I have some questions about this:

1) In Lib/test/string_tests.py it says:
“Common tests shared by test_str, test_unicode, test_userstring and
test_string”

but

   a) I cannot find test_str

   b) string is imported and only some constants ascii_letters
   and digits are used

   c) In test_join there is a comment “see the test in
   test.test_string.StringTest.test_join” Is that obsolete?
   (I cannot find StringTest in the test_string test)

2) Is there more tests for the build in format (in
test_buildin.BuiltinTest.test_format only the basic machinery is tested.
I would expect something like in test_format.py somewhere

3) it is true that all tests for the build in 'format' should also pass 
in 'string.Formatter().format'

Thanks in advance!

francis