')
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/shutil.py", line 110, in copytree
  File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/os.py", line 172, in makedirs
OSError: [Errno 17] File exists: 'foo'

I want this to work the same way as if I had typed:

$ mkdir foo
$ cp bar/* foo/
$ cp baz/* foo/

Do I need to use shutil.copy() to copy each file in baz into foo? (After I’ve already copied the contents of ‘bar’ into ‘foo’ with shutil.copytree()?) Or is there an easier/better way?