New submission from Craig McQueen <pyt...@craig.mcqueen.id.au>: I have just been trying to figure out how string interpolation works for "%s", when Unicode strings are involved. It seems it's a bit complicated, but the Python documentation doesn't really describe it. It just says %s "converts any Python object using str()".
Here is what I have found (I think), and it could be worth improving the documentation of this somehow. Example 1: "%s" % test_object >From what I can tell, in this case: 1. test_object.__str__() is called. 2. If test_object.__str__() returns a string object, then that is substituted. 3. If test_object.__str__() returns a Unicode object (for some reason), then test_object.__unicode__() is called, then _that_ is substituted instead. The output string is turned into Unicode. This behaviour is surprising. [Note that the call to test_object.__str__() is not the same as str(test_object), because the former can return a Unicode object without causing an error, while the latter, if it gets a Unicode object, will then try to encode('ascii') to a string, possibly generating a UnicodeEncodeError exception.] Example 2: u"%s" % test_object In this case: 1. test_object.__unicode__() is called, if it exists, and the result is substituted. The output string is Unicode. 2. If test_object.__unicode__() doesn't exist, then test_object.__str__() is called instead, converted to Unicode, and substituted. The output string is Unicode. Example 3: "%s %s" % (u'unicode', test_object) In this case: 1. The first substitution causes the output string to be Unicode. 2. It seems that (1) causes the second substitution to follow the same rules as Example 2. This is a little surprising. ---------- assignee: d...@python components: Documentation messages: 109516 nosy: cmcqueen1975, d...@python priority: normal severity: normal status: open title: Improve docs for string interpolation "%s" re Unicode strings versions: Python 2.7 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue9196> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com